Astronomy

Cen A

11 million light-years away a giant elliptical galaxy - the closest active galaxy to Earth Centaurus A's central region is a jumble of gas, dust, and stars in optical light, but both radio and x-ray telescopes trace a remarkable jet of high-energy particles streaming from the galaxy's core. The cosmic particle accelerator's power source is a black hole with about 10 million times the mass of the Sun coincident with the x-ray bright spot at the galaxy's center. Blasting out from the active galactic nucleus toward the upper left, the energetic jet extends about 13,000 light-years. A shorter jet extends from the nucleus in the opposite direction. Other x-ray bright spots in the field are binary star systems with neutron stars or stellar mass black holes. Active galaxy Centaurus A is likely the result of a merger with a spiral galaxy some 100 million years ago. This galaxy has long been considered an example of an elliptical galaxy disrupted by a recent collision with a smaller-companion spiral galaxy. The black hole in the active nucleus is so dense it contains the mass of about 55 million suns, compacted into a small region of space not much larger than our solar system. Mid-IR images show dust in the inner disk of the spiral galaxy Radio images show plasma jets It lies nestled between a pair of giant radio-emitting gas plumes ejected by its supersized black hole X-Ray images show high temperature gases and matter likely originating from an event in the center peculiar structure is the result of a merge event of a giant elliptical galaxy and a small spiral galaxy Centaurus A is a peculiar galaxy located in the southern constellation Centaurus. It is the fifth brightest galaxy in the night sky, the nearest giant galaxy to the Milky Way, and one of the nearest radio galaxies to Earth. Its exact type is uncertain, but it is usually classified either as a giant elliptical or lenticular galaxy. The galaxy's exact distance from Earth is also uncertain, but estimates generally range from 10 to 16 million light years. When observed in optical wavelengths, Centaurus A looks like an elliptical galaxy. However, ellipticals don't typically have dust bands because they are dust poor and contain mostly old stars. Astronomers believe that Centaurus A is in fact a giant elliptical galaxy currently in the process of devouring a dusty barred spiral galaxy about the same size as the nearby Triangulum Galaxy (Messier 33). The elliptical galaxy is believed to have assimilated its smaller neighbour about 200 to 700 million years ago. As one of the closest radio galaxies to the solar system, NGC 5128 and its active galactic nucleus have often been an object of study, while amateur astronomers like to observe it because it is one of the brightest galaxies in the sky and relatively easy to find. Centaurus A lies far to the south and can only be seen from the southern hemisphere and low northern latitudes. The galaxy lies about 4 degrees north of the globular cluster Omega Centauri, which can be seen without binoculars. The galaxy's central bulge and the dust lane can be seen in large binoculars and amateur telescopes, but additional details are only visible in larger telescopes. The galaxy is located near the centre of Centaurus constellation. It lies to the southwest of the bright star Menkent (Theta Centauri) and northwest of the even brighter Alpha and Beta Centauri Centaurus A is the nearest active galaxy to the solar system. It is a starburst galaxy, with more than 100 regions of star formation identified in the galaxy's disk. The galaxy's bulge, on the other hand, contains mostly evolved red stars. The starburst activity in the disk is believed to be a result of a collision with another galaxy. Centaurus A was first discovered by the Scottish astronomer James Dunlop from the Parramatta observatory in New South Wales in Australia on August 4, 1826. Dunlop included the galaxy as number 482 in his "A Catalogue of Nebulae and Clusters of Stars in the Centaurus A was identified as a radio galaxy by astronomers John Bolton, Bruce Slee, and Gordon Stanley in 1948-49. They had studied the galaxy using a sea interferometer at Dover Heights in Australia. The galaxy's radio emissions were among the first discovered to be associated with an extragalactic object. German astronomers Walter Baade and Rudolph Minkowski studied Centaurus A at the Palomar Observatory in California and confirmed that it was indeed a galaxy. They also suggested that the galaxy's peculiar morphology, with a superimposed dark dust lane, was the result of a merger between a giant elliptical and a small spiral galaxy. American astronomer Stuart Bowyer detected X-rays emanating from NGC 5128 in 1969-70. The UHURU satellite confirmed that the X-ray emissions were indeed coming from Centaurus A in 1970. An X-ray jet coming from the galaxy's central region was first discovered in the 1970s. Astronomers later observed the jet in radio wavelengths Centaurus A is our nearest giant galaxy, at a distance of about 13 million light-years in the southern constellation of Centaurus, and as such, it is one of the most extensively studied objects in the southern sky. It is an elliptical galaxy, currently merging with a companion spiral galaxy, resulting in areas of intense star formation and making it one of the most spectacular objects in the sky. Centaurus A hosts a very active and highly luminous central region, caused by the presence of a supermassive black hole with a mass of about 100 million solar masses (see ESO 04/01), and is the source of strong radio and X-ray emission. Thick dust layers almost completely obscure the galaxy's centre. Image: ESO/IDA/Danish 1.5 m/R. Gendler, J.-E. Ovaldsen & S. Guisard (ESO) Centaurus A is one of the few deep sky objects that can be detected across all energy ranges of the entire electromagnetic spectrum. The galaxy is even sometimes considered to be a source of Ultra High Energy Cosmic Rays (UHECR). The galaxy's core spans only 10 light days, which makes it one of the smallest extragalactic radio sources known. Centaurus A is the central galaxy in one of the two subgroups of the Centaurus A/M83 Group. The Southern Pinwheel Galaxy (Messier 83) is at the centre of the M83 Group. The Centaurus A/M83 Group is a located in the Virgo Supercluster, relatively near to us. It is sometimes identified as a single group of galaxies and sometimes as two groups. The galaxies in the group are gravitationally bound and do not appear to move relative to each other. A supernova was detected in Centaurus A in May 1986. Designated SN 1986G, the supernova was discovered by R. Evans and later classified as a Type Ia supernova, a violent explosion of a white dwarf. The supernova was discovered in the south-eastern part of the galaxy's dust lane. The centre of Centaurus A contains a supermassive black hole with an estimated mass of about 55 million solar masses. Material in the galaxy's dense core region releases enormous amounts of energy as it spirals toward the black hole. The jet ejected by the black hole is a source of X-ray and radio emissions. The inner parts of the relativistic jet are moving at an approximate speed of 9 million kilometres per minute, or half the speed of light. The galaxy's radio jets span more than a million light years. In X-ray wavelengths, the jet extending to the northwest is about 25,000 light years long, while the one extending southwest is significantly less conspicuous. Astronomers suspect that the more prominent jet is moving in our direction, while the other one is moving away from us. The galaxy's central region also has a number of smaller X-ray sources. These are suspected to be neutron stars and stellar-mass black holes that are accumulating material from nearby stars. Type: S0 pec or Ep Distance: 10 - 16 million light years (3 - 5 Mpc) Apparent magnitude: 6.84

Lilian Date

A Lilian date is the number of days since the beginning of the Gregorian Calendar on October 15, 1582, regarded as Lilian date 1. It was invented by Bruce G. Ohms of IBM in 1986 and is named for Aloysius Lilius, who devised the Gregorian Calendar

Sun-like star formation

A star forms when part of the interstellar medium (a cold dark cloud) begins to collapse under its own weight The cloud heats up as it shrinks and eventually its center becomes hot enough for fusion to begin At this point, the contraction stops and the star is born In order for these steps to happen, gravitational force must be able to overcome effect of heat

S type star

An S-type star (or just S star) is a cool giant with approximately equal quantities of carbon and oxygen in its atmosphere

Death of a low mass star

Before the carbon core of the low-mass star can attain the temperatures needed for carbon ignition, its density reaches a point beyond which it cannot be compressed further At this density, the electrons in the core become degenerate, the contraction of the core ceases, and the temperatures stop rising Some oxygen is formed via reactions between carbon and helium, but these reactions are not frequent or violent enough to create heavier elements Inner carbon core no longer generates energy and outer core continues to burn hydrogen and helium As more of the inner core reaches its final, high-density state, the nuclear burning increases in intensity The envelope continues to expand and cool, reaching the maximum radius of about 300 times that of the sun The burning now becomes unstable The helium-burning shell is subject to a series of explosive helium shell flashes These are caused by the enormous pressure in the helium burning shell and the extreme sensitivity of the triple-alpha burning rate to small changes in temperature These flashes produce large fluctuations in the intensity of the radiation reaching the star's outermost layers, causing these layers to pulsate violently as the envelope repeatedly heats, expands, cools, and contracts The amplitude of these pulsations grows as the core's temperature continues to increase and the nuclear burning increases in surrounding shells Surface layers are increasingly unstable Around the peak of every pulsation, the surface temperature drops below the point at which electrons can recombine with nuclei to form atoms Each recombination produces additional photons, giving the gas a little outward push and allowing some of it to escape As a result of increasingly intense radiation from within and instabilities in the core and outer layers, pretty much all of the star's envelope is ejected into space in less than a few million years The "star" now consists of a well-defined core of mostly carbon ash and an expanding cloud of dust and cool gas (the ejected envelope of the giant) The core is hot, dense, and very luminous, and only the outer layers still fuse helium into carbon and oxygen As the core exhausts the last of its remaining fuel, it contracts and heats up and moves to the left of the HR diagram Eventually, it becomes so hot that its ultraviolet radiation ionizes the inner parts of the surrounding cloud, creating a planetary nebula (planetary nebulas have no association with planets, were named back in the 18th century when viewed at poor resolution) The central star cools and the gas cloud disperses into space During the final stages of the current red giant's life, nuclear reactions between carbon and unburned helium in the core produce oxygen and heavier elements such as neon and magnesium Some of these reactions release neutrons which can interact with existing nuclei to form heavier elements as they have no electrostatic barrier to overcome The carbon core continues to evolve and becomes visible as the envelope recedes This core is about the size of the earth, has a mass about that of the sun, and has a white-hot surface The core is now a white dwarf Helium dwarf is when a star is so small that helium fusion never occurs and eventually all of its hydrogen is converted to helium (theory predicts this) In some stars more massive than the sun, temperatures in the core might become high enough for the reaction 16O + 4He —> 20Ne + energy to occur, leading to the formation of a neon-oxygen white dwarf Most white dwarfs are composed of carbon and oxygen Once a star becomes a white dwarf, its evolution is over The white dwarf continues to cool and dim, eventually becoming a black dwarf

Spiral galaxies

Flattened galactic disk Spiral arms Central galactic bulge with a dense nucleus Extended halo of old, faint stars Type Sa have largest bulges and more open spiral arms, type Sc have smallest bulges and loose, poorly defined spiral structures, Sb in the middle The tightness of the spiral pattern correlated with the size of the bulge Bulges and halos of these galaxies contain large numbers of reddish old stars and globular clusters Galactic disk populated by A-G type stars Flat disks rich in gas and dust Spiral arms contain many emission nebulae and newly formed O and B type stars they have more orderly, rotational motion than random motion (the rotation refers to the disk as a whole and means that the star orbits are closely confined to a narrow range of angles and are fairly circular) they have some or a lot of gas and dust between the stars this means they can have new star formation occurring in the disk, particularly in the spiral arms they have a spiral structure.

Andromeda Galaxy

Furthest object that can be seen with the naked eye 2.25 million (2,250,000) light years Spiral galaxy

Julian Date

Julian dates (abbreviated JD) are simply a continuous count of days and fractions since noon Universal Time on January 1, 4713 BC (on the Julian calendar). Almost 2.5 million days have transpired since this date. Julian dates are widely used as time variables within astronomical software

Reddening

Less blue light reaches us, so the object appears redder than it should This effect is called reddening, though perhaps it should be called "de-blueing" If the dust particles were much larger (say, the size of grains of sand), reddening would not be observed If the dust particles were much smaller (say, the size of molecules), the scattering would behave as 1/l4 Trumpler showed that a given spectral type of star becomes increasingly redder with distance This discovery was further evidence for dust material between the stars If the Sun is in a typical spot in the Galaxy, then Trumpler's observation means that more distant stars have more dust between us and them

Barred spiral galaxies

Like spiral galaxies but with an elongated bar of stellar and interstellar matter passing through the center and extending beyond the bulge into the disk

Antennae Galaxies

Merging pair of spiral galaxies During the course of the collision, billions of stars will be formed. The brightest and most compact of these star birth regions are called super star clusters The two spiral galaxies started to interact a few hundred million years ago, making the Antennae galaxies one of the nearest and youngest examples of a pair of colliding galaxies By age dating the clusters in the image, astronomers find that only about 10 percent of the newly formed super star clusters in the Antennae will survive beyond the first 10 million years. The vast majority of the super star clusters formed during this interaction will disperse, with the individual stars becoming part of the smooth background of the galaxy. It is however believed that about a hundred of the most massive clusters will survive to form regular globular clusters, similar to the globular clusters found in our own Milky Way galaxy. The Antennae galaxies take their name from the long antenna-like "arms" extending far out from the nuclei of the two galaxies, best seen by ground-based telescopes. These "tidal tails" were formed during the initial encounter of the galaxies some 200 to 300 million years ago 45 million light years away Pink and red represents clouds of gas Blue areas are star forming regions High rate of star formation that will not last Will eventually become one elliptical galaxy In a paper published in The Astrophysical Journal in July 2014, researchers noted that the region around SN 2014J was relatively devoid of material and that there were no X-ray emissions in the vicinity of the site of the supernova. If the progenitor star had been surrounded by enough material, it would have produced a bright X-ray source after the explosion. The lack of material around the site is unusual. It could be explained by a merger of two white dwarf stars. If this had been the cause of the explosion, there may have been relatively little mass transfer from one star to another and less material around the progenitor star. Another possible explanation is that there had been a series of smaller eruptions of the surface of the star, which had cleared the region before the explosion. The Antennae Galaxies' nuclei are in the process of joining to form a single giant galaxy. This will happen within the next 400 million years. Simulations of the galactic collision indicate that as the galaxies' nuclei join to form a single core, the two galaxies will eventually form a single giant elliptical galaxy. NGC 4038 and NGC 4039 were two separate galaxies some 1.2 billion years ago. NGC 4039 was a spiral galaxy and the larger of the two, while NGC 4038 was a barred spiral galaxy. The two galaxies started approaching each other roughly 900 million years ago. At this point, the pair appeared similar to the colliding spiral galaxies NGC 2207 and IC 2163, located in Canis Major constellation. The Antennae Galaxies are believed to have passed through each other about 600 million years ago, when they may have appeared similar to the Mice Galaxies (NGC 4676), a pair of interacting spiral galaxies lying in the direction of the constellation Coma Berenices. 300 million years later, the stars in both galaxies started being released into intergalactic space. As a result, there are now two streamers of expelled stars extending far beyond NGC 4038 and NGC 4039, giving the pair the appearance of the antennae. The Antennae Galaxies are believed to have passed through each other about 600 million years ago, when they may have appeared similar to the Mice Galaxies (NGC 4676), a pair of interacting spiral galaxies lying in the direction of the constellation Coma Berenices. 300 million years later, the stars in both galaxies started being released into intergalactic space. As a result, there are now two streamers of expelled stars extending far beyond NGC 4038 and NGC 4039, giving the pair the appearance of the antennae. The Antennae Galaxies are believed to have passed through each other about 600 million years ago, when they may have appeared similar to the Mice Galaxies (NGC 4676), a pair of interacting spiral galaxies lying in the direction of the constellation Coma Berenices. 300 million years later, the stars in both galaxies started being released into intergalactic space. As a result, there are now two streamers of expelled stars extending far beyond NGC 4038 and NGC 4039, giving the pair the appearance of the antennae. The NASA/ESA Hubble Space Telescope has snapped the best ever image of the Antennae Galaxies. Hubble has released images of these stunning galaxies twice before, once using observations from its Wide Field and Planetary Camera 2 (WFPC2) in 1997, and again in 2006 from the Advanced Camera for Surveys (ACS). Each of Hubble's images of the Antennae Galaxies has been better than the last, due to upgrades made during the famous servicing missions, the last of which took place in 2009. The galaxies — also known as NGC 4038 and NGC 4039 — are locked in a deadly embrace. Once normal, sedate spiral galaxies like the Milky Way, the pair have spent the past few hundred million years sparring with one another. This clash is so violent that stars have been ripped from their host galaxies to form a streaming arc between the two. In wide-field images of the pair the reason for their name becomes clear — far-flung stars and streamers of gas stretch out into space, creating long tidal tails reminiscent of antennae. This new image of the Antennae Galaxies shows obvious signs of chaos. Clouds of gas are seen in bright pink and red, surrounding the bright flashes of blue star-forming regions — some of which are partially obscured by dark patches of dust. The rate of star formation is so high that the Antennae Galaxies are said to be in a state of starburst, a period in which all of the gas within the galaxies is being used to form stars. This cannot last forever and neither can the separate galaxies; eventually the nuclei will coalesce, and the galaxies will begin their retirement together as one large elliptical galaxy. This image uses visible and near-infrared observations from Hubble's Wide Field Camera 3 (WFC3), along with some of the previously-released observation

Population II stars

Old stars with few heavy elements are referred to as population II stars and are found in the halo and in globular clusters, 10-13 billion years old, masses less than or equal to 0.8 solar masses, metal poor

Photodisintegration

Photodisintegration (also called phototransmutation) is a nuclear process in which an atomic nucleus absorbs a high-energy gamma ray, enters an excited state, and immediately decays by emitting a subatomic particle.

Luminosity

Total energy radiated by a star each second

ULXs

Ultraluminous X-ray sources millions of times brighter than the Sun At least some powered by neutron stars, not black holes

Radial Velocity

Up until the launch of the planet-hunting spacecraft Kepler in 2009, radial velocity was the most effective method for locating extrasolar planets. The vast majority of Exoplanets detected from Earth were discovered by this method.

Red Giant Star

When a star has consumed its stock of hydrogen in its core, fusion stops and the star no longer generates an outward pressure to counteract the inward pressure pulling it together A shell of hydrogen around the core ignites continuing the life of the star, but causes it to increase in size dramatically The aging star has become a red giant star, and can be 100 times larger than it was in its main sequence phase When this hydrogen fuel is used up, further shells of helium and even heavier elements can be consumed in fusion reactions The red giant phase of a star's life will only last a few hundred million years before it runs out of fuel completely and becomes a white dwarf.

M51

Whirlpool Galaxy M51 is located 31 million light-years from Earth in the constellation Canes Venatici. It has an apparent magnitude of 8.4 and can be spotted with a small telescope most easily during May Some astronomers think that the Whirlpool's arms are particularly prominent because of the effects of a close encounter with NGC 5195, the small, yellowish galaxy at the outermost tip of one of the arms In Hubble's captivating image of M51, the red represents infrared light as well as hydrogen within giant star-forming regions. The blue color can be attributed to hot, young stars while the yellow color is from older stars The interaction between the pair also results in compression of hydrogen gas which in turn leads to formation of starburst regions, seen in pictures as bright blue knots across the galaxy's spiral arms. this deep sky object was the very first to be classified as a spiral galaxy Lots of fuzzy x-ray emission that comes from gas that has been superheated by supernova explosions of massive stars Contains a ULX The Whirlpool Galaxy is approximately 60 light years across. It has an angular diameter of about 11.2′. The galaxy's bright circular disk is believed to have a radius of approximately 43,000 light years. The estimated mass of M51 is about 160 billion solar masses. The galaxy's compact nucleus is classified as of Seyfert type 2.5. With a diameter of approximately 75,000 light years, the Whirlpool is about 25 percent smaller than our own galaxy, the Milky Way. The angular diameter of the galaxy is about 1/3rd the width of the full Moon. Three supernovae have been observed in the galaxy. SN 1994I was spotted on April 2, 1994. It peaked at magnitude 12.8 and was classified as a type Ic supernova. SN 2005cs, a type II supernova was seen on June 27, 2005, and peaked at visual magnitude 14. SN 2011dh was detected on May 31, 2011. It had an apparent visual magnitude of 14.2, peaked at magnitude 13.5, and was classified as a type II supernova. The central region of the Whirlpool Galaxy is undergoing a period of intense star formation at a rate it will likely not be able to sustain for more than another 100 million years. In NGC 5195, however, there are almost no new stars being formed. This means that the smaller galaxy was either dust-poor even before the encounter with the Whirlpool or it has been stripped of dust as a result of the interaction with the larger galaxy Both NGC 5194 and NGC 5195 are believed to contain supermassive black holes, each emitting intense X-rays. The current spiral structure of the larger galaxy is believed to be a result of the smaller galaxy passing through its main disk some 500 to 600 million years ago and making another crossing about 50 to 100 million years ago. NGC 5195 is now located slightly behind the Whirlpool Galaxy. The two galaxies are gravitationally bound and approaching each other for another interaction. They will eventually merge, but not before they have made a few more passes, which will likely take hundreds of millions of years. The Whirlpool Galaxy is the brightest member of the M51 Group, a group of galaxies that includes several notable members located in the same region of the sky: the Sunflower Galaxy (Messier 63), NGC 5023, NGC 5229, UGC 8313, and UGC 8331. The nucleus of Messier 51 contains a cross, or an X-structure, which suggests that the galaxy has two dust rings surrounding the central black hole.

Giant molecular cloud

a large, dense gas cloud (with dust) that is cold enough for molecules to form

Emission Spectra

a spectrum of the electromagnetic radiation emitted by a source.

Chandra deep field south

an image taken by the Chandra X-ray Observatory satellite. The location was chosen because, like the Lockman Hole, it is a relatively clear "window" through the ubiquitous clouds of neutral hydrogen gas in our Milky Way galaxy, which allows us to clearly see the rest of the universe in X-rays represents the deepest ever x-ray image of the Universe Points of light most likely due to supermassive black holes in distant galaxies

Blackbody

an object that absorbs all radiation falling on it

Distance modulus

m - M = 5log(r/10)

Major mergers

mergers between two galaxies of similar mass

Eclipsing Variable Stars

stars that vary in brightness due to our view being obscured by another object. Just as astronomers can detect the minute difference in brightness of exoplanet transits in transit photometry, they can detect the variations in brightness. As the secondary star travels around the primary, the primary star's brightness appears to dim, even though the star itself may not be undergoing any changes to its properties

Apparent magnitude

the brightness measured by an observer at a specific distance from the object m

Extinction

Starlight passing through a dust cloud can be affected in a couple of ways The light can be totally blocked if the dust is thick enough or it can be partially scattered by an amount that depends on the color of the light and the thickness of the dust cloud All wavelengths of light passing through a dust cloud will be dimmed somewhat This effect is called extinction The amount of extinction is proportional to 1/(wavelength of the light)

Visual Binary

Stars can be distinguished using a telescope Five to ten percent of the stars visible to us are visual binary stars. Careful spectroscopic studies of nearby solar-type stars show that about two-thirds of them have stellar companions. We estimate that roughly half of all stars in the sky are indeed members of binaries.

Monolithic Collapse

The term 'monolithic collapse' is often used as an alternative to primordial collapse. In this theory for galaxy formation, all galaxies were formed at the same time through the collapse of a cloud of primordial gas. In theory, however, a subtle distinction can be made between primordial collapse and monolithic collapse. Primordial collapse (by definition) involved the collapse of a primordial gas cloud early in the history of the Universe, while monolithic collapse could still occur today

SB0 galaxies

Thin disk and flattened bulge No gas No spiral arms Bar

47 Tucanae

a globular cluster located about 15,000 light years away in the outskirts of the Milky Way the second brightest globular cluster 20,000 light years away Has few binary systems Contains at least 20 neutron stars

Emission Nebula

a nebula that shines with its own light

Blackbody curve

the radiation distribution curve of a blackbody

Faber-Jackson relation

velocity dispersion = 220 × (L/L*)0.25, where L* is a characteristic galaxy luminosity (around 10 billion solar luminosities)

Stellar Nursery

An area of outer space within a dense nebula in which gas and dust are contracting, resulting in the formation of new stars

Open Clusters

An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitationally bound to each other. In contrast, globular clusters are very tightly bound by gravity. Open clusters are found only in spiral and irregular galaxies, in which active star formation is occurring. They are usually less than a few hundred million years old: they become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center, as well as losing cluster members through internal close encounters.

Parallax

Apparent displacement of a foreground object relative to the background as the observer's location changes is known as this Amount of this is inversely proportional to an object's distance

Irregular galaxies

Appearances distinguish themselves from all other categories Tend to be rich in interstellar matter and young blue stars Lack any regular structure Irr I galaxies and Irr II galaxies Irr I look like misshapen spirals Irregular galaxies are smaller than spirals and larger than dwarf ellipticals Smallest of these are called dwarf irregulars Most irregulars are small and faint The dwarf irregulars may be the most common type of galaxy in the universe (or maybe the dwarf ellipticals are)

X-ray binary system

Binary system (two stars that orbit around their common center of mass) that emit X-Rays Made up of a normal star and collapsed star (white dwarf, neutron star, or black hole) Produce X-rays if the stars are close enough together that material is pulled off the normal star by the gravity of the dense, collapsed star X-rays come from the area around the collapsed star where the material that is falling towards it is heated to very high temperatures

X9

Black hole & star system Closest star to a black hole Star changes in X-ray brightness every 28 minutes, making it likely the companion star makes one complete orbit around the black hole in this amount of time Located in 47 tucanae A low-mass X-ray binary

Stellar mass black holes

Black holes with masses comparable to that of the sun When the mass of a neutron star exceeds three solar masses, its core collapses on itself and a black hole is formed Only things that can be measured from the outside are mass, charge, and angular momentum Schwarzschild radius is the radius at which the escape speed is the speed of light Sphere with this radius centered on a collapsing star is the event horizon -- the point of no return The event horizon is the one-way filter in the black hole: anything can enter, but nothing can leave. A black hole is a very simple object: it has only three properties mass, spin and electrical charge. Because of the way in which black holes form, their electrical charge is probably zero, which makes them simpler yet As a black hole radiates via this virtual particle method it will lose mass and get smaller. Using Equation 4 we see that a smaller mass means a bigger temperature, so as the black hole radiates it gets smaller and hotter. If it gets hotter then the rate of radiation will increase and so you'll have a chain reaction with the black hole getting ever smaller and hotter until it gets to zero mass.

supermassive black holes

Black holes with millions or billions of solar masses (solar mass = mass of sun) Often in the centers of galaxies When the mass of a neutron star exceeds three solar masses, its core collapses on itself and a black hole is formed Only things that can be measured from the outside are mass, charge, and angular momentum Sphere with Schwarzschild radius as radius centered on a collapsing star is the event horizon -- the point of no return

Stage 3 of star formation: fragmentation ceases

By the beginning of this stage, the fragment has shrunk to roughly the size of our solar system Density in the inner regions becomes high enough so that the gas is opaque to the radiation it emits, causing the core of the fragment to heat up Temperature on outer parts still hasn't increased much Dense opaque region in the center is called a protostar Protostar's mass increases as more material rains down on it from the surrounding, shrinking fragment Protostar's radius continues to decrease because pressure is still unable to overcome gravity After this stage, the protostar has a surface called the photosphere

Galactic Structure and Interactions

Collisions and interactions between galaxies provide opportunities for gas to mix, which can set off rapid star formation When the gravitational tug from one galaxy is enough to disrupt the other one, they are interacting Major mergers and minor mergers Major mergers are mergers between two galaxies of similar mass Minor mergers are mergers where one galaxy is significantly more massive than the other one When two galaxies collide the stars will pass right on by each other without colliding The stars may be flung out from the colliding galaxies to form long arcs

Comet

Comets are cosmic snowballs of frozen gases, rock and dust that orbit the Sun. When frozen, they are the size of a small town. When a comet's orbit brings it close to the Sun, it heats up and spews dust and gases into a giant glowing head larger than most planets. The dust and gases form a tail that stretches away from the Sun for millions of miles. There are likely billions of comets orbiting our Sun in the Kuiper Belt and even more distant Oort Cloud. Gas tails and dust tails

First stage of star formation: interstellar cloud

Dense interstellar cloud such as the core of a dark dust cloud or a molecular cloud Must become unstable, start to collapse under the weight of its own gravity, and break up into small pieces for star formation to start Most astronomers think that this happens when an external event pushes a cloud beyond the point where pressure can resist gravity's inward pull This whole process can take a few million years

Direct Imaging

Direct imaging of exoplanets is extremely difficult, and in most cases impossible. Being small and dim planets are easily lost in the brilliant glare of the giant stars they orbit. Nevertheless, even with existing telescope technology there are special circumstances in which a planet can be directly observed

Eclipsing Binary

Eclipsing binaries are systems in which the orbital plane is oriented exactly edgewise to the plane of the sky so that the one star passes directly in front of the other, blocking out its light during the eclipse. Eclipsing binaries may also be visual or spectroscopic binaries. The variation in the brightness of the star is called its light curve.

Radio Galaxies

Emit huge amounts of radio energy The radio emission comes from the core AND from very large regions on either side of the optical part of the galaxy called "radio lobes'' The radio lobes can extend for millions of light years from the center of the galaxy The radio emission from normal galaxies is thousands to millions of times less intense and is from the gas between the stars Most radio galaxies are elliptical galaxies

Hubble's law

Except for a few nearby systems, every galaxy takes part in a general motion away from us Galaxy clusters also have an overall recessional motion although their individual galaxies move randomly with respect to each other The greater the distance that the galaxy is from us, the greater the redshift (rate it moves away from us) is Redshift resulting from universal recession is called cosmological redshift recessional velocity(km/s) = H0 x distance (megaparsecs) H0 = 70 (km/s/Mpc)

Supernovae

Explosive death of star Core of star collapses, then expands Produce bursts of light billions of times brighter than the sun Stars can only become this once

Herbig Haro objects

Herbig-Haro objects are turbulent looking patches of nebulosity associated with newborn stars. They are formed when narrow jets of partially ionized gas ejected by said stars collide with nearby clouds of gas and dust at speeds of several hundred kilometers per second.

Hierarchical clustering

Hierarchical clustering, also known as hierarchical cluster analysis, is an algorithm that groups similar objects into groups called clusters. The endpoint is a set of clusters, where each cluster is distinct from each other cluster, and the objects within each cluster are broadly similar to each other

Evolution of high mass stars

High mass stars evolve much faster than low mass stars All evolutionary changes happen more quickly for higher mass stars because their larger mass and stronger gravity generate more heat After the star runs out of hydrogen, it leaves the main sequence Red giants move horizontally across the HR diagram after leaving the main sequence Helium burning happens smoothly and stably (no helium flash) High mass stars can fuse heavier elements and so the inner core continues to contract and the central temperature continues to rise The rate of burning accelerates As each element is burned to depletion, the core contracts, heats up, and fusion starts again A new inner core forms, contracts again, heats again, etc. The star's radius increases as its surface temperature drops and so the star swells to become a red giant Star is destined to die in a supernova

Type 1 supernovae

Hydrogen poor Caused by sudden onset of nuclear burning Light curve similar to that of a nova Results from the detonation of a carbon white dwarf, has little hydrogen because these stars have little hydrogen Appearance of light curve results from radioactive decay of unstable elements produced in the explosion Only a small fraction of low mass stars evolve into white dwarfs that explode as type 1 supernovae Also known as carbon detonation supernovae Novae eject matter from a white dwarf's surface, but they don't necessarily expel or burn all material that has accumulated since the last outburst, so white dwarfs have a tendency to increase mass with each new nova cycle The maximum mass of a white dwarf is 1.4 solar masses (Chandrasekhar Mass), when the mass exceeds that, the star starts to collapse because the degeneracy pressure of electrons that holds up the white dwarf is unable to withstand the force of gravity, causing the star to collapse This causes the internal temperature to rise, which causes carbon fusion, which causes the star to explode Binary star system where a white dwarf is gaining matter from a companion White dwarfs have burnt up all of the hydrogen, so no hydrogen lines in the spectra Type I supernovae happen in close binary systems and do not show strong hydrogen emission lines. Type I (especially Ia) supernova create most of the iron and nickel found in the interstellar medium. Type Ia supernovae are several times more luminous than Type Ib, Ic, and Type II supernovae, leave no core remnant behind, and result from when a low-mass star's core remnant (a white dwarf) detonates.

Starburst galaxies

Luminous galaxies, luminosities more than 10^10 times the solar value Non Stellar emission, meaning that most energy is emitted as invisible wavelengths Previously normal galaxies now characterized by widespread episodes of star formation most likely caused by violent events such as interactions with a neighbor galaxies that are observed to be forming stars at an unusually fast rate (about 10^3 times greater than in a normal galaxy) At these high levels of star formation it is estimated that the supply of gas and dust within the galaxy would be exhausted within about 10^8 years, meaning that these episodes of intense star formation must have started relatively recently and will end in the not too distant future Interactions between galaxies that do not merge can trigger unstable rotation modes, such as the bar instability, which causes gas to be funneled towards the nucleus and ignites bursts of star formation near the galactic nucleus It was initially detected in 2010 using the Sunyaev-Zel'dovich effect Although hidden from us at optical wavelengths by the enshrouding dust, massive stars are formed out of the available gas. They emit copious amounts of ultraviolet wavelengths which are absorbed by the surrounding dust and reemitted at infrared wavelengths, making starburst galaxies among the most luminous infrared objects in the Universe.

Population III stars

Population III consisting of the very first stars with little to no metal content, as they did not exist near the beginning of the universe. They did not last very long, but helped the metals to form for the later populations.

Stage 5 of star formation: protostar evolution

Protostar approaches the main sequence It has shrunk to about 10 times the mass of the sun, surface temperature is about 4000 K, luminosity is about 10 times the solar value, central temperature is 5,000,000 K Gas is completely ionized but the protons do not have enough energy to overcome the electromagnetic repulsion Core is still too cool for nuclear fusion to start

Stage 6 of star formation: newborn star

Protostar becomes a true star Radius is about 1,000,000 km, central temperature is about 10,000,000 K, surface temperature is about 4,500 K Luminosity is ⅔ of the solar value

Rotation Powered Pulsars

Pulsars (in general, isolated radio pulsars) with powerful magnetic fields that cause accelerated charged particles to emit radiation. This radiation is channeled into two conical-shaped regions centered about the pulsar's magnetic poles (which are not aligned with the pulsar's axis of rotation). As these conical-shaped regions sweep through space -- much like a lighthouse beam -- they produce the pulses of electromagnetic radiation that are the hallmark of pulsars.

Secular Evolution

Secular evolution is defined as slow, steady evolution. In galaxies, such evolution is either the result of long-term interactions between the galaxy and its environment (such as gas accretion or galaxy harassment), or it is induced by internal processes such as the actions of spiral arms or bars

Binary System

Stars stay on the opposite side of the center of mass from each other. The massive star moves slower than the low-mass star. The center of mass is also the point where mass1 × velocity1 = mass2 × velocity2 A binary star system consists of two stars which orbit around a common point, called the center of mass following Kepler's Laws.

Tolman-Oppenheimer-Volkoff limit

The Tolman-Oppenheimer-Volkoff limit (or TOV limit) (also referred to as the Landau-Oppenheimer-Volkoff limit (or LOV limit)) is an upper bound to the mass of stars composed of neutron-degenerate matter (i.e. neutron stars). The TOV limit is analogous to the Chandrasekhar limit for white dwarf stars

M82

The cigar galaxy shines brightly at infrared wavelengths and is remarkable for its star formation activity Cigar galaxy experiences gravitational interactions with its galactic neighbor, M81, causing it to have an extraordinarily high rate of star formation — a starburst Around the galaxy's center, young stars are being born 10 times faster than they are inside our entire Milky Way galaxy. Radiation and energetic particles from these newborn stars carve into the surrounding gas, and the resulting galactic wind compresses enough gas to make millions of more stars. The rapid rate of star formation in this galaxy eventually will be self-limiting. When star formation becomes too vigorous, it will consume or destroy the material needed to make more stars. The starburst will then subside, probably in a few tens of millions of years Located 12 million light-years from Earth in the constellation Ursa Major, M82 has an apparent magnitude of 8.4 and is best observed in April The red in the image represents hydrogen and infrared light, indicating starburst activity. The blue and greenish-yellow color represent visible wavelengths of light

Stage 7 of star formation: main sequence

The star contracts a little more over the next 30 million years, causing the central density to rise up to about 10^32 particles/m^3 Central temperature increases to 15,000,000 K and surface temperature reaches 6000 K Star finally reaches at the main sequence (band on the HR diagram where most stars are found that runs from the top left to bottom left of the diagram) Pressure and gravity are finally balanced, rate at which nuclear energy is generated in the core matches the rate at which energy is radiated from the surface

S0 galaxies

Thin disk and flattened bulge No gas No spiral arms No bar

Transit Photometry

This method detects distant planets by measuring the minute dimming of a star as an orbiting planet passes between it and the Earth. The passage of a planet between a star and the Earth is called a "transit." If such a dimming is detected at regular intervals and lasts a fixed length of time, then it is very probable that a planet is orbiting the star and passing in front of it once every orbital period.

Globular clusters

Tightly bound star clusters Spherical clusters found away from the milky way plane that contain hundreds of thousands of stars spread over about 50 pc Found on halos of galaxies Lack upper main sequence stars Biggest stars are 0.8 times the sun's mass Spectra show few heavy elements which implies that they formed in the past before heavy elements were as abundant as they are now Cluster color-magnitude diagrams change with age. More massive stars evolve quicker than low-mass stars. The hot, luminous main sequence stars will die before the cool, dim main sequence stars. This means that an old cluster will have only the low-mass stars still on the main sequence, but a young cluster will have both high and low-mass stars on the main sequence. The most massive star still on the main sequence tells us the age of the cluster. That point on the main sequence is called the main sequence turnoff. All stars in a cluster are assumed to have formed at about the same time (observations of current star formation do show that stars form in batches) For the common lower mass stars (< 10 solar masses), age of the cluster = (1010)/(MST mass3) years. Use solar masses! For the rare massive stars (> 30 solar masses), use age of the cluster = (1010)/(MST mass2) years.

Stellar parallax

To determine this, observe the object from some baseline and measure the angle through which the line of sight to the object shifts Distance in parsecs = 1/this in arcseconds Defined as half of its apparent angle shift relevant to the background To measure this, observe the star at different points in the year to get a larger parallactic shift and thus get a more accurate calculation of this

Period luminosity relationship

Used to infer the luminosity of a variable star Cepheids that have long periods have large luminosities and Cepheids with short periods have small luminosities Once distances are known to variable stars, can use relationship between luminosity and distance to calculate luminosity This works for the nearby galaxies Mv = - [2.76 (log10(P) - 1.0)] - 4.16, M is absolute magnitude, P is period

White Dwarf Star

When a star has completely run out of hydrogen fuel in its core and it lacks the mass to force higher elements into fusion reaction, it becomes a white dwarf star The outward light pressure from the fusion reaction stops and the star collapses inward under its own gravity A white dwarf shines because it was a hot star once, but there's no fusion reactions happening any more A white dwarf will just cool down until it reaches the background temperature of the Universe This process will take hundreds of billions of years, so no white dwarfs have actually cooled down that far yet.

M100

a Grand Design spiral galaxy A large galaxy of over 100 billion or so stars with well defined spiral arms Has bright blue star clusters and intricate winding dust lanes 9.3 apparent magnitude 60,000 kly Barred galaxy Grand design spiral galaxy with two small companion galaxies

Rotation Curve

a plot of the orbital velocity of the clouds around the galactic center vs. their distance from the Galaxy center The term "rotation" in this context refers to the motion of the galactic disk as a whole---the disk made of stars and gas clouds appears to spin The gas clouds are assumed to move in the plane of the disk on nearly circular orbits Jan Oort (lived 1900--1992) found in 1927 that stars closer to the galactic center complete a greater fraction of their orbit in a given time than stars farther out from the center This difference in the angular speeds of different parts of the galactic disk is called differential rotation

Planetary nebula

a ring-shaped nebula formed by an expanding shell of gas around an aging star

Color index

a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature. The smaller this is, the bluer the object is. Conversely, the larger this is, the redder the object is

Absorption spectra

a spectrum of electromagnetic radiation transmitted through a substance, showing dark lines or bands due to absorption of specific wavelengths.

Seyfert Galaxies

a spiral galaxy with a compact, very bright nucleus that produces a non-thermal continuous spectrum with broad (fat) emission lines on top Some of the emission lines are produced by atoms that have several electrons removed from them Such highly ionized atoms are found only in regions of intense energy Many Seyfert nuclei are in disks with distorted spiral arms and a companion galaxy nearby that is probably gravitationally interacting with the galaxy

Pulsars

astronomical objects emitting radio radiation in the form of pulses Each one has its one pulse period and duration compact spinning neutron stars that periodically flash radiation towards earth Radiation caused by charged particles flowing along magnetic field lines Two hotspots on the neutron star that emit radiation Tend to have high speeds are all neutron stars but not all neutron stars are these This is because the factors that make them pulse -- a strong magnetic field and rapid rotation -- decrease with time so that the pulses weaken and become less frequent Sometimes, they aren't observed as being these from earth because their beams aren't oriented the right way

Delta Scuti Variables

The delta Scuti stars reside near the point where the instability strip crosses the main sequence in the HR diagram Stars in a variety of evolutionary states -- including pre-main sequence stars -- can lie within the instability strip, so long as they have spectral types between (roughly) F8 and A2, and luminosity classes between V (dwarf) and III (subgiant) For stars with solar metal abundances, this corresponds to masses between about 1.5 and 2.5 solar masses, and between 1.0 and 2.0 solar masses for metal-poor stars They are all short period stars, with individual periods lying in the range of 0.03 to 0.3 day.

Interstellar Dust

The dust is made of thin, highly flattened flakes or needles of graphite (carbon) and silicates (rock-like minerals) coated with water ice. Each dust flake is roughly the size of the wavelength of blue light or smaller. The dust is probably formed in the cool outer layers of red giant stars and dispersed in the red giant winds and planetary nebulae.

21 cm line

The hydrogen line, 21-centimeter line or H I line refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms.

Supergiant Stars

The largest stars in the Universe are supergiant stars These are monsters with dozens of times the mass of the Sun Unlike a relatively stable star like the Sun, supergiants are consuming hydrogen fuel at an enormous rate and will consume all the fuel in their cores within just a few million years Supergiant stars live fast and die young, detonating as supernovae; completely disintegrating themselves in the process

Main Sequence Star

The majority of all stars in our galaxy, and even the Universe, are main sequence stars. Our Sun is a main sequence star, and so are our nearest neighbors, Sirius and Alpha Centauri A. Main sequence stars can vary in size, mass and brightness, but they're all doing the same thing: converting hydrogen into helium in their cores, releasing a tremendous amount of energy. A star in the main sequence is in a state of hydrostatic equilibrium. Gravity is pulling the star inward, and the light pressure from all the fusion reactions in the star are pushing outward. The inward and outward forces balance one another out, and the star maintains a spherical shape. Stars in the main sequence will have a size that depends on their mass, which defines the amount of gravity pulling them inward. The lower mass limit for a main sequence star is about 0.08 times the mass of the Sun or 80 times the mass of Jupiter. This is the minimum amount of gravitational pressure you need to ignite fusion in the core. Stars can theoretically grow to more than 100 times the mass of the Sun

Kepler's laws

The orbital paths of the planets are elliptical with the sun at the focus An imaginary line connecting the sun to any planet sweeps out equal areas of the ellipse in equal intervals of time The square of a planet's orbital period is proportional to the cube of its semimajor axis (half the length of the long axis of the ellipse)

Tully-Fisher relationship

The relationship between rotational speeds and luminosities of spiral galaxies within a few tens of megaparsecs of the milky way Published by Tully and Fisher We can calculate rotational speed because radiation from the side of the galaxy whose matter is approaching us and radiation from the side of the galaxy that is receding from us is redshifted by a similar amount Can use this relationship to measure distances up to 200 Mpc L* is 10 billion solar luminosities The Tully-Fisher relation for the infrared luminosity is: circular velocity = 220 × (L/L*)0.22

Stellar classification

classified using the letters O, B, A, F, G, K, and M, where O is hottest and M is coolest Numbers after letters indicate the tenths of the range between the two star classes Roman numerals often also used afterwards to indicate luminosity (see bottom picture) Class "W" stars are very hot stars known as "Wolf-Rayet" Stars. "R", "N", "S" stars are cool stars with particular types of molecular bands. "L" stars are possibly not truly stars at all, in the sense that they may not have nuclear reactions at their cores. No green or purple stars because of the way the humans percieve light A green star is radiating right in the center of the visible light spectrum, which means it is emitting some light in all the possible colors. The star would therefore appear white — a combination of all colors. Earth's sun emits a lot of green light, but humans see it as white. Purple stars are something the human eye won't easily see because our eyes are more sensitive to blue light. Since a star emitting purple light also sends out blue light — the two colors are next to one another on the visible light spectrum — the human eye primarily picks up the blue light.

Gamma ray binaries

composed of a massive star and a compact object most of their emission is radiated in the 1-1000 MeV band Most gamma ray bursts occur in distant galaxies

Population I stars

contain more heavy elements than globular cluster and halo stars, are typically younger and found in the disk, and are especially concentrated in the spiral arms

Hawking Radiation

electromagnetic radiation which, according to theory, should be emitted by a black hole. The radiation is due to the black hole capturing one of a particle-antiparticle pair created spontaneously near to the event horizon.

Schwarzschild radius

radius at which the escape speed is the speed of light

H II regions

regions of hot (several thousand K), thin hydrogen emission nebulae that glow from the fluorescence of hydrogen atoms An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically a cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic cm.

Absolute magnitude

the magnitude of brightness that the star would have if it was 10 parsecs away from earth M

Signal to Noise Ratio

the ratio of the strength of an electrical or other signal carrying information to that of interference, generally expressed in decibels

Rotating Variable Stars

vary in brightness due to its rotation, potentially causing sunspots to appear into view. These darker regions on the star reduce the luminosity, and thus appear to have variable luminosity

Spectroscopic Binary

we determine the orbit by measuring Doppler shifts

Revolution

when the axis of rotation is outside of the object (ex: the earth around the sun)

SPT 0346-52

12.7 billion light years from Earth data from the Atacama Large Millimeter/submillimeter Array (ALMA) revealed extremely bright infrared emission from this galaxy. This suggested that the galaxy is undergoing a tremendous explosion of star birth The distorted galaxy shown here results from a collision between two galaxies followed by them merging Once the two galaxies collide, gas near the center of the merged galaxy (shown as the bright region in the center of the simulation) is compressed, producing the burst of new stars seen forming in SPT0346-52. The dark regions in the simulation represent cosmic dust that absorbs and scatters starlight An extremely old hyper-starburst galaxy approximately 12.7 billion ly away

IC 10

2.3 million light-years distant light is dimmed by intervening dust, the irregular dwarf galaxy still shows off vigorous star-forming regions that shine with a reddish glow Closest known starburst galaxy has a large population of newly formed stars that are massive and intrinsically very bright, including a luminous X-ray binary star system thought to contain a black hole. Located within the boundaries of the northern constellation Cassiopeia, IC 10 is about 5,000 light-years across An irregular starburst galaxy featuring a large amount of Wolf-Rayet stars. 10.4 Apparent magnitude An irregular dwarf galaxy known for many X-ray binaries

ESO 137-001

220 million light years away As the spiral speeds along at nearly 7 million kilometers per hour, its gas and dust are stripped away when ram pressure with the cluster's own hot, tenuous intracluster medium overcomes the galaxy's gravity Evident in Hubble's near visible light data, bright star clusters have formed in the stripped material along the short, trailing blue streaks. Chandra's X-ray data shows off the enormous extent of the heated, stripped gas as diffuse, darker blue trails stretching over 400,000 light-years toward the bottom right. The significant loss of dust and gas will make new star formation difficult for this galaxy These streaks are actually hot young stars, encased in wispy streams of gas that are being torn away from the galaxy by its surroundings as it moves through space Barred spiral galaxy

T Tauri Stars

A T Tauri star is stage in a star's formation and evolution right before it becomes a main sequence star. This phase occurs at the end of the protostar phase, when the gravitational pressure holding the star together is the source of all its energy. T Tauri stars don't have enough pressure and temperature at their cores to generate nuclear fusion, but they do resemble main sequence stars; they're about the same temperature but brighter because they're a larger. T Tauri stars can have large areas of sunspot coverage, and have intense X-ray flares and extremely powerful stellar winds. Stars will remain in the T Tauri stage for about 100 million years. Named for the first of their type observed, T Tauri stars are variable stars which show both periodic and random fluctuations in their brightnesses. They are newly-formed (< 10 million years old) low to intermediate mass stars (< 3 solar masses) with central temperatures too low for nuclear fusion to have started. Indeed, for up to another ~100 million years, the emitted radiation will come entirely from the gravitational energy released as the star contracts under its own self-gravity. T Tauri stars therefore represent an intermediate stage between real protostars (e.g. YY Orionis stars) and low-mass main sequence (hydrogen burning) stars like the Sun. The nearest T Tauri stars to us are in the Taurus and ρ-Ophiuchus molecular clouds, both about 400 light years away. The prototypical T Tauri star - T Tauri itself - is part of a close binary system with a smaller, fainter companion. This is visible in the high resolution infrared image below. This image also reveals the complexity of this environment, with hints of stellar winds and jets showing that at least some T Tauri stars are interacting with their environments. Both the winds and jets of T Tauri stars are thought to be powered by material falling onto the central star via the accretion disk (or protoplanetary disk) observed to surround many of them. Planets will also form from this protoplanetary disk and some may survive to form a planetary system surrounding the newborn star. The variability of T Tauri stars has a number of potential sources: The random variations (with time-scales from minutes to years) may be caused by instabilities in the accretion disk (which also produce the 'bullets' of material seen in the jet of HH-30 above), flares on the stellar surface, or simple obscuration by nearby dust and gas clouds. The periodic (regular) variations (with timescales of days) are almost certainly associated with huge sunspots on the stellar surface which pass into and out of view as the star rotates. The variations of T Tauri stars provide a rich source of information about the various components of these newly formed star systems. In addition, as a phase of stellar evolution through which our Sun and Solar System passed about 5 billion years ago, the study of T Tauri stars allows a glimpse into the past of both our central star and its planetary system.

Blue Straggler

A blue straggler is a main-sequence star in an open or globular cluster that is more luminous and bluer than stars at the main sequence turnoff point for the cluster. Blue stragglers were first discovered by Allan Sandage in 1953 while performing photometry of the stars in the globular cluster M3

Second stage of star formation: Collapsing cloud fragment

A fragment of an interstellar cloud that will form a star like a sun is about 1 to 2 solar masses at this point and spans a few hundredths of a parsec across Central density is about 10^12 particles/m^3 Average temperature not that different from that of the initial star despite the fact that the fragment has shrunk a lot This is because the gas constantly radiates large amounts of energy into space and the material of the fragment is so thin that photons can easily escape without being reabsorbed, allowing virtually all of the energy released in the collapse to escape without increasing the temperature The temperature only rises significantly at the center because radiation must traverse the greatest amount of material to escape This process is eventually stopped by the increasing density within the shrinking cloud As the fragment continues to contract, it eventually becomes so dense that radiation cannot get out of the cloud easily The trapped radiation causes the temperature and pressure to rise, which causes the fragmentation to cease

Magnetar

A magnetar is a type of neutron star believed to have an extremely powerful magnetic field (G). The magnetic field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays

Microquasar

A microquasar, the smaller version of a quasar, is a compact region surrounding a black hole with a mass several times that of our sun, and its companion star. The matter being pulled from the companion star forms an accretion disk around the black hole

Molecular cloud

A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit the formation of molecules, most commonly molecular hydrogen (H2)

Proplyd

A proplyd, a syllabic abbreviation of an ionized protoplanetary disk, is an externally illuminated photoevaporating disk around a young star. Nearly 180 proplyds have been discovered in the Orion Nebula

Protostar

A protostar is what you have before a star forms. A protostar is a collection of gas that has collapsed down from a giant molecular cloud The protostar phase of stellar evolution lasts about 100,000 years Over time, gravity and pressure increase, forcing the protostar to collapse down All of the energy released by the protostar comes only from the heating caused by the gravitational energy - nuclear fusion reactions haven't started yet.

Reflection Nebula

A reflection nebula is created when light from a star is scattered or reflected off a neighbouring dust cloud. The scattered light is slightly polarised and has a spectrum similar to that of the illuminating star, only bluer. This shift in colour arises because the typical size of dust grains in the cloud are comparable to the wavelength of blue light. The result is that blue light is scattered more efficiently than longer, red wavelengths giving the characteristic blue colour for these nebulae. Reflection nebulae are usually less dense than dark nebulae, and have sizes that are determined by the source of illumination. Their extent is not defined by the size of the dust cloud but rather the area over which their brightness remains above the point of detection

Neutron star

A remnant of the core of a star that experienced a type 2 supernova A dense ball of neutrons that remains at the core of a star after a supernova has destroyed the rest of the star Extremely small and very dense Composed of neutrons packed into a tight ball about 20 km across Solid Normally about 20 km across Supported by neutron degeneracy pressure Rotate extremely rapidly Have very strong magnetic fields The original field of the original star is amplified because contracting material squeezes electric field lines closer together If a star has between 1.35 and 2.1 times the mass of the Sun, it doesn't form a white dwarf when it dies Instead, the star dies in a catastrophic supernova explosion, and the remaining core becomes a neutron star As its name implies, a neutron star is an exotic type of star that is composed entirely of neutrons This is because the intense gravity of the neutron star crushes protons and electrons together to form neutrons If stars are even more massive, they will become black holes instead of neutron stars after the supernova goes off.

Standard Candles

A standard candle is an astronomical object that has a known absolute magnitude. They are extremely important to astronomers since by measuring the apparent magnitude of the object we can determine its distance using the formula: m-M = 5 log d - 5 where m is the apparent magnitude of the object, M is the absolute magnitude of the object, and d is the distance to the object in parsecs. The most commonly used standard candles in astronomy are Cepheid Variable stars and RR Lyrae stars. In both cases, the absolute magnitude of the star can be determined from its variability period. Type Ia supernovae are also normally classed as standard candles, but in reality they are more standardisible candles since they do not all have the same peak brightness. However, the differences in their peak luminosities are correlated with how quickly the light curve declines after maximum light via the luminosity-decline rate relation, and they can be made into standard candles by correcting for this effect.

Abell 400/NGC 1128/3C 75

Abell 400 is a galaxy cluster which contains the galaxy NGC 1128 with two supermassive black holes (3C 75) spiraling towards merger. These two supermassive black holes are contained in NGC 1128. The galaxy, microwave radio jets, multi-million degree X-ray producing gas and resultant radio source is known as 3C 75 The black holes are an estimated 25,000 light years apart, and thus will take millions of years to collide. Should the two supermassive black holes merge, they will form a single super-supermassive black hole

Evolution of a sun-like star

After about 10 billion years of hydrogen burning, the star runs out of hydrogen As the amount of hydrogen decreases, the amount of helium increases As the hydrogen becomes depleted, an inner core of non-burning helium starts to grow Without nuclear burning to maintain it, the outward-pushing gas pressure weakens in the helium inner core However, the inward pull of gravity does not weaken, so structural changes occur in the star As the hydrogen is consumed, the inner core begins to contract, once all of the hydrogen in the core is depleted, this process accelerates The helium at the center cannot burn yet because although the core is hot, it is still too cold to fuse helium into anything heavier The shrinkage of the helium core releases gravitational energy, driving up the central temperature and heating the overlying burning areas This causes hydrogen nuclei to fuse even more rapidly than before Hydrogen is burning quickly in a shell surrounding the non-burning core of helium ash This is known as the hydrogen-shell-burning stage The hydrogen shell generates energy faster than did the original main-sequence star's hydrogen-burning core The shell's energy production continues to increase as the helium core continues to shrink Star's temperature and luminosity change Surface temperature drops, luminosity increases slightly, leaves the main sequence Star's radius has increased to three times that of sun Star is now a subgiant The path from its main sequence location to its subgiant location on the main sequence is called the subgiant branch Star is now far from the main sequence and no longer in equilibrium Helium core is unbalanced and shrinking, hydrogen is being fused into helium at an increasing rate Surface temperature has fallen Convection now carries star's energy output to the surface, resulting in a stable surface temperature Radius and luminosity increase Star is now a red giant Helium begins to burn Helium is transformed into carbon through the triple-alpha process Electron degeneracy pressure occurs, meaning the electrons are incompressible This makes the core unable to respond to the changing conditions and the temperature changes rapidly in a helium flash The helium burns rapidly for a few hours and then the core expands, density drops, and equilibrium is restored Helium flash causes a reduction in the star's energy output Star is now stably burning helium in the core and fusing hydrogen in the shell surrounding it Helium is consumed and a new carbon-rich inner core is formed Helium becomes depleted at the center of the star and fusion ceases The non-burning carbon core shrinks in size, its mass increases due to helium fusion, and it heats up as gravity pulls it inward This causes the hydrogen and helium burning rates to increase Star is now once again a red giant Radius and luminosity are even bigger then they were before Carbon core grows in mass and continues to shrink in radius

Stage 4 of star formation: protostar

As it evolves, the protostar shrinks, density grows, and temperature rises Luminosity much brighter than the sun Nuclear reactions have not yet begun, luminosity solely due to the release of gravitational energy Can now be plotted on HR diagram(plot of surface temperature and luminosity) The evolutionary track is a graph of a star's development on the HR diagram As the fragment contracts, it spins faster and flattens into a rotating protostellar disk Internal heat gradually diffuses, slowing the contraction After this stage, luminosity decreases and temperature increases, at this stage, violent surface activity is often present, resulting in extremely strong protostellar winds

Astrometry

Astrometry is the science (and art!) of precision measurement of stars' locations in the sky. When planet hunters use astrometry, they look for a minute but regular wobble in a star's position. If such a periodic shift is detected, it is almost certain that the star is being orbited by a companion planet.

Chemical Composition

Astronomers use the letters X, Y and Z to denote the fraction of material (by mass) which made up by hydrogen, helium, and everything else: X = 1.0 Y = 0.0 Z = 0.0 pure hydrogen X = 0.5 Y = 0.5 Z = 0.0 hydrogen/helium mix X = 0.0 Y = 0.5 Z = 0.5 helium/heavy mix

Cataclysmic Variable Stars

Cataclysmic variable stars (CV) are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state

Stellar evolution

Core hydrogen burning -- when a star fuses hydrogen into helium in its core Hydrostatic equilibrium -- when pressure's outward push counteracts gravity's inward pull, a stable balance where a small change in one results in a compensating change in the other As the hydrogen in the star's core is consumed, the internal balance starts to shift and the internal structure and outward appearance; the star leaves the main sequence Evolution from this point on depends on a star's mass

Brown Dwarfs

Failed stars Cloud fragments too small to become stars because they lack the mass to initiate nuclear burning Instead of turning into stars, they continue cooling, eventually becoming compact, dark, cold fragments of unburned matter

Light curves

Graphs that show the brightness of an object over a period of time Can be used to identify the object being observed A star's diameter is found from speed = (distance traveled)/(time it takes). The speed comes from the Doppler shift and the time is the length of the eclipse. The distance traveled during the eclipse is equal to the diameter of the star = 2 × radius. The light curve---plot of brightness vs. time---is used to derive the star diameters

Gravitational waves

Gravitational counterpart of an electromagnetic wave Results from changes in the strength of a gravitational field Objects most likely to produce these are binary systems containing black holes, neutron stars, and white dwarfs

Normal galaxies

Gravitationally bound collections of large groups of stars All galaxies are about the same age ~ 15 billion years old Overall energy emission is consistent with the summed light of many stars The linear distance from the galaxy center = [(2p × (distance to the galaxy) × (angular distance in degrees)] / 360°

InterstelIar medium

In astronomy, the interstellar medium (ISM) is the matter and radiation that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space

Glitch

In pulsar astronomy, a glitch is a sudden discontinuity in the rotation period of a pulsar. There are two physical mechanisms thought to be responsible for the glitch of a pulsar - either they are caused by starquakes, in which case the neutron star's crust cracks, and there is a fundamental reorganization of the matter within the star, or they are due to a catastrophic unpinning of vortices in the neutron star superfluid.

Jean's Instability

In stellar physics, the Jeans instability causes the collapse of interstellar gas clouds and subsequent star formation. It occurs when the internal gas pressure is not strong enough to prevent gravitational collapse of a region filled with matter

Blackbody radiation

Intensity as a function of frequency Intensity specifies amount or strength of radiation at a point in space No real object absorbs and radiates as a perfect blackbody The frequency at which emitted intensity is highest is indicative of the temperature of an object

Type 2 supernovae

Lots of hydrogen Caused by enormously energetic shock wave Plateau in light curve a few months after the maximum Caused by the explosion of the core of a massive star Light curve caused by expansion and cooling of the star's outer envelope as it is blown into space by shock waves Expanding envelope consists mainly of unburned gas such as hydrogen and helium All high mass stars become type 2 supernovae Also known as core collapse supernovae Elements in the core of a star are burnt to depletion within the star's lifetime, once the star starts to burn iron, gravity overwhelms the pressure of the star's hot gas and the star implodes Fuel is exhausted so it collapses This happens because iron lies at the dividing line between elements that engage in fusion and elements that engage in fission, and so it engages in neither, causing it to dampen the inferno in the star's core Have retained their hydrogen and helium layers, so strong hydrogen emission lines Type II supernovae happen in single star systems (or at least far enough away from any companion star to retain their hydrogen outer layers) and have strong hydrogen emission lines. Type II create most of the oxygen found in the interstellar medium.

Russell Vaught Theorem

M* < 0.01 M - Planet. Jupiter, for example, has a mass of about 0.001 x M. Jupiter's temperature is slightly warmer than would be expected from the amount of solar energy it receives; this is interpreted as due to gravitational potential energy stored as heat from Jupiter's contraction out of the proto-solar nebula. But the energy balance for Jupiter and other planets is largely determined by the energy received from the sun and central temperatures never come close to the 1 million K required for even the simplest nuclear reactions. 0.01 M < M* < 0.085 M - Brown Dwarf; these objects will never become hot enough in their cores to ignite the P-P Chain. Release gravitational potential energy will cause them to heat up to core temperatures as hot as 3 million K, hot enough for the first stages of nuclear reactions, perhaps, but never hot enough to establish stable hydrogen burning. With atmospheric temperatures Tsurface < 2000K, brown dwarfs will be very faint, radiating the vast majority of their luminous energy in the infrared, and very hard to detect. A new near-infrared survey of the sky called 2MASS has detected a large number of cool stars, now classified as L-stars which are likely to be brown dwarfs. 0.085 M < M* < 0.4 M - these stars will be very long lived, but will never reach temperatures hot enough for the Triple-alpha process to occur. They will not have a helium flash in the red giant stage nor a helium-burning main-sequence phase. 0.4 M < M* < 1.2 M - these stars like the sun will burn hydrogen to helium via the P-P Chain and will burn helium to carbon via the Triple-alpha process M* > 1.2 M - these stars will reach high enough core temperatures to burn hydrogen via the CNO cycle. M* > 8 M - stars more massive than about 8 solar masses (this number is very uncertain compared with those above) will have a larger number of nuclear burning cycles and their cores will be more massive than the limiting mass of 1.4M, the largest mass that can be supported by electron degeneracy, and thus the largest possible mass for a white dwarf. As we shall see these stars end their lives with a cataclysmic explosion called a supernovae

Spectroscopic parallax

Measurement of apparent brightness of a light source, combined with some knowledge of its luminosity, can yield an estimate of its distance Steps to use this Measure the star's apparent brightness and spectral type without knowing how far away it is Use the spectral type to estimate the star's luminosity Use the inverse square law to determine the distance to the star (apparent brightness is directly related to luminosity/(distance2)) Spectral type is letter classification (OBAFGKM — corresponds to luminosity (O is most luminous, M is least luminous)) followed by a number between 0-9 that corresponds to how hot a star is (0 is hottest)

Microlensing

Microlensing is the only known method capable of discovering planets at truly great distances from the Earth. Whereas radial velocity searches look for planets in our immediate galactic neighborhood, up to 100 light years from Earth, and transit photometry can potentially detect planets at a distance of hundreds of light-years, microlensing can find planets orbiting stars near the center of the galaxy, thousands of light-years away.

Mira Variable Stars

Mira variables are red giant stars that have grown to enormous sizes after they finished "burning up" all the hydrogen in their outer atmosphere. These stars pulsate and change surface temperature and size by about 20%. While this is a large change for a star, it cannot directly account for the extremely large changes in visual light.

NGC 4993

NGC 4993 has several concentric shells of stars and large dust lane with diameter of approximately a few kiloparsecs which surrounds the nucleus and is stretched out into an "s" shape. The dust lane appears to be connected to a small dust ring with a diameter of ~330 ly (0.1 kpc). These features in NGC 4993 may be the result of a recent merger with a gaseous late-type galaxy that occurred about 400 million years ago. However, Palmese et al. suggested that the galaxy involved in the merger was a gas-poor galaxy NGC 4993 has a dark matter halo with an estimated mass of 193.9×10^10 M☉ NGC 4993 has an estimated population of 250 globular clusters The luminosity of NGC 4993 indicates that the globular cluster system surrounding the galaxy may be dominated by metal-poor globular clusters NGC 4993 has a supermassive black hole with an estimated mass of roughly 80 to 100 million solar masses (8×10^7 M☉).

Elliptical galaxies

No spiral arms, often no obvious galactic disk Stellar density increases in nucleus E0 are most circular, E7 are most elongated, anything else is in between Giant ellipticals and dwarf ellipticals Dwarf ellipticals much more common Little to no cool gas or dust Generally no evidence of young stars or star formation Mostly made up of old, reddish, low mass stars Lots of hot interstellar gas in the interiors they have much more random star motion than orderly rotational motion (star orbits are aligned in a wide range of angles and have a wide range of eccentricities) they have very little dust and gas left between the stars this means that they have no new star formation occurring now and no hot, bright, massive stars in them (those stars are too short-lived) they have no spiral structure. The elliptical galaxy's mass = k × (velocity dispersion)2 × (the distance the stars are from the galaxy center)/G, where k is a factor that depends on the shape of the galaxy and the angle the galaxy is from Earth.

Quasars

Quasars tend to be found at great distances from us; there are no nearby quasars When we look at quasars, we see them as they were billions of years ago The number of them increases at greater distances, so that must mean they were more common long ago The number of quasars peaks at a time when the universe was about 20% of its current age Back then the galaxies were closer together and collisions were more common than today Also, the galaxies had more gas that had not been incorporated into stars yet The number of quasars was hundreds of times greater than the time closer to the present At very great distances the number of quasars drops off The light from the most distant quasars are from a time in the universe before most of the galaxies had formed, so fewer quasars could be created A quasar is an extremely luminous active galactic nucleus It has been theorized that most large galaxies contain a supermassive central black hole with mass ranging from millions to billions of times the mass of our Sun In quasars and other types of AGN, the black hole is surrounded by a gaseous accretion disk

Red Dwarf Star

Red dwarf stars are the most common kind of stars in the Universe These are main sequence stars but they have such low mass that they're much cooler than stars like our Sun Red dwarf stars are able to keep the hydrogen fuel mixing into their core, and so they can conserve their fuel for much longer than other stars Astronomers estimate that some red dwarf stars will burn for up to 10 trillion years The smallest red dwarfs are 0.075 times the mass of the Sun, and they can have a mass of up to half of the Sun.

Galactic Clusters

Some clusters have only a handful of galaxies and are called poor clusters Other clusters with hundreds to thousands of galaxies are called rich clusters The low mass of a poor cluster prevents the cluster from holding onto its members tightly The poor cluster tends to be a bit more irregular in shape than a rich cluster Galaxy clusters attract each other to produce superclusters of tens to hundreds of clusters. Their mutual gravity binds them together into long filaments (thin, stringlike structures) 300 to 900 million light years long, 150 to 300 million light years wide, and 15 to 30 million light years thick on average. The discovery of these huge structures was made recently from years of taking doppler shifts of thousands of galaxies.

SN2014J

Spectra indicate it is a Type Ia supernova caused by the explosion of a white dwarf accreting matter from a companion star. By some estimates one week away from its maximum brightness, SN 2014J is already the brightest part of M82 and visible in small telescopes in the evening sky SN 2014J was one of the brightest and nearest supernova events seen in recent decades, along with SN 1993J, spotted in Messier 81 in 1993, and SN 1987A, which was even closer to us, only 168,000 light years away, in the outskirts of the Tarantula Nebula, an H II region in the Large Magellanic Cloud in Dorado constellation. SN 2014J is the nearest Type Ia supernova event observed in the last 42 years. It reached peak magnitude of 10.5 and was bright enough to be seen by amateur astronomers. Astronomers have been unable to identify the progenitor star and may never be able to do so because stars that end their life in Type Ia supernova explosions are usually white dwarfs in binary systems, not nearly luminous enough to be seen from a distance of over 11 million light years. In a paper published in The Astrophysical Journal in July 2014, researchers noted that the region around SN 2014J was relatively devoid of material and that there were no X-ray emissions in the vicinity of the site of the supernova. If the progenitor star had been surrounded by enough material, it would have produced a bright X-ray source after the explosion. The lack of material around the site is unusual. It could be explained by a merger of two white dwarf stars. If this had been the cause of the explosion, there may have been relatively little mass transfer from one star to another and less material around the progenitor star. Another possible explanation is that there had been a series of smaller eruptions of the surface of the star, which had cleared the region before the explosion.

Lifetime

Star main sequence lifetime = [star's mass / star's luminosity] × 1010 years. Star main sequence lifetime = 1010 / (star's mass)(p - 1), where p = 3 for stars more massive than 30 solar masses and p = 4 for stars less massive than 10 solar masses

Sagittarius A

Supermassive black hole in the center of our galaxy Material around it is faint in X-Rays 26,000 light years from earth One of the few black holes where we can witness the flow of matter nearby 4 million times the mass of the sun less than 1% of the material initially within the black hole's gravitational influence reaches the event horizon, or point of no return, because much of it is ejected. Consequently, the X-ray emission from material near Sgr A* is remarkably faint, like that of most of the giant black holes in galaxies in the nearby Universe The captured material needs to lose heat and angular momentum before being able to plunge into the black hole. The ejection of matter allows this loss to occur A radio source corresponding to a supermassive black hole Sagittarius A (Sgr A) is a complex radio source located at the centre of the Milky Way Galaxy. It lies in the direction of Sagittarius constellation, near the border with Scorpius. The radio source consists of the supernova remnant Sagittarius A East, the spiral structure Sagittarius A West, and a bright compact radio source at the centre of the spiral structure, called Sagittarius A*. Sgr A can't be seen in optical wavelengths because it is hidden from view by large dust clouds in the Milky Way's spiral arms. Sagittarius A* has a diameter of 44 million kilometres, roughly equalling the distance from Mercury to the Sun (46 million km). Sgr A* emits a large amount of IR, gamma-rays and X-rays. It appears motionless, but there are clouds of dust and gas orbiting it, which provides a clue to the nature of the object. Astronomers calculated its mass using Kepler's laws and measuring the period and semi-major axis of the orbit of a star that came within 17 light hours of the object. They arrived at approximately 4 million solar masses. The only kind of object that can be that massive and have a radius of about 100 astronomical units is a black hole

Blazhko Effect

The Blazhko effect, which is sometimes called long-period modulation, is a variation in period and amplitude in RR Lyrae type variable stars. ... The physics behind the Blazhko effect is currently still a matter of debate, with there being three primary hypotheses.

Eddington Limit

The Eddington luminosity, also referred to as the Eddington limit, is the maximum luminosity a body (such as a star) can achieve when there is balance between the force of radiation acting outward and the gravitational force acting inward

Cepheids

Variable stars, meaning that their luminosities change with time Pulsation periods vary greatly Pulsate due to changes in opacity of interior Opacity is the degree to which gas hinders the passage of light through it Rising opacity causes radiation to become trapped, which causes the pressure to increase, causing the star to puff up If the opacity falls, radiation can escape more easily, causing the star to shrink These conditions occur because of the star's temperature and luminosity placing it on the instability strip of the H-R diagram, when the star is on the instability strip, it is internally unstable Both the temperature and radius vary in a regular way, causing the pulsations High mass stars become Cepheids Pulsating variable stars are normal stars passing through a million-year phase of instability Period related to luminosity Type I: classical Cepheids are from young high-metallicity stars and are about 4 times more luminous than Type II Cepheids Type II: W Virginis Cepheids are from older low-metallicity stars and are about 4 times less luminous than Type I Cepheids have pulsation periods of 1 to 50 days

RR Lyrae Stars

Variable stars, meaning that their luminosities change with time Shorter periods and lower luminosities than cepheids RR Lyrae stars all pulsate similarly with only small differences in the period between them RR Lyraes are pulsating horizontal branch aging stars of spectral class A or F, with a mass of around half the Sun's. They are thought to have previously shed mass during the Red-giant branch phase, and consequently, they were once stars with similar or slightly less mass than the Sun, around 0.8 solar masses All RR Lyrae stars in a cluster have the same average apparent magnitude All RR Lyrae stars have the same average absolute magnitude of 0.6 All of them have approximately the same luminosity Pulsate due to changes in opacity of interior Opacity is the degree to which gas hinders the passage of light through it Rising opacity causes radiation to become trapped, which causes the pressure to increase, causing the star to puff up If the opacity falls, radiation can escape more easily, causing the star to shrink These conditions occur because of the star's temperature and luminosity placing it on the instability strip of the H-R diagram, when the star is on the instability strip, it is internally unstable Both the temperature and radius vary in a regular way, causing the pulsations Low mass horizontal branch stars that lie in the lower part of the instability strip become RR Lyrae stars Pulsating variable stars are normal stars passing through a million-year phase of instability This type of low-mass star has consumed the hydrogen at its core, evolved away from the main sequence, and passed through the red giant stage. Energy is now being produced by the thermonuclear fusion of helium at its core, and the star has entered an evolutionary stage called the horizontal branch They pulsate with a period between 5 and 15 hours (Cepheid pulsation periods are greater than 24 hours) Low-mass stars will go through a RR Lyrae pulsation stage while the high-mass stars will go through a Cepheid stage Because low-mass stars live longer than high-mass stars, the Cepheid stars as a group are younger than the RR Lyrae stars.

Degenerate Gas

When gas becomes super-compressed, particles bump right up against each other to produce a kind of gas, called a degenerate gas, that behaves more like a solid Normal gas exerts higher pressure when it is heated and expands, but the pressure in a degenerate gas does not depend on the temperature The laws of quantum mechanics must be used for gases of ultra-high densities. Degenerate gases strongly resist compression The degenerate particles (electrons or neutrons) are locked into place because all of the lower energy shells are filled up The only way they can move is to absorb enough energy to get to the upper energy shells This is hard to do! Compressing a degenerate gas requires a change in the motions of the degenerate particle But that requires A LOT of energy. The pressure in a degenerate gas depends only on the speed of the degenerate particles NOT the temperature of the gas But to change the speed of degenerate particles requires A LOT of energy because they are locked into place against each other Adding heat only causes the non-degenerate particles to move faster, but the degenerate ones supplying the pressure are unaffected Increasing the mass of the stellar core increases the compression of the core The degenerate particles are forced closer together, but not much closer together because there is no room left A more massive stellar core remnant will be smaller than a lighter core remnant This is the opposite behavior of regular materials: usually adding mass to something makes it bigger!

Accretion Powered Pulsar

X-ray binary pulsars whose pulses are generated by the accretion flow striking the neutron star. Instead of falling uniformly onto the neutron star, the steady flow (accretion) of matter from the companion star is channeled by the pulsar's magnetic field onto the magnetic poles of the neutron star, resulting in a pair of "hot spots" on the pulsar surface. As the pulsar spins, these hot spots are brighter in X-rays than the rest of the star, giving rise to X-ray pulsations at the spin rate. In accretion-powered millisecond pulsars, the magnetic field (and thus the channeling) is relatively weak, so that the pulsations can be difficult to detect.

NGC 5195

located about 26 million light years from Earth Contains one of the most massive black holes that is currently undergoing powerful outbursts Just outside the outer X-ray arc is a slender region of hydrogen emission detected in an optical image. This suggests that the X-ray emitting gas has "snow-plowed" or swept-up the hydrogen gas from the center of the galaxy The outbursts of the supermassive black hole in NGC 5195 may have been triggered by the interaction of this galaxy with the large spiral galaxy in M51, causing gas to be disrupted and then funneled down towards the black hole This arc of hydrogen gas contains what appears to be two or three small "HII regions." An HII (pronounced "H-two") region is created when the radiation from hot, young stars strips away the electrons from neutral hydrogen atoms (HI) to form clouds of ionized hydrogen (HII). This suggests that the outer arc has plowed up enough material to trigger the formation of new stars An irregular dwarf galaxy (NGC 5195) that is interacting with a large grand design spiral galaxy (M51)

Phoenix cluster

located about 5.7 billion light years from Earth The Chandra data reveal hot gas in the cluster and the optical and UV images show galaxies in the cluster and in nearby parts of the sky Stars are forming in the Phoenix Cluster at the highest rate ever observed for the middle of a galaxy cluster. The object is also the most powerful producer of X-rays of any known cluster, and among the most massive of clusters. The data also suggest that the rate of hot gas cooling in the central regions of the cluster is the largest ever observed Phoenix contains a vast reservoir of hot gas -- containing more normal matter than all of the galaxies in the cluster combined This hot gas is giving off copious amounts of X-rays and cooling quickly over time, especially near the center of the cluster, causing gas to flow inwards and form huge numbers of stars. These results are striking because most galaxy clusters have formed very few stars over the last few billion years. Astronomers think that the supermassive black hole in the central galaxy of clusters pumps energy into the system the supermassive black hole in the central galaxy of Phoenix is growing very quickly, at a rate of about 60 times the mass of the Sun every year. This rate is unsustainable, because the black hole is already very massive, with a mass of about 20 billion times the mass of the Sun. Therefore, its growth spurt cannot last much longer than about a hundred million years or it would become much bigger than its counterparts in the nearby Universe. A similar argument applies to the growth of the central galaxy. Eventually powerful jets should be produced by the black hole in repeated outbursts, forming the deep notes seen in objects like Perseus and stopping the starburst

Dark Matter

material that does not produce detectable amounts of light but it does have a noticeable gravitational effect

Minor mergers

mergers where one galaxy is significantly more massive than the other one

M81

spiral galaxy about 12 million light years away that is both relatively large in the sky and bright One of the densest known galaxies It can be evidenced as pink areas of light where the HII regions exist - while the blue areas are home to countless new stars. Found in Ursa Major The galaxy is located approximately 11.8 million light years from Earth and has an apparent magnitude of 6.94 M81's active galactic nucleus contains a supermassive black hole with about 70 million solar masses, or 15 times the mass of the black hole in the Milky Way Galaxy, and has been an object of extensive study. The angular size of M81 roughly corresponds to that of the full Moon. The central bulge of M81 is home to significantly older stars, red in colour, and much larger than the central bulge of our own galaxy, the Milky Way. Bode's Galaxy is the largest member of the M81 Group, a group of 34 galaxies lying in Ursa Major constellation. It has a strong gravitational effect on Messier 82 (Cigar Galaxy) and NGC 3077, two other prominent members of the group. The close encounter between the galaxies occurred about 300 million years ago. As a result, all three galaxies have had hydrogen gas stripped away. The gravitational interaction has also resulted in the formation of filamentary structures in the group. The filaments are made of the gas that is being stripped away from the three galaxies. The interaction has also caused the gas to fall into the central regions of Messier 82 and NGC 3077, resulting in increased star-forming activity. Bode's Galaxy's infrared emissions mostly come from the interstellar dust found within its spiral arms. The dust is associated with starburst regions. The hot, young blue stars heat the dust, increasing the level of emissions in the infrared.

Rotation

the circular movement of an object about a point in space (ex: the earth on its axis)

Roche Limit

the closest distance from the center of a planet that a satellite can approach without being pulled apart by the planet's gravitational field

Accretion

the coming together and cohesion of matter under the influence of gravitation to form larger bodies