Astronomy Test 3

Briefly describe the basic chain of events leading to the formation of a star like our Sun.

1) Interstellar Cloud: spins and collapses 2) Protostar: Fragmentation stops, fragments start collapsing 3) Protostellar Evolution: shrinks to 10x the size of the Sun. contraction rate decreases. 4) A New-Born Star: Eventually the star moves into the main sequence as it reaches hydrostatic equilibrium

What is 21-cm radiation? With what element is it associated? Why is it useful to astronomers?

21-cm radiation is the radiation emitted when a hydrogen atom switches from a high energy state to a low energy state by doing an electron spin-flip. It can be used to detect any part of the universe that is high in hydrogen, because the radiation can pass unimpeded and arrive at earth where researches can detect it. This is useful, because astronomers can detect dark clouds of matter in the universe.

What are the ingredients and the end result of the proton-proton chain in the Sun? Why is energy released in the proton-proton chain?

4H => 1He + positron + neutrino + gamma (radiation energy) The energy is released because mass is missing and E=MC^2

What are the potential ending states for a star?

<0.4M turns into brown dwarf 0.4-8M core shrinks, heats up, unstable outer layers at high temperatures=helium shell flashes, planetary nebula (the outer layers are ejected), leaves a white dwarf (a ball of glowing ball of carbon), cool into black dwarf OR white dwarfs may transfer mass with other stars, and if they reach 1.4M, it will explode again as a nova or supernova (type I) >8M internal fusion begins in outer layers due to the heat created during the shrinking process. Fuses up to iron. The core cools and shrinks until in an instant, the great pressure breaks the carbon down into its particles. (Supernova type II) forcing the neutrons to interact with each other. the collapsing outer layers bounce of the rigid core. THEN: 8-25M Neutron "star" (a ball of neutrons) >25M With enough mass after the supernova (2-3M) the neutron star collapses to a point of singularity called a Black Hole

The time between successive sunspot maxima is about A month A year A decade A century

A decade

What is a planetary nebula? Why do many planetary nebulae appear as rings?

A planetary nebula is the result of a star that is shedding incredible amounts of its mass. Those "rings" stay roughly circular/spherical because the matter being ejected from the star is coming out at about equal speeds in all directions.

The telescope best suited to observing dark dust clouds is An X-ray telescope A large visible-light telescope An orbiting ultraviolet telescope A radio telescope

A radio telescope

If a newly forming star has an excess of heat, then it will likely have More gravity Less gravity A slower contraction rate A rapid contraction rate

A slower contraction rate

What is the cause of sunspots, flares, and prominences?

All of these phenomena are caused by activity in the magnetic field of the Sun. -Sunspots are caused by kinks or loops of magnetic field extending through the lower atmosphere. These areas of concentrated magnetic field repel hot material trying to rise up from the Sun's interior, so the section of the Sun underneath the knot cools off and darkens. -Flares, by contrast, are areas where large amounts of energy are released in a short amount of time. Their origin is mysterious, but they are somehow connected to instabilities in the magnetic field. -Prominences are caused by material ejected from the Sun's surface that follows along huge loops of magnetic field that carry the luminous gas far above the solar surface.

Why does the core of a massive star collapse?

As the core of a red giant collapses, the core can heat up even more, and these more massive stars will create a heat that fuses all the way up to iron, but as soon as iron is fused, fusion stops, radiation pressure decreases rapidly, and gravity collapses the star.

How can we see a black hole if it doesn't emit light/ describe two specific observations astronomers look for when trying to find a black hole. What does each of these observations tell us about the unseen object?

Astronomers look for areas with stron X-ray emissions coming from the accretion disks, and that helps determine the size of the blakc hole. they also use gamma, optical, and x-rays to observe the orbit of another star the black hole's binary system; the orbit of the companion star and the wobble in its orbit help to determine the mass of the black hole.

How do astronomers go about measuring stellar luminosities? What is the difference between luminosity and apparent brightness?

Astronomers measure stellar luminosity by first measuring the amount of energy detected through a telescope in a given amount of time (the stars apparent brightness). Second, the star's distance must be measured—by stellar parallax for nearby stars (the measuring of the stars movement in the sky when the earth is on opposite ends of its orbit) and by Spectroscopic parallax for more distant stars (The estimating of a stars luminosity by what the stars spectral class is). The luminosity can then be found using the inverse-square law [apparent intensity=(actual intensity)/radius2].

As a star evolves, why do heavier elements tend to form by helium capture rather than by fusion of like nuclei?

At higher and higher temperatures, heavier and heavier nuclei can gain enough energy to overcome the electrical repulsion between them. At about 600 million K (reached only in the cores of stars much more massive than the Sun), carbon nuclei can fuse to form magnesium. The formation of most heavier elements occurs by way of an easier path. For example, the repulsive force between two carbon nuclei is three times greater than the force between a nucleus of carbon and one of helium. Thus, carbon-helium fusion occurs at a lower temperature than that at which carbon-carbon fusion occurs.

On the main sequence, massive stars Conserve their hydrogen fuel by burning helium Burn their hydrogen fuel more rapidly than the Sun Burn their fuel more slowly than the Sun Evolve into stars like the Sun

Burn their hydrogen fuel more rapidly than the Sun

An observable supernova should occur in our galaxy about once every Year Decade Century Millennium

Century

What makes Cygnus X-1 a good black-hole candidate?

Cygnus X-1 is a good candidate because it is easily observed through its interaction with its partner star that is slowly being pulled towards it. X-ray images reveal the accretion disk around where either a black hole or a neutron star is, but the center of the accretion disk has to high of a mass limit to be a neutron star.

A typical open cluster will dissolve in about the same amount of time as the time since North America was first visited by Europeans Dinosaurs walked on Earth Earth was formed The universe formed

Dinosaurs walked on Earth

A millisecond pulsar is actually a very old neutron star that has been recently spun up by interaction with a neighbor.

False

A nova is a sudden outburst of light coming from an old main-sequence star.

False

A star of apparent magnitude 5 looks brighter than a star of apparent magnitude 2.

False

Although visible light cannot escape from a black hole, high-energy radiation, like gamma rays, can escape.

False

An emission nebula is a cloud of dust reflecting the light of a nearby group of stars.

False

Convection involves cool gas rising toward the solar surface, and hot gas sinking into the interior.

False

Gamma-ray bursts mainly occur within the Milky Way Galaxy.

False

Interstellar matter is quite evenly distributed throughout the Milky Way Galaxy.

False

Most interstellar matter exists in the form of molecular clouds.

False

Neutrinos are hypothetical particles that are believed to exist but have never been detected experimentally.

False

On the main sequence M-type stars are more massive than O-type stars

False

Once on the main sequence, gravity is no longer important in determining a star's internal structure.

False

Red giants are very bright because they are extremely hot.

False

Star A appears brighter than star B, as seen from Earth. Therefore, star A must be closer to Earth than star B.

False

The faintness of the chromosphere is a direct result of its low temperature.

False

Thousands of black holes have now been identified in our Galaxy.

False

A massive star becomes a supernova when it Collides with a stellar companion Forms iron in its core Suddenly increases in surface temperature Suddenly increases in mass

Forms iron in its core

The primary source of the Sun's energy is Fusion of light nuclei to make heavier ones Fission of heavy nuclei into lighter ones The slow release of heat left over from the Sun's formation The solar magnetic field

Fusion of light nuclei to make heavier ones

Four formulas you should know, Gravity, luminosity, the star distance using the HR diagram, and Star lifetime

Gravity: F=GMm/R^2 Luminosity: L α R^2 * T^4 Star distance: D=10 x 10^[(m-M)/5] Star lifetime: Lifetime α 1/(stellar mass^3) (aka greater the mass, shorter the lifetime)

The best place to search for black holes is in a region of space that Is dark and empty Has recently lost some stars Has strong X-ray emission Is cooler than its surroundings

Has strong X-ray emission

The X-ray emission from a neutron star in a binary system comes mainly from The hot surface of the neutron star itself Heated material in an accretion disk around the neutron star The neutron star's magnetic field The surface of the companion star

Heated material in an accretion disk around the neutron star

High-mass stars start off with much more fuel than low-mass stars. Why don't high-mass stars live longer?

High mass stars use up the fuel much faster because a higher mass means more gravity which means more pressure in the core which means a higher core temperature which means a higher fusion rate.

What is hydrostatic equilibrium?

Hydrostatic equilibrium is the stable relationship between the inward force of gravity and the outward force of radiation.

Under what circumstances will a binary star produce a nova?

If the binary contains a white dwarf star and another larger star, a nova could occur. The large star must be close enough to the w.d. so that its atmosphere will get pulled onto the w.d.. When that material hits the w.d., it heats and gives off extra light.

Why do astronomers think that gamma-ray bursts are very distant and very energetic?

In combining the gamma-ray, X-ray and optical observations of the gamma-ray bursts and the redshifts observed showed that they were cosmological distances away. Because these bursts are so far away and we can still detect them, we can assume that they are very energetic.~

What evidence is there for black holes much more massive than the Sun?

In using high-resolution observations at wavelengths ranging from radio to ultraviolet, stars and gas can be seen orbiting rapidly around a dark, unseen object, and according to Newton's laws, the object should be millions to billions times the mass of the Sun.

What is the composition of interstellar gas? What about interstellar dust?

Interstellar gas is mostly hydrogen, but about 9% is helium and less than 1% is heavier elements. Interstellar dust is harder to detect, but we have evidence of some silicates, graphite, and probably water ice with traces of methane and ammonia.

Give a brief description of the interstellar medium. How dense is it, and how is it distributed throughout space?

Interstellar medium is made up of gas and dust that is interspersed thinly throughout all of space. It has an extremely low density.

What is an emission nebula?

It is a region of glowing, ionized gas with at least one O or B-type star (on the main sequence) near the center

What is luminosity, and how is it measured in the case of the Sun?

Luminosity is the quantity of energy leaving the Sun's surface. It is measured with a light-sensitive device held perpendicular to the Sun's rays. With it, we can determine the solar energy per square meter and then use a lot of math to determine the Sun's luminosity.

Make a sketch of the H-R diagram. Specify the divisions of the axes and label each with the quantity they represent. Indicate BRIGHT, DIM, HOT, and COOL on axes. Indicate the regions of the three main types of stars. On your diagram, locate the Sun and its current state and draw lines representing its evolutionary path.

Luminosity left (bright up, dim down) spectral class down (OBAFGKM; hot left, cool right)

The gravitational contraction of an interstellar cloud is primarily the result of its Mass composition diameter pressure

Mass

Molecular clouds are routinely studied using spectral lines from all but which of the following? Molecular hydrogen Carbon monoxide Formaldehyde Water

Molecular hydrogen

The Sun spins on its axis roughly once each Hour Day Month Year

Month

A star will evolve off the main sequence when it uses up All of its hydrogen Half of its hydrogen Most of the hydrogen in the core All of its gas

Most of the hydrogen in the core

What is the order of the Stellar Spectra

OBAFGKM

What are pulsars, and how are they related to neutron stars? Why aren't all neutron stars seen as pulsars?

Pulsars are neutron stars that have a strong magnetic field and rapid rotation oriented so that the pulse can be seen from earth.

The best evidence for supermassive black holes in the centers of galaxies is The absence of stars there Rapid gas motion and intense energy emission Gravitation redshift of radiation emitted from near the center Unknown visible and X-ray spectral lines

Rapid gas motion and intense energy emission

List some characteristics of red-giants and white-dwarf stars.

Red-giants are cooler in nature, but they are larger in radius and they have fainter Balmer lines. White dwarf stars are hotter in nature, but they are smaller in radius and they have bright Balmer lines. Both Red-giants and white-dwarfs are off of the main sequence on the H-R diagram.

Why are scientists so interested in solar neutrinos? What is the most likely solution to the solar neutrino problem?

Scientists are interested in neutrinos, because they do not react with anything and, thus, are a good way to tell what happens in the core. With our detections, we receive only about half of the neutrinos that we should, but the neutrinos can probably spontaneously change to a different kind of neutrino. There are three different kinds, but we can only detect one, so the neutrinos probably changed to an undetectable type of neutrino.

How do observations of the sun's surface tell us about conditions in the solar interior?

Scientists can use helioseismology to study the vibrations on the surface of the sun to study conditions below the surface. Simple temperature measurements can tell us the interior temperature. the solar granulation shows that there is convection which tells us that the energy is made in the core. Any sort of active region behavior tells us that the sun has an intense magnetic field that that the sun is made of a liquid-plasma-like material.

Why don't stars live forever? Which stars live the longest?

Stars need hydrogen to fuel the fusion in their core, and when the hydrogen runs out, the star begins to collapse in on itself because there is no longer enough radiation to counter the force of gravity to keep the star in hydrostatic equilibrium. The lowest mass stars live the longest because their fusion process is slower.

Describe how astronomers measure stellar radii.

Stellar radii can be measured in two different ways 1) by determining the angular size of the star, and in combining that with a known distance of the star, simple geometry can be used to calculate the radius [d×tan(θ⁄2)=r]. 2) by using the formula r=√L/T^2 which shows the relationship between the radius, the luminosity and the temperature.

What are T Tauri stars?

T Tauri stars are stars that are not in equilibrium and are starting to shrink. They have lower luminosity and higher temperature. They are located in the instability strip. They are protostars that have heated up to the point in which extreme activity on the surface generates a powerful "protostellar wind". These winds can drive away material in the outer portions of the cloud.

Briefly describe the basic chain of events leading to the formation of a star like the Sun.

The Sun-like star begins as a large interstellar cloud containing cold molecular hydrogen. Gravity makes it start collapsing and overdensities within the big cloud make it fragment into smaller clouds. These clouds collapse fairly quickly (10^4 yrs) because the grav potential energy goes into breaking H_2 bonds instead of increasing the gas temperature and pressure. After those bonds are broken, collapse goes more slowly as it is accompanied by increased temperature and pressure. The protostar gets hot and produces winds which blow away the outer parts of the cloud. Gas and dust remain the longest in a protoplanetary disk but eventually gets cleared. The center of the protostar finally starts fusion when its temperature gets above about 10 million K. After that there is some more slow collapse and heating of the center as the star settles onto the main sequence.

How can we tell whether a star cluster is young or old?

The age of a star cluster is determined by the type of star that is about to leave its main sequence. The more massive a star is, the faster it uses up hydrogen fuel on the main sequence, so as the star ages, the more massive stars leave the main sequence first. If we can identify the star with the largest mass still on the main sequence, the cluster's age will be the same as the main sequence lifetime of that star.

Most of the carbon in our bodies originated in The Sun The core of a red-giant star A supernova A nearby galaxy

The core of a red-giant star

Which of the following is not evidence for supernovae in our Galaxy? The rapid expansion and filamentary structure of the Crab Nebula Historical records from China and Europe The existence of binary stars in our Galaxy The existence of iron on Earth

The existence of binary stars in our Galaxy

What is the helium flash?

The explosive burning of helium in the case of a star of low mass that occurs when the core is so dense that the matter has become degenerate. The burning causes a rapid rise in temperature until it is so high that the gas ceases to be degenerate, after which there is a rapid expansion.

What is the main sequence? What basic property of a star determines where it lies on the main sequence?

The main sequence is the band of stars in the H-R diagram where most stable stars are located. The mass determines where the stars lie on the main sequence.

Which stars are most common in our Galaxy? Why don't we see many of them in H-R diagrams? Which stars are least common in our Galaxy?

The most common stars in our Galaxy are red-dwarfs. We don't see them on many H-R diagrams because they have a very low luminosity, so they are difficult to see. The least common stars in our galaxy are the Red and Blue giants that are not located nearby us.

Compare and contrast the observed properties of open star clusters and globular star clusters

The open clusters are found in the plane of the galaxy, while globular clusters (GC's) are found in its halo. OC's have many fewer stars than GC's. OC's are usually less compact. OC's contain young, blue stars, while GC's generally do not. OC's are young, GC's are old. OC's are higher in metals than GC's.

On the H-R Diagram, which side is cooler? less massive?

The right. The right.

21-cm radiation can pass unimpeded through the entire Milky Way Galaxy.

True

As a red giant, the Sun will have a core that is smaller than it was when the Sun was on the main sequence.

True

As a star evolves away from the main sequence, it gets larger.

True

In a core-collapse supernova, the outer part of the core rebounds from the inner, high-density core, destroying the entire outer part of the star.

True

It takes less and less time to fuse heavier and heavier elements inside a high-mass star.

True

More massive stars form more rapidly.

True

Most stars form as members of groups or clusters of stars.

True

Observations of sunspots indicate that the Sun rotates differentially.

True

Stellar nucleosynthesis can account for the existence of all elements except hydrogen and helium.

True

Sunspots are regions of intense magnetic fields.

True

The brightest stars visible in the night sky are generally all found in the upper part of the H-R diagram.

True

The gas in an emission nebula eventually dissipates into space leaving behind a star cluster.

True

The time a solar-type star spends in formation is relatively short compared to the time it spends as a main-sequence star.

True

The various stages of stellar evolution predicted by theory can best be tested by observations of stars in clusters.

True

There are no other stars within 1 pc of the Sun.

True

How do the mechanisms responsible for Type I and Type II supernovae explain their observed differences?

Type I and type II supernovae are distinguished by the absence or the presence of hydrogen lines in the spectrum. Type Ia supernova are white dwarf supernova. As these consist mainly of carbon and oxygen, with the hydrogen atmosphere of a normal star, no hydrogen lines are visible. Type II are massive star supernovae. These stars have atmospheres made of mostly hydrogen. However, some very massive stars have such strong stellar winds that much of the atmosphere had already blown by the time the star explodes. These of course will not show hydrogen spectral lines. These supernovae are often called Type Ib supernovae, despite that they are really massive star supernovae.~

Overall, the Sun's average density is roughly the same as that of Rain clouds Water Silicate rocks Iron-nickel meteorites

Water

Describe some ways in which we can "see" a dark interstellar cloud.

We can "see" a dark interstellar cloud by detecting 21-cm radiation from that area of space, by detecting the infrared radiation they give off, or by visually seeing them due to a brighter stellar body behind it.

The solar neutrino problem is that We detect more solar neutrinos than we expect We detect fewer solar neutrinos than we expect We detect the wrong type of neutrinos We can't detect solar neutrinos

We detect fewer solar neutrinos than we expect

A star like the Sun will end up as a Blue giant White dwarf Binary star Red dwarf

White dwarf

What are white dwarfs? Why are they hard to observe?

White dwarfs are actually the left over cores of red giants after all of the outer layers have dispersed, so they are really big balls of carbon. They are hard to observe because there is no fusion occurring in them, so they don't create any radiation.

How does the way in which a neutron star forms determine some of its most basic properties?

a neutron star forms in the core of a supernova, where there is so much pressure in the core that all of the iron that has been fused breaks down and collapses into its constituent particles, mainly neutrons. This origin accounts for the small size and extreme density, and the strong magnetic fields that often occur because the magnetic field lines are closer together. Also, rapid rotation occurs because the star has gone from being very big to being very little (just like an ice skater).

According to the inverse-square law, if the distance to a lightbulb increases by a factor of 5, the bulb's apparent brightness stays the same becomes 5 times less becomes 10 times less becomes 25 times less

becomes 25 times less

The mass of a star may be determined by measuring its luminosity by determining its composition by measuring its Doppler shift by studying its orbit around a binary companion

by studying its orbit around a binary companion

Compared to other stars on the H-R diagram, red-giant stars are so named because they are Cooler Fainter Denser Younger

cooler

Stars interact with emission nebulae by Exciting their atoms enough to emit light Illuminating them like an advertising billboard Causing them to contract Heating them so they explode

exciting their atoms enough to emit light

Compared with a star of absolute magnitude -2 at a distance of 100 pc, a star of absolute magnitude 5 at a distance of 10 pc will appear brighter fainter to have the same brightness bluer

fainter

Cool stars can be very luminous if they are very small hot large close to our solar system

large

A proto star that will eventually turn into a star like the Sun is significantly smaller more luminous fainter less massive than the Sun

more luminous

The most rapidly "blinking" pulsars are those that Spin fastest Are oldest Are most massive Are hottest

spin fastest

How do supernovae help "recycle" galactic matter?

supernovae recycle galactic matter by fusing lots of atoms together (up to iron) but then when the core shrinks rapidly and the supernova occurs, the outer layers in all its particles are scattered far across the universe.

The chemical composition of the interstellar medium is basically similar to that of the Sun Earth Venus Mars

the Sun

Stars of spectral class M do not show strong lines of hydrogen in their spectra because they contain very little hydrogen their surfaces are so cool that most hydrogen is in the ground state their surfaces are so hot that most hydrogen is ionized the hydrogen lines are swamped by even stronger lines of other elements

their surfaces are so cool that most hydrogen is in the ground state