Astronomy 3- Final 3/3
Why do stars leave the main sequence?
Because they exhaust the supply of hydrogen in their central cores
What are the properties of the Milky Way bulge?
M= 10^10Msun
What is the relationship between velocity and radius for the planets in our solar system?
v=2πr(radius)/P(period) Relationship b/w v and r comes from Newton's laws, based on the bulk of the mass in the solar system being concentrated in the Sun
How do objects move in the disk versus the bulge or halo of the Milky Way?
. Disk stars orbit the galaxy's center in orderly circles that all . Halo stars swoop high above and below the disk on randomly oriented orbits . Bulge stars are weirder, with some orbiting like halo stars and others orbiting like disk stars
What is a WIMP?
. Stands for Weakly Interacting Massive Particles; These particles are theoretical, they have not yet been discovered; They would be massive enough to exert gravitational influence; they would emit no electromagnetic radiation (light) or be bound to any charged matter which could emit light; As weakly interacting particles, they wouldn't collapse with a galaxy's disk, yet would remain gravitationally bound in the galaxy's halo .One ex: The neutralino, 10-1000 times the mass of a proton
What is the Chandrasekhar limit for a white dwarf?
1.4Msun
What is the maximum for a neutron star?
3Msun; beyond the neutron star limit, no known force can resist the crush of gravity
What is the inferred mass of the super massive black hole?
4,000,000(4mill)Msun
How do we measure the size of such a small central object?
?? We look at timescales? AGN are seen to vary on timescales as small as 1 hour in X-ray emission; such short-time variability limits size of central region to be R< cΔt=7.2AU, where Δt=1 hour, or else variability would be smeared out by difference in light-travel times
What is a quasar, and how were they first discovered?
A quasar is the brightest type of active gigantic nucleus (10-1000x as luminous as entire galaxy);initially discovered by the huge red shift they made
What is the typical radius of a white dwarf?
About the same radius of the Earth
What is an active galaxy?
Active galactic nuclei are unusually luminous centers of some galaxies, thought to be powered by accretion onto super massive black holes; Quasars are the brightest type of active galactic nuclei; radio galaxies also contain active galactic nuclei
What is adaptive optics ?
Air turbulence in the atmosphere distorts light. This is why the stars appear to "twinkle" and angular resolution is degraded. It is possible to de-twinkle a star; By monitoring the distortions of the light from a nearby star, a computer can deform the secondary mirror in the opposite way, and then the wave fronts are restored to their original state
What is a singularity?
Beyond the neutron star limit, no known force can resist the crush of gravity; as far as we know, gravity crushes all the matter into a single point known as a singularity
Which constellation has the Milky Way's Galactic Center in it?
Center of the Milky Way Galaxy is in Constellation Sagittarius
What are the structural components of the Milky Way?
Disk Bulge Halo Globular Clusters
What is a giant star? Describe their process.
Giants/ and supergiants are stars that are nearing the ends of their lives. They have used up all the hydrogen fuel in their central cores and are trying to stave off the inevitable crushing force of gravity. This causes them to release fusion energy at a really high rate, which is why they're very luminous (which means they also must have a bigger radii). Conservation of energy requires these stars to radiate this huge amount of energy, so they have to expand to a huge size
What is the Hubble diagram?
Ho, the slope of the line, is called Hubbles constant [km/s/Mpc]
Has Hubble's constant stayed the same throughout history?
Hubble's constant has changed throughout history
How does the model explain the huge AGN luminosities observed?
Huge luminosities result because only matter falling into black hole releases that much energy from so small a volume
As you increase the mass of a white dwarf, what happens to its size?
It becomes smaller. In a degenerate gas, increasing the density does not increase the pressure (opposite to a normal gas). But increased density does increase gravity. So, as you add mass to a white dwarf, the gravity increases, but the pressure only changes a small amount. Gravity wins and the star shrinks.
What do we think that dark matter is composed of ?
It could be made out of the "ordinary" matter like protons, neutrons, and electrons, but then the only unusual thing about dark matter is that it's dim; So we think that some or all of dark matter could be made of particles which we have yet to discover AKA extraordinary matter; Baryonic matter: Protons and neutrons Nonbaryonic matter: Extraordinary matter
What happens when a high-mass star explodes?
It explodes as a supernova, scattering all the newly made material into interstellar space; once gravity pushes the electrons past the quantum mechanical limit, they can no longer exist freely. The degeneracy pressure provided by the electrons vanishes, and gravity has free rein. The star collapses into a ball of neutrons. The collapse only stops because the neutrons have a degeneracy pressure of their own. The collapse releases a shit ton of energy which drives the other layers out into space in a huge explosion called a supernova
What is Cygnus X-1 and what does it provide evidence for?
It is a binary star system; It was the first good candidate for a black hole
What is a star doing when it is on the main sequence?
It is fusing hydrogen into helium in their cores
What is the expression for redshift?
Remember that red shifts occur when the object emitting the radiation is moving away from us; The expression for red shift is _z_?
What is a Seyfert galaxy?
Seyfert galaxies are active galaxies (have bright centers that produce strong emission lines)
What is the best method for measuring close distances?
The best methods for measuring close distances are parallax and main-sequence fitting aka comparing the entire main sequence (Parallax is used to measure distances to nearby stars, and the next link would be to use the next method)
What is the luminosity of the center of an AGN?
The luminosity is equivalent to >1000 times the luminosity of the entire galaxy, but originated from a tiny volume;
What is the rotation curve for our Galaxy? Why is our Galaxy's rotation curve weird at large radii? What does this mean for dark matter?
The milky way has a flat rotation curve. At large radii, material is rotating more quickly than expected based on distribution of ordinary matter (stars+gas). This means that : Most of the mass in the Milky Way galaxy: - Does not radiate - Is not ordinary "baryonic" matter, such as protons, neutrons, electrons On the largest scales in the universe (clusters of galaxies, the cosmic web), we estimate over 80% of matter is dark.
What is the Cosmic Microwave Background radiation (CMB)?
The remnant radiation from the Big Bang, which we detect using radio telescopes sensitive to microwaves
What are the properties of spiral galaxies?
This includes our own Milky Way; They look like flat white disks w/ yellowish bulges at their centers; The disks are filled with cool gas and dust, interspersed w/ hotter ionized gas, and usually display beautiful spiral arms; 2 components: the disk component (including stars and dust clouds) and the halo component/ spheroidal component (which includes both the halo and the central bulge)
What is a black hole?
This is a region of space in which the matter is so dense that nothing can escape from it, even light;It is the result of a neutron star surpassing the 3 Solar M limit.
What is dark matter?
This is matter than we infer too exists from its gravitational effects but from which we have not detected any light (it neither emits nor absorbs light); it dominates the total mass of the universe ;)
What is cosmological redshift?
This is the redshift we see from distant galaxies, caused by the fact that expansion of the universe stretches all the photons within it to longer, redder wavelengths
How large is a neutron star?
Typically just 10 kilometers in radius yet more massive than the Sun
How do we know that the universe is expanding?
We know the universe is expanding through motions of galaxies (redshifts) and distances of galaxies (distance indicators/ standard candles).
What is the relationship between redshift and the expansion of the universe?
We measure the spectra of galaxies and the spectra of external galaxies are systematically redshifted
Which wavelengths do we use to measure the properties of the Galactic center?
We use X-ray, radio, and infrared wavelengths to study the properties of the galactic center
How do we measure the mass of such a small central object? (the center of the AGN?)
We use the motions of stars and gas to measure the mass of the center object, it indicated the object to be 10^6 (1 mill) to 10^9 (1 bill) solar mass; also more specifically, the mass of a galaxy's central black hole is typically about 1/500 of the mass of the galaxy's bulge; ?We can also measure using uniform circular motion using the equation M=v2r/G
What happens when a high mass star leaves the main sequence?
When a high mass star leaves the main sequence it continues to zig zag back and forth in the H-R diagram b/c the core changes so quickly that the outer layers have virtually no time to respond, and the star progresses steadily to becoming a red giant
What is an X-ray binary?
When an object accreting extra mass is a Neutron star or black hole, infalling material releases lots of energy as it falls in, heats up surrounding accretion disk so that it radiates a lot in the X-ray region of the spectrum
What does it mean to say that the universe expansion is bound or unbound?
(Critical density refers to the mass density required for this gravitational pull to equal the kinetic energy of the Universe) If density> critical density, the Universe will stop expanding and then contract (bound) If the density< critical density, the Universe will expand forever (unbound); If the density=critical density, the universe will stop expanding at time=infinity (critical)
What are the properties of the Milky Way halo?
. 3.5x 10^5 l.y. across . M= 10^9 Msun . Halo stars are very old . The stars contain smaller proportions of heavy elements than disk stars
What does the study of galaxy rotation say about dark matter?
. When you look at the rotation curve of the Milky Way Galaxy: -Atomic gas clouds beyond our Sun orbit faster than predicted by Newton's Laws if most of mass comes from stuff we see -At the same time, most of the Galaxy's light comes from stars closer to the center than the Sun . So either we don't understand gravity on a galaxy-size scale, or the gas velocities are caused by the gravitational attraction of unseen matter
What are the properties of the Milky Way disk?
.1000 l.y. thick . 100,000 l.y. across . M=several x 10^10 Msun . Fairly flat . Most of the galaxy's stars are up in here (many different ages) . Contain higher proportion of heavy elements
So overall, what techniques make up the chain of measurements that allow us to determine distances across the universe?
.Radio ranging .Parallax .Main-sequence fitting .Cepheid variables . Standard candles .Distant standards (supernovae explosions) .Hubble's law
Why do we think that MACHOs aren't leading cause of dark matter?
.So we're able to detect MACHOs if they pass in front of a star where they gravitationally lens the star's light, the star gets much brighter for a few days to weeks, and we can measure the MACHO's mass . These events occur to only one in a million stars per year -but the # of MACHOs detected so far does not account for the Milky Way's dark matter
What are the pieces of evidence that we have for what is at the center of the AGN?
.The only way to account for all the features of the AGN (have very small central objects that somehow generate extreme luminosities, emit radiation over a broad range of wavelengths) is with the model assuming that the central object is a super massive black hole surrounded by a swirling accretion disk of very hot gas . So the extreme luminosities prob arise from the conversion of gravitational potential energy into thermal energy and radiation as matter falls toward the black hole .The broad wavelength range is explained by the environment around the super massive black hole. Hot gas in the accretion disk produced UV and X-ray photons; the radiation ionizes surrounding interstellar gas, creating ionization nebulae that emit visible light ; dust grains in the molecular clouds around the AGN absorb high-energy light and re-emit is as infrared light ; and then radio emission comes from fast-moving electrons in the jets of plasma shooting outward
What are the 4 models for the future of the universe?
1. Re-collapsing universe: the expansion will someday halt and reverse 2. Critical universe: will not collapse, but will expand more slowly w. time 3. Coasting universe: will expand forever w/ little slowdown 4. Accelerating universe (currently favored): the expansion will accelerate w/ time
What is a Type Ia supernova, and how do they occur? What is a Type II supernova?
A Type Ia supernova is when a WD explodes due to accretion from the companion star, while a Type II supernova is an iron core collapse in high-mass star.
How long do stars spend on the horizontal branch?
A few billion years; for the Sun, it's 10 billion years
What is a radio galaxy?
A galaxy that emits unusually large quantities of radio waves; thought to contain active galactic nucleus powered by a super massive black hole
How massive is a neutron star?
A neutron star is supported by neutron degeneracy pressure and has maximum mass of 3 solar masses.
What is a neutron star?
A neutron star is the ball of neutrons created by the collapse of the iron core in a massive star supernova
What is a pulsar, and how do they form? And why do we observe a pulsation?
A pulsar is a neutron star left behind by the supernova explosions from which we see rapid pulses of radiation as it rotates; The pulsations arise because the neutron star is spinning rapidly as a result of the conservation of angular momentum, As an iron core collapses into a neutron star, its rotation rate must increase as its shrinks in size; The collapse bunches the magnetic field lines running through the core far more tightly, greatly amplifying the strength of the magnetic field; These intense magnetic fields direct beams of radiation out along the magnetic poles; If the neutron star's magnetic poles are not aligned w/ its rotation axis, the beams of radiation sweep round and round; The neutron stars emit a steady beam of light, but we see a pulse of light each time the beam sweeps past Earth
What are the properties of the Milky Way globular clusters?
A spherical cluster of up to a million+ stars; found mostly in the halos of galaxies and contain only very old stars
What is a standard candle?
A standard candle is an object w/ known constant luminosity
How does the mass of a star determine its lifetime?
A star's lifetime depends on both its mass and its luminosity. A star's mass determines how much hydrogen fuel the star initially contains in its core. Its luminosity (which is determined by mass; more mass= higher luminosity) determines how rapidly the star uses up the fuel. Massive stars start their lives with a larger supply of hydrogen, but they fuse this hydrogen into helium so rapidly that they end up with shorter lives.
What do we think is at the center of an AGN?
A super massive black hole :)
How do low-mass stars end their life?
After the helium core-fusion star converts all its core helium into carbon, the star expands again, just as it did to become a red giant (This time it is the helium fusion in a shell around the inert carbon core that makes it expand. At the same time, hydrogen fusion is continuing in a shell on top of the helium layer= Double shell-fusion star). Keeps expanding and becoming more luminous until the fusion ends. Finally, through winds and other processes, the star will eject its outer layers into space, creating a huge shell of gas expanding away from the inert carbon core. (B/c its so hot, it'll have high UV radiation which will make it glow like crazy) This radiation will ionize the gas in the expanding shell, making it glow brightly as a planetary nebula
What makes up the density of the universe? How much of the universe's mass is in stars? Gas?
All of the ordinary matter (baryons: stars, planets, gas dust, etc.) make up only 4-5% of critical density; Stars only make up p<1% of pcrit (critical density), while the rest is gas So we believe there is other matter that is present in the universe
Are all neutron stars pulsars?
All pulsars must be neutron stars, but not all neutron stars are pulsars
How can we see the center of the Galaxy?
Although it is hidden by layers of dusty spiral arms... We can see it using radio, infrared, and X-ray light, which can pass through dust and gas better
What happens in the evolution of a high mass star? (broad af)
An intermediate mass star undergoes a similar evolution to low mass stars with the exception that it does not have a helium flash and develops into a blue giant as well as begins to burn carbon.
How were active galaxies first discovered?
Astronomers had noticed that about 1% of nearby galaxies (s/t called Seyfert galaxies) have bright centers- active gigantic nuclei- that produce unusually strong emission lines (The emission lines are broader than absorption lines in normal galaxies, implying fast motions of gas clouds at the center of the galaxy); the bright centers are so compact that even the sharpest images do not resolve them, and they can produce substantial amounts of light in all bands of the electromagnetic spectrum, from radio waves to gamma rays
What are Cepheid variables? How can they be used to measure distances?
Cepheids are pulsating stars with pulsation periods ranging from one to a few hundred days; Period of Cepheid is closely related to luminosity. Measure the period which tells you luminosity which can measure flux (energy) and you can then compare flux (energy) and luminosity to infer distance to galaxy containing Cepheids.
What percentage of the mass/ energy in the universe is dark matter? Dark energy? Regular matter?
Cold dark matter: 23% Dark energy: 73% Regular matter (Atoms): 4%
How do we study galaxies back in time in the early universe?
Computer models of the formation and evolution of galaxies; .Cold Dark Matter Framework: Galaxy formation and evolution is a consequence of growth of density fluctuations by gravitational instability, in a universe dominated by dark matter (~85% of all matter is unknown form of "dark matter"). .Such models predict evolution of dark matter distribution through merging and accretion under the effects of gravity alone . It's hard to model real galaxies because we need to account not only for dark matter skeleton, but also "baryons" and photons . Another observational approach is to look at the nearby spectra which shows integrated light from constituent stars and gas; through these spectra, we can excavate the "fossil record"
What are some of the different lines of evidence that we have for the existence of dark matter?
Dark matter can be observed by means of its gravitational effect on stars and galaxies, and on rays of light
What is a white dwarf supernova, and how do they occur?
Each time a nova subsides, a white dwarf begins to accrete matter again. A white dwarf's mass might gradually approach the white dwarf limit, or it may reach that limit by merging w/ its binary-star companion. As the mass approaches the limit, its temp rises enough for carbon fusion to ignite When the carbon fusion stars, it ignites almost instantly t/o the star, and the white dwarf explodes completely into a white dwarf supernova
What happens as a high-mass star uses up the available fuel?
Each time the core depletes the elements it is fusing, it shrinks and heats up until it becomes hot enough for other fusion reactions (this repeats multiple times); meanwhile, a new type of shell fusion ignites between the core and the overlying shells
What were some early theories as to where we were in the Milky Way? Where are we actually in the Milky Way?
Early theories were that the sun was at the center of the Milky Way and that the Milky Way was small; Our Sun is located in the disk, about 27,000 light-years from the galactic center, a little more than halfway out from the center to the edge of the disk
What is dark energy? How does dark energy affect the expansion of the universe?
Einstein's equations for general relativity imply that expansion can speed up if exotic form of repulsive energy exists (dark energy), dark energy has the equivalent of negative gravity; The expansion rate will accelerate w/ time, galaxies will recede from w/ e/o (tracing the expansion of space-time) w/ increasing speed, The universe will become cold and dark, We will lose ability to communicate w/ galaxies as they expand out our "horizon" (such that their light can reach us)
What causes the different types of AGN that we've seen?
First off, the main kinds of AGN are Seyfert Galaxies, Radio Galaxies, and QSOs. The cause of the different types of AGN is accretion which is the most efficient process for releasing energy possible. AGNs are powered by accretion onto super massive black hole.
How can we discover white dwarfs?
First white dwarf was discovered using binary star motion with Sirius B.
What is the asymptotic giant branch?
For low-mass stars, this the last major phase of life: in the Asymptotic giant branch. This final phase of hydrogen burning happens after the star has moved from the Main sequence, through the red giant phase, and past the Horizontal Branch. At this point, they are characterized by an inert carbon-oxygen core, surrounded by 2 separate nuclear burning layers- an inner layer of Helium and an outer later of Hydrogen. As the star goes through the AGB phase, it cools, expands, and grows in brightness, burning its nuclear fuel faster and faster
What is differential rotation?
In a rotating solid body, regions that are adjacent at one point in time will remain adjacent as the body rotates. This means that points further from the rotation centre will travel at greater speeds than those closer in.
What is the Schwarzchild Radius, and how do we derive this? What is the event horizon?
In dealing with black holes:The Schwarzschild Radius is the distance from the center of a non-rotating black hole to the event horizon.(The event horizon being the boundary between the black hole and the rest of the universe where the escape velocity is just equal to the speed of light)
How do neutron stars form?
In high mass stars, Iron core is supported by electron degeneracy pressure. If mass exceeds 1.4 Solar M, electron degeneracy pressure cannot balance gravity. Electrons combine with protons to make neutrons,no more electron degeneracy pressure. Pressure support lost, core collapses from 10,000 km down to 10 km in 0.1 second. Core of solid neutrons, which is a neutron star.
What initially made quasars so interesting?
Interesting due to their huge redshifts. They also have large distances and their luminosities are hugeeee, even though they're very small.
Why does nuclear fusion in high-mass stars stop at iron?
Iron is the one element from which it is not possible to generate any kind of nuclear energy Iron has the lowest mass per nuclear particle of all nuclei and therefore cannot release by either fusion or fission, so once the core turns into iron, it can't generate any more energy
How long do stars generally spend on the main sequence?
It ranges from like 10 million years for very luminous stars to like 300 billion years for stars that are much less luminous
What is Hubble's Law?
It was known that galaxy spectra tended to show redshifts; Hubble found that more distant galaxies move away from us faster; the corresponding law is v=Ho x d (so the galaxy's recession velocity was proportional to the distance away from us) cz=Ho x d (v=cz) V= a galaxy's speed away from us d= distance Ho=Hubble's constant
How did we discover pulsars?
J Bell discovered some radio waves that pulsed on and off in precise clockwork intervals
What is the equation for the luminosity of an AGN based on its mass accretion rate?
L=0.1×(Δm/Δt)×c2
What is main-sequence fitting?
Main sequence fitting: You measure the distance to a cluster of stars by comparing the apparent brightness of the cluster's main sequence with that of the standard main sequence
What is the helium flash?
Marks the sudden onset of helium fusion in the previously inert helium core of a low-mass star
What is uniform rotation?
Material at all radii rotate with constant angular velocity; So at each radii, it takes the same time to complete one circle
What is the structure of a radio galaxy?
Most of the radio emission comes from pairs of huge radio lobes that extend beyond the galaxy's stars; Long jets of plasma connect the lobes to the very center of the galaxy, where we find an active galactic nucleus that is firing the jets of plasma outwards at speeds close to the speed of light. Ya, the lobes lie at the end of the jets, where the jets ram into intergalactic gas and inflate the radio lobes
Is the super massive black hole actively accreting?
Most suspected black holes are thought to accumulate matter through accretion disks that radiate brightly in X rays ; But with our latest observations (such as with Sgr*), the explanation may be that matter falls into it in big chunks instead of from a smooth, swirling accretion disk
What is the Milky Way?
Name of our galaxy; refers to the band of light we see in the sky when we look into the plane of the Milky Way Galaxy
How do astronomers infer the presence of a super massive black hole at the center of the Milky Way?
Orbits of stars indicate a mass of about 4 million Msun (packed into a region a little larger than the solar system); Stars appear to be orbiting something massive but invisible; Super-massive Black hole fits the bill
How much dark matter exists in the universe?
Our best estimate of the matter density in the universe (remember, the total matter content is ~85% unidentified dark matter, and ~15% ordinary baryonic matter) is that it makes up density=0.3xpcrit (critical density); ordinary baryonic matter has a density of density=0.05xpcrit; At 0.3xdensitycrit, the matter density is less than the critical density, and if we ignore the effects of dark energy, the universe should expand forever, but decelerate until it reaches some constant, coasting expansion rate
How do we measure the redshift of a distant galaxy?
Radiation from young, massive stars ionize Hydrogen, gas is heated to 10,000 K, emission from H, O, and other metals is highlighted. We then measure the redshift using the formula: z = λobs − λrest/λrest
How can we measure the distances to Type Ia supernova?
Since every Type 1a SN progenitor is likely the same size (~1.4 solar masses), they may be good standard candles for measuring distance
What are white dwarfs composed of ?
Since white dwarfs are the core left over after a star has stopped nuclear fusion, its composition reflects the products of the star's final fusion stage. For instance, for a star like our Sun, the white dwarf would consist mostly of carbon, since stars like the Sun fuse helium into carbon in their final stage of life
What do galaxy clusters say about dark matter? What do hot gas in galaxies tells us about hot matter? What does gravitational lensing by galaxy clusters tell us about dark matter?
So galaxies are found in clusters. There are 3 independent ways to measure galaxy cluster mass: 1.Measure the speeds and positions of the galaxies w/i the cluster 2.Measure the temp and distribution of the hot gas b/w the galaxies 3.Observe how clusters bend light as gravitational lenses; The cluster masses which are measured by all three of these independent methods agree that galaxy clusters contain far more mass in dark matter than in stars and gas
What does it mean for matter to be degenerate?
So in a white dwarf, there is no fusion to maintain heat and pressure, so it has to rely on degeneracy pressure to oppose gravity. This (electron) degeneracy pressure comes from matter (particles) being packed very closely together. The electron degeneracy pressure makes it so that white dwarfs live in a state of balance b/c the outward push of EDP matches the inward crush of gravity.
What do stars do on the horizontal branch?
So on the horizontal branch, we have stars that have already undergone the helium flash and become a helium core-fusion star. What it does now is convert all of its core helium into carbon.
What is a MACHO?
Stands for MAssive Compact Halo Objects; Our galactic halo should contain baryonic matter (MACHO) which is dark: -Low-mass M dwarfs, brown dwarfs, and Jovian-sized planets -They're too faint to be seen at large distances
What is a subgiant star?
Subgiant stars are stars that have just began their expansion into red giants as their cores have shut down and hydrogen shell fusion has begun
What is the Hubble constant?
The Hubble constant expresses the current rate of expansion of the universe; The reciprocal of Hubble's constant is the age the universe would have been if the expansion rate had never changed; Measured in km/s/Mpc; Our best measurement of the Hubble constant is 72 km/s/Mpc; So the age of the universe ~1/Ho=13.6 billion years
How can we relate the redshift of an object and the size of the universe when that light was emitted?
The important formula is: observed wavelength/ emitted wavelength= (size of the universe now/size when the light was emitted); The redshift tells you by how much the Universe has expanded in the time its taken for the light to get to us
What type of hydrogen fusion occurs in high mass stars? Why is the process different than w/ low-mass stars?
The type of Hydrogen fusion that occurs in high-mass stars is the CNO (carbon-nitrogen-oxygen) cycle. It is different than low mass stars, because in high mass stars, gravity compresses the hydrogen core to a higher temp, making it possible for protons to slam into carbon, oxygen, or nitrogen atoms as well as in other protons
What is the critical density of the universe? What does the critical density of the universe mean for the eventual fate of the universe?
The value of Ho tells us about the current kinetic energy of the universe. Based on what we know about the current rate of expansion in the universe, the critical density is p(density)crit~10^-26 kg/m3 (several atoms per cubic meter, on average).; It determines the number based off whether the universe will be bound, unbound, or equal it. Space time can only be flat when the average density of matter is close to the critical density
What makes active galaxies so interesting?
Their emission lines:Emission lines are broader than absorption lines in normal galaxies. Absorption lines in normal galaxies trace motions of stars. Broad emission lines imply fast motions of gas clouds at the center of the galaxy.
How are galaxies classified?
There are 3 major types of galaxies: Spiral galaxies Elliptical galaxies Irregular galaxies
What is a white dwarf?
These are the hot, compact corpses of low-mass stars
What are the properties of irregular galaxies?
They appear neither disk-like nor rounded; these areas are usually white and dusty; like the disks of spirals; and contain many young massive stars
What are the properties of elliptical galaxies?
They are redder, rounder, and often elongated like a football; Compared with spiral galaxies, they contain very little cool gas and dust, though they often contain very hot ionized gas; So they have only a (Spheroidal) halo component, but not a disk component
What do white dwarfs do during their lifetime?
They just cool off.
What happens when a star leaves the main sequence?
They no longer generate energy in the same way that our Sun does; and they now go through specific phases until the end of its life
How massive are white dwarfs typically?
They typically have the mass of the Sun compressed into the volume of the Earth
What is a nova?
This occurs to a white dwarf in a binary star system. So in a binary star system, gas from a companion star can spill toward a white dwarf, forming a swirling accretion disk around it. The gas gradually spins inward and eventually settles onto the white dwarf. The white dwarf's strong gravity compresses this hydrogen gas into a thin surface layer. The temperature and pressure rise as the layer builds up w/ more accreting gas, until hydrogen fusion ignites. The fusion supplies energy. This thermonuclear flash causes the binary system to shine for a few weeks as a nova. It ejects most of the material that has accreted onto the white dwarf (s/t leaving a nova remnant) The process is now able to happen again.
How can we interpret the expansion of the universe with kinetic and potential energies?
To understand dynamics of expansion of universe, think of projectile; It has kinetic energy (KE) and gravitational potential energy (PE), and the sum of KE+PE remains constant; Kinetic energy of sphere is related to Hubble. Mass inside the sphere is M=densityxvolume=pxV; Gravitational potential energy is related to density (p) inside sphere;We refer to the mass density required for this gravitational pull to equal the kinetic energy of the Universe as the critical density.
Describe the 2 types of Seyfert galaxies.
Type 1: Broad hydrogen lines (1000-5000 km/s), narrower oxygen lines (500 km/s); with the broad emission lines varying on short timescales (weeks) Type 2: Have narrower hydrogen and oxygen lines (500 km/s)
What does the Hubble constant say about the expansion of the universe throughout time?
Use redshifts and distances to probe how size of universe and H(t) evolve with time i.e. redshift v. distance can be re-cast as expansion factor (size of universe) v. lookback time
How do we measure the distance to far-away galaxies?
We can measure the distances to far away galaxies by using standard candles, main-sequence fitting, cepheid variables, white dwarf supernovae
What happens to a white dwarf when you add more mass than the Chandrasekhar limit?
When more mass is added, the electrons approach the speed of light, but nothing can move faster than light, so electron degeneracy can no longer support the weight So gravity wins?
Why do we need adaptive optics (AO) to see the center of the Galaxy?
Without AO, we see only one star; With AO, angular resolution improves