astro final

Consider all of the observations shown in the video. Which of the following are reasonable conclusions?

-There are strong magnetic fields in the central region of the galaxy. -Gas orbits the radio source called Sgr A*. -Stars near the galactic center are much closer together than stars around our Sun. These individual conclusions all point to a deeper conclusion: There is a great deal of gas and many stars orbiting around the tiny central source known as Sgr A*, which must be a very massive object to hold so much material in orbit around it.

What is the approximate range of masses that newborn main-sequence stars can have?

0.1 to 150 solar masses The lower limit is 0.08 solar masses: Below that, the core is not hot enough to sustain nuclear fusion. The upper limit of around 150 solar masses is as a result of the fact that higher-mass objects would be so luminous that they would blow themselves apart with their own radiation pressure.

This diagram shows the life tracks from protostar to the main sequence for several stars of different masses. Which track represents the star that takes the longest amount of time to reach the main sequence?

1 The lowest mass star takes the longest time.

Which star spends the longest time in the protostellar phase of life?

1 solar mass star Lower mass stars take longer in all phases of life.

According to the diagram, the approximate abundance of oxygen atoms in the galaxy is __________.

1/1000 that of hydrogen Looking along the vertical axis, you can see that the point for oxygen lies about halfway between the points labeled 10−4 and 10−2, which means about 10−3, which is 0.001 or 1/1000. Therefore, the abundance of oxygen atoms is about 1/1000 that of hydrogen.

Approximately what core temperature is required before hydrogen fusion can begin in a star?

10 million K

Approximately what core temperature is required before hydrogen fusion can begin in a star?

10 million k

What is the minimum temperature for a cloud to excite emission lines from H2?

100 k

Look again at the orbit of the star with the highlighted orbit. By comparing the orbit to the scale bar shown on the diagram, you can estimate that this orbit has a semimajor axis of about _____.

1150 au Notice that, in its longest direction, this orbit is a little less than 1 1/2 times the length of the scale bar, which represents 1600 AU. Therefore the entire orbit is about 2300 AU long. The semimajor axis is about half of this length, about 1150 AU

What are Magellanic Clouds?

2 small galaxies that orbit the milky way The two are called the Large Magellanic Cloud and the Small Magellanic Cloud.

Elements heavier than hydrogen & helium constitute about _____ of mass of milky way's interstellar medium.

2% The overall chemical composition is about 70% hydrogen, 28% helium, and 2% everything else.

To determine the mass of the central object, we must apply Newton's version of Kepler's third law, which requires knowing the orbital period and average orbital distance (semimajor axis) for at least one star. We could consider any of the stars shown in the figure, so let's consider the star with the highlighted orbit (chosen because its dots are relatively easy to distinguish). What is the approximate orbital period of this star?

20 years The 19 dots visible on the highlighted orbit span nearly the entire orbit, so 20 years is a reasonable estimate of the orbital period.

Approximately how long does it take the Sun to orbit the Milky Way Galaxy?

230 million years

Approximately how far is the Sun from the center of the galaxy?

27,000 light-years

How many helium nuclei fuse together when making carbon?

3

Based on current understanding, the minimum mass of a black hole that forms during a massive star supernova is roughly _________. View Available Hint(s)

3 solar masses A collapsing stellar core must contain about 3 solar masses (or more) for the crush of gravity to overcome neutron degeneracy pressure.

How long does the protostellar stage last for a star like our Sun?

30 million years

Ten parsecs is about _________.

32.6 light-years You find this by remembering that 1 parsec = 3.26 light-years.

This H-R diagram shows the life track of a 1 M Sun star from the time it first becomes a main-sequence star. Which numbered point represents the star when it has both hydrogen-burning and helium-burning shells around an inert carbon core?

4 It is a double shell burning giant in its last stage of life, just before the planetary nebula.

Based on the measurements discussed in part D, the mass of the central black hole is calculated to be about __________ times that of the Sun.

4 million The central object, Sgr A*, is a black hole with a mass about 4 million times that of the Sun.

The following equation, derived from Newton's version of Kepler's third law, allows us to calculate the mass (M) of a central object, in solar masses, from an orbiting object's period (p) in years and semimajor axis (a) in astronomical units: M=a3p2 Using this formula with the values you found in Parts C and D, what is the approximate mass of the central object?

4 million solar masses From Part D you know that the semimajor axis distance is about a=1150AU, and from Part C you know that the orbital period is about p=20years. Plugging these values into the given formula gives a mass of about 4 million solar masses.

This figure shows the life track of a 1 solar-mass star from its beginnings in a collapsing cloud fragment until it becomes a main-sequence star. At which of the numbered points is the object generating energy primarily from hydrogen fusion?

4 only Hydrogen fusion marks the beginning of the star's main-sequence life.

What is the smallest mass a newborn star can have?

80 times the mass of Jupiter

According to present scientific understanding of the Milky Way's formation, which of the following statements are true?

=The protogalatic cloud(s) contained essentially no elements besides hydrogen and helium. =Halo stars formed before disk stars. In the basic picture, a protogalactic cloud collapses so that its gas collects into a spinning disk. Halo stars are those born before the gas formed a disk, and disk stars can form only after the formation of the disk. The picture is more complex, however, because evidence indicates that there were several distinct protogalactic clouds that merged to become our galaxy.

Both photos show the same field of view containing a star-forming molecular cloud. One of the photos was taken in visible light and the other in infrared light. Which one is the visible-light photo, and how do you know?

A is the visible-light photo, and we can tell because dust in the molecular cloud absorbs visible light, making the cloud appear dark. Notice that we can see stars within the cloud in Photo B because infrared light can penetrate through dusty clouds.

What law explains why a collapsing cloud usually forms a protostellar disk around a protostar?

A protostellar disk forms as a consequence of conservation of angular momentum, which makes the cloud rotate faster as it shrinks in size.

common trait of all main sequence stars

A star becomes a main-sequence star when it first starts fusing hydrogen into helium, and it ends its main-sequence life when it exhausts its central core supply of hydrogen for fusion.

Which of the following statements about spectral types of stars is true?

A star with spectral type A is cooler than a star with spectral type B. The spectral type of a star can be used to determine its color. A star with spectral type F2 is hotter than a star with spectral type F3. The spectral type of a star can be used to determine its surface temperature.

Listed following are several stars found in the disk and halo of the Milky Way Galaxy. Assume that both the blue and yellow disk stars are members of the same open cluster. Rank the stars based on the abundance of elements heavier than carbon that you would expect to find in each of the stars, from highest to lowest. If you expect two (or more) stars to have approximately the same abundance, rank them as equal by dragging one on top of the other(s).

A star's abundance of heavy elements (heavier than helium) depends primarily on when it was born. Very old stars lacked earlier generations of stars manufacturing heavy elements when they were born, and thus have very low levels. Globular cluster stars (very old stars) therefore have a very small abundance of heavy elements — typically less than 0.1 percent. Open clusters in the disk are generally quite young, and their stars therefore have more heavy elements — typically about 2 percent.

What is a Cepheid variable? View Available Hint(s)

A type of luminous star that makes an excellent standard candle. Cepheids are pulsating variable stars for which we can infer the luminosity from the time between peaks of brightness, which makes them valuable as standard candles. Part H the period between its peaks of brightness and its luminosity The period-luminosity relation allows us to determine its luminosity from the period between its peaks of brightness.

This photo shows an object located in the halo of our Milky Way galaxy. Notice the many bright red dots in the photograph. What are they?

All the stars in a globular cluster are old and therefore low in mass, so the brightest ones must be red giants.

This photo shows the famous Crab Nebula. What is it?

An expanding cloud of remains from a star that died in a supernova.

What causes the radio pulses of a pulsar?

As the star spins, beams of radio radiation sweep through space. If one of the beams crosses Earth, we observe a pulse What is the ultimate fate of an isolated pulsar? It will slow down, the magnetic field will weaken, and it will become invisible.

Assuming that we can measure the apparent brightness of a star, what does the inverse square law for light allow us to do? View Available Hint(s)

Calculate the star's luminosity if we know its distance, or calculate its distance if we know its luminosity. That is, we can calculate luminosity if we have a way to measure the distance (such as by stellar parallax), or we can calculate the distance if we have some way to estimate the star's luminosity (such as by knowing what type of star it is).

Which cosmic distance measurement techniques are considered standard candle techniques? Select all that apply. View Available Hint(s)

Cepheids white dwarf supernovae (distant standards) main-sequence fitting A standard candle technique requires knowing the luminosities of the objects whose distances we are trying to find. This is the case for main-sequence fitting, which relies on knowing the luminosities of main-sequence stars; it is the case for Cepheids, because we know their luminosities from the period-luminosity relation; and it is the case for white dwarf supernovae, which are all thought to have about the same luminosity. In contrast, radar and parallax are direct measurement techniques based on applying geometry, and Hubble's law is an estimation technique based on a rule about how galaxy distances depend on recession velocities.

Why are Cepheid variables important?

Cepheids are pulsating variable stars, and their pulsation periods are directly related to their true luminosities. Hence, we can use Cepheids as "standard candles" for distance measurements.

What causes the glow of the reddish areas visible primarily in the spiral arms?

Clouds of gas are being heated by ultraviolet light from nearby, recently formed stars. The red regions are ionization nebulae, and their glow is arises because their gas is ionized by ultraviolet light from young, hot stars. (Small, cool stars to not emit enough ultraviolet light to have much of an effect on these clouds.)

Cluster ages can be determined from

Cluster ages can be determined from main sequence turnoff.

Which of the following best explains the current state of agreement between theoretical models of star formation and observations of star formation?

Current models can reproduce general features of star birth, but not all the details.

What prevents a brown dwarf from undergoing nuclear fusion?

Degeneracy pressure halts the contraction of a protostar so the core never becomes hot or dense enough for nuclear fusion.

Which of the following statements about electron degeneracy pressure and neutron degeneracy pressure is true? View Available Hint(s)

Electron degeneracy pressure is the main source of pressure in white dwarfs, whereas neutron degeneracy pressure is the main source of pressure in neutron stars. Both types of degeneracy pressure arise for different reasons than the thermal pressure that supports the structures of stars that still have ongoing nuclear fusion.

When a newly forming star is at its greatest luminosity, what is its energy source?

From Part C, you know that a newly forming star is most luminous during its protostar stage. The source of energy for protostars is the energy released by gravitational contraction.

When a star exhausts its core fusion fuel so that the core begins to contract, which of the following can stop the contraction?

Fusion of another element. Degeneracy pressure. When as star exhausts a fuel for fusion in its core, the core will no longer be in energy balance and therefore will contract either until it becomes hot enough to ignite fusion of another element or its contraction is stopped by degeneracy pressure.

Which of the following statements correctly summarize key differences between the disk and the halo?

Gas and dust are abundant in the disk but not in the halo. Disk stars come in a broad range of masses and colors, while halo stars are mostly of low mass and red. Clusters of young stars are found only in the disk. Stars in the disk all orbit in the same direction and nearly the same plane, while halo stars have more randomly oriented orbits.

The most abundant molecule in molecular clouds is

H2

Which of the following types of molecule is the most abundant in an interstellar molecular cloud?

H2 Hydrogen is the most abundant element, and molecular hydrogen is the most abundant molecule.

Why are we unlikely to find Earth-like planets around halo stars in the Galaxy?

Halo stars formed in an environment where there were few heavy elements to create rocky planets.

How are magnetic fields thought to affect star formation in molecular clouds?

How are magnetic fields thought to affect star formation in molecular clouds? View Available Hint(s) They can help resist gravity, so that more total mass is needed before the cloud can collapse to form stars. The magnetic fields that thread clouds inhibit the movement of gas, thereby adding to the resistance to gravity provided by thermal pressure (and turbulent gas motions).

Why does a star grow larger after it exhausts its core hydrogen?

Hydrogen fusion in a shell outside the core generates enough thermal pressure to push the upper layers outward.

According to this diagram, how much more abundant is hydrogen in the universe than nitrogen?

Hydrogen is about 10,000 times as abundant as nitrogen. Notice that the dot for nitrogen is at 10-4 = 1/10,000 on the vertical (abundance) axis.

This is the H-R diagram for stars in a 12-billion-year-old globular cluster. Based on our understanding of stellar lives, which of the four labeled stars is currently generating energy through hydrogen fusion in a shell around an inert (nonburning) helium core?

III This is a red giant, which means it has exhausted its core hydrogen (and left the main sequence) and is now generating energy through hydrogen fusion in a shell around the inert core.

What is interstellar reddening?

Interstellar dust absorbs more blue light than red light, making stars appear redder than their true color.

Consider the portion of the video that starts with the all-sky view of the Milky Way and then zooms in to the galactic center. All of the images except the first two show radio, infrared, or X-ray light. Why don't these images show visible light?

Interstellar dust in the galactic disk prevents us from seeing the galactic center with visible light. Although dust blocks visible light and most ultraviolet light, it does not block most infrared, radio, or X-ray wavelengths. We can therefore study the galactic center using these other wavelengths of light.

What happens to the core of a star after a planetary nebula occurs?

It becomes a white dwarf.

prostar

It generates energy by gravitational contraction because it is not yet hot enough for nuclear fusion it its core. powerful "jets" shooting out along the rotation axis of a protostar strong winds of particles blowing out into space from a protostar the formation of a spinning disk of material around a protostar energy generated by gravitational contractionpressure and gravity are not precisely balancedluminosity much greater than the Sunradius much larger than the Sun

What is the eventual fate of a brown dwarf?

It gradually cools down and becomes ever dimmer.

This star map shows stars as we see them in our sky from Earth, centered around the constellation Canis Major. Larger dots represent brighter stars, and a few of the brightest stars are identified. From this view alone, what can you conclude about Sirius?

It has the greatest apparent brightness of any star in this region of the sky. It is the largest dot, and larger means brighter in this diagram.

In what way is iron unique among the elements?

It has the lowest mass per nuclear particle. Because iron has the lowest mass per nuclear particle, neither fission nor fusion of iron can release any energy.

What happens to the visible radiation produced by new stars within a molecular cloud?

It is absorbed by dust grains and heats up the cloud.

This photo shows gas associated with a protostar. What is this gas doing?

It is flowing outward in two opposite directions from the protostar The long, mostly straight paths shows that these are jets flowing outward from the protostar.

What is happening inside a star while it expands into a subgiant?

It is fusing hydrogen into helium in a shell outside the core.

Which of the following best describes the status of the Milky Way in our Local Group of galaxies? View Available Hint(s)

It is one of the two largest galaxies in the group. The other large galaxy is the Andromeda Galaxy.

Where does gold (the element) come from? end of ch.17

It is produced during the supernova explosions of high-mass stars.

What do we mean by the main-sequence turnoff point of a star cluster, and what does it tell us?

It is the spectral type of the hottest main sequence star in a star cluster, and it tells us the cluster's age.

This photo shows the Large Magellanic Cloud, or LMC for short. Based on what you have learned in about the LMC from your text, which of the following statements about the LMC is not true?

It orbits the Milky Way Galaxy. It is an irregular galaxy. Its color indicates active star formation. Previous Answers Correct The LMC is an irregular galaxy, and although irregular galaxies are less common than spirals and ellipticals, they are not extremely rare.

What happens to the core of a high-mass star after it runs out of hydrogen?

It shrinks and heats up.

The arrow in the photo on the left shows the star that we see as a supernova in the photo on the right. What can we conclude about this star

It was a high-mass star with at least 8 times the mass of the Sun. Only high-mass stars explode as supernovae.

According to the inverse square law of light, how will the apparent brightness of an object change if its distance to us triples?

Its apparent brightness will decrease by a factor of 9. The inverse square law for light tells us that the light gets dimmer with increasing distance by the square of the distance, so tripling the distance decreases the brightness by a factor of 32 = 9.

What happens when a star exhausts its core hydrogen supply?

Its core contracts, but its outer layers expand and the star becomes bigger and brighter.

What happens to the surface temperature and luminosity when gravity first assembles a protostar from a collapsing cloud?

Its surface temperature and luminosity increase.

Watch the red dot representing the protostar in the video. After it reaches its highest point on the diagram, how do the protostar's surface temperature and luminosity change as it approaches the main sequence?

Its surface temperature increases, but its luminosity decreases Notice that the red dot in the video moves left and down after it passes its highest point and begins to approach the main sequence. Left means increasing surface temperature, and down means decreasing luminosity.

What are the letters that follow the spectral sequence OBAFGKM?

LT

lenticular galaxies

Lenticular galaxies are disk galaxies with no obvious spiral structure. or spiral arms

Suppose you had looked with your naked eye at the supernova on the night it was photographed in the "after" photo. Assuming that it was bright enough to see (it was), what would it have looked like to your naked eye?

Like a single star in the night sky The size of the blob in the photo is an artifact of the photography; the star is so far away that even as it explodes it would appear only as a point of light and not even as bright as some nearby stars in our night sky.

How do the properties of long-lived stars compare to those of short-lived stars?

Long-lived stars begin their lives with less mass and a smaller amount of hydrogen fuel. Long-lived stars are less luminous during their main-sequence lives. The key point is that longer lifetime goes with less massive stars. Stellar lifetime depends on how much fuel is available for fusion and how rapidly that fuel is burned. More massive stars have more fuel, but they burn through it at far higher rates and hence have shorter lives.

What is the main idea captured by this graph?

Low-mass stars are much more common than higher-mass stars. For example, it shows that for every star with mass above 10 MSun, there are 200 stars with mass less than 0.5 MSun.

What is the key observation needed to determine whether the compact object in Part C is a neutron star or a black hole?

Measure Doppler shifts in the spectrum of the main-sequence star so that you can determine the mass of the compact object. The Doppler shifts will allow you to determine the speed (or at least the component of the speed that is in your line of sight) of the main-sequence star as it orbits the compact object. You can then use this speed along with the known mass of the main-sequence star to determine the compact object's mass (or a lower limit on its mass). If this mass is greater than the neutron star limit of about 3 solar masses, then the object must be a black hole. Provide Feedback

Which of the following statements about a globular cluster is true?

Most stars in the cluster are yellow or reddish in color.

Is a protostar in energy balance? Why or why not?

No, because its core generates less energy than its surface radiates into space. The fact that the core generates less energy than the surface radiates into space means that the protostar remains relatively cool inside, which allows gravity to continue to compress it in size.

The presence of many massive young stars in the spiral arms. Spiral arms are regions of enhanced density in the disk, which means gas tends to clump into star-forming clouds along the spiral arms, and star forming regions are where we expect to find massive, hot, young stars. (These stars also ionize surrounding gas, explaining the many ionization nebulae visible in the spiral arms.)

Notice the well-defined spiral arms in this photograph of the galaxy M51. What makes the spiral arms so much brighter than regions between the arms?

From hottest to coolest, the order of the spectral types of stars is ________

OBAFGKM

The following figures give the approximate speeds at which five galaxies are moving away from Earth due to the expansion of the universe. Rank the galaxies based on the amount of redshift that would be observed in each galaxy's spectrum,

Recall that a redshift tells us that an object is moving away from us, and the larger the redshift, the higher the speed. Note that expansion applies to the whole universe, so astronomers tend to think of galaxies being carried along with the expansion rather than moving "through" the universe. For this reason, redshifts due to expansion are often called "cosmological redshifts." Hubble's law tells us that the more distant a galaxy is from Earth, the faster it is moving away from us.

The following figures show various stages during the life of a star with the same mass as the Sun. Rank the stages based on when they occur, from first to last.

Remember that these stages take very different amounts of time. A one-solar-mass star spends about ten billion years as a hydrogen-burning main-sequence star, making this by far the longest stage of its life.

Which constellation lies in the direction toward the galactic center?

Sagittarius

If you wanted to observe the center of our galaxy, you would need to point a telescope in the direction of the constellation __________.

Sagittarius As discussed in the video, the center of our galaxy lies in the direction of the constellation Sagittarius. That is why the central object includes "Sagittarius" in its name (Sagittarius A*).

Which of the following statements about galaxies is true?

Small galaxies outnumber large galaxies but large galaxies produce most of the light in the universe.

Spiral galaxies Elliptical galaxies

Spiral galaxies have a flattened disk of stars are rare in central regions of galaxy clusters contain many bright, hot stars contain abundant clouds of cool gas and dust have significant, ongoing star formation Elliptical galaxies contain primarily old, low-mass stars are more reddish in color The most fundamental difference between spiral and elliptical galaxies is that spiral galaxies have a disk and elliptical galaxies do not. In other words, spiral galaxies have both a disk and halo while elliptical galaxies are "all halo." Because star formation occurs in a disk, a lack of disk means very little star formation — and hence very few (if any) young, hot stars — in elliptical galaxies. Spiral galaxies are rare in the central regions of clusters because galaxies in those regions tend to have suffered collisions, which either strip out the cool gas and dust needed to make a disk, or induce a burst of star formation that uses up the remaining cool gas.

If stars A and B are both main-sequence stars and star A has a greater fusion rate than star B, which of the following statements hold(s)?

Star A must be more luminous than star B. Star A must be more massive than star B. A more massive main-sequence star has a higher rate of fusion, which makes it more luminous. Provide Feedback

In Part D, you saw that elements with even atomic numbers tend to be more abundant than neighboring elements with odd atomic numbers. What nuclear process explains why this is the case?

Starting from carbon (atomic number is 6), the most common nuclear reactions involve the fusion of an additional helium nucleus. Helium nuclei have two protons—which means it has an atomic number of 2—so fusing a helium nucleus into some other element increases the atomic number by two. Carbon is formed by the fusion of three helium nuclei, which is why carbon has an atomic number of 6. Fusing another helium nucleus to carbon makes oxygen, with an atomic number of 8; fusing a helium nucleus to oxygen makes neon, with an atomic number of 10; and so on. That is why even-numbered elements tend to be more common.

The observational data for the element abundances agree quite well with what we expect based on our current understanding of nuclear fusion and stellar evolution. But imagine the data had turned out to be different. Which of the following differences, if it had actually been observed, would have forced us to rethink our entire picture of stellar evolution? View Available Hint(s)

The abundance of elements heavier than uranium turned out to be greater than the abundance of carbon. We expect carbon to be relatively common because it is formed by the fusion of three helium nuclei, and helium is the second most abundant element. We expect uranium and heavier elements to be quite rare because they form only in rare reactions associated with supernova explosions. Therefore, if elements heavier than uranium turned out to be more common than carbon, we would be forced to conclude that our model of element creation is incorrect.

Listed following are several locations in the Milky Way Galaxy. Rank these locations based on their distance from the center of the Milky Way Galaxy, from farthest to closest. View Available Hint(s)

The central bulge extends out about 5,000 light-years from the galactic center. Our solar system is about 28,000 light-years from the center, which is about halfway through the disk. The halo extends to much greater distances than the 50,000 light-year diameter of the disk, which is why this choice is ranked first.

This figure shows frames from a computer simulation of star formation, starting at the left with a large molecular cloud measuring more than a light-year across. What is happening as time passes (from left to right)?

The cloud is fragmenting into smaller pieces that will form stars.

This figure shows frames from a computer simulation of star formation, starting at the left with a large molecular cloud measuring more than a light-year across. What is happening as time passes (from left to right)?

The cloud is fragmenting into smaller pieces that will form stars. This type of fragmentation is why most clouds give birth to many stars.

What happens after a helium flash?

The core quickly heats up and expands.

Based on observations (both images and spectra), which of the following statements are true about star-forming clouds?

The darkness of these clouds (in visible light) is due primarily to light absorption by tiny grains of interstellar dust. Star-forming clouds have the same overall chemical composition Most of the hydrogen in star-forming clouds is in the form of hydrogen molecules (H2). as the galaxy as a whole. The densest cloud regions appear dark to visible light telescopes but we can see into these regions with Infrared telescopes. Star-forming clouds glow with visible light in regions where the gas is heated by radiation from nearby stars.

Assume that all four H-R diagrams below represent a star in different stages of its life, after it starts to fuse hydrogen in its core. Rank the HR diagrams based on when each stage occurs, from first to last.

The diagram at the left represents the Sun (or any other one-solar-mass star) as a hydrogen-burning main-sequence star, with spectral type G and one solar luminosity. The next diagram shows the Sun after it has exhausted its core hydrogen and left the main sequence, making it a subgiant with energy generated by hydrogen burning in a shell around an inert helium core. The third diagram shows the Sun a little later; its energy source is still hydrogen shell burning, but at this point it has expanded in size so much that it is a red giant. The final diagram (far right) shows the white dwarf corpse of a one-solar-mass star; it is hot because it is the exposed core of the dead star, but dim because it is small in size.

Notice that some of the stars on the diagram are represented by a series of dots that are very close together, while others have their dots farther apart. Keeping in mind that all the stellar positions were measured at approximately one-year intervals, which stars are moving the fastest in their orbits during the time period indicated by the dots?

The fastest stars are the ones with the dots farthest apart

Imagine a photon of light traveling the different paths in the Milky Way described in the following list. Rank the paths based on how much time the photon takes to complete each journey, from longest to shortest.

The light-travel time converts directly to a distance in light-years, so you can think of this question as a simple variation on Part A, but with the added choice of the thickness of the disk (from top to bottom). The correct answer shows that the halo is larger in diameter than the disk, which is larger than the bulge, and that the Sun is within the disk but outside the bulge. The disk is quite thin compared to its diameter (typically only about 1,000 light-years in thickness), which makes it relatively flatter than a typical pancake.

What is the approximate range of masses that newborn main-sequence stars can have?

The lower limit is 0.08 solar masses: Below that, the core is not hot enough to sustain nuclear fusion. The upper limit of around 150 solar masses is as a result of the fact that higher-mass objects would be so luminous that they would blow themselves apart with their own radiation pressure. 0.1 to 150 solar masse

The main photo and the smaller inset both show the same field of view containing a star-forming molecular cloud. One of the photos was taken in visible light and the other in infrared light. Which one is the infrared photo, and how do you know?

The main (larger) photo is the infrared because molecular clouds emit much more infrared light than visible light. Molecular clouds are too cool to emit visible light. Notice that the bright region in the large, infrared image is the dark region in the smaller, visible inset.

Why is there an upper limit to the mass of a white dwarf?

The more massive the white dwarf, the greater the degeneracy pressure and the faster the speeds of its electrons. Near 1.4 solar masses, the speeds of the electrons approach the speed of light, so more mass cannot be added without breaking the degeneracy pressure.

What event brings the forming star into energy balance?

The onset of nuclear fusion in the core The onset of nuclear fusion brings the star into energy balance and marks the beginning of its long, main-sequence lifetime.

This famous image from the Hubble Space Telescope shows what is sometimes called the "pillars of creation." Which of the following best describes what it shows?

The pillars are clouds of gas and dust in which many new stars are forming; the edges of the pillars are sculpted by ultraviolet radiation from stars outside the pillars The ultraviolet radiation heats and erodes the dark gas, giving the star-forming clouds of the pillars their distinctive shape.

This famous image from the Hubble Space Telescope shows what is sometimes called the "pillars of creation." Which of the following best describes what it shows?

The pillars are clouds of gas and dust in which many new stars are forming; the edges of the pillars are sculpted by ultraviolet radiation from stars outside the pillars. he ultraviolet radiation heats and erodes the dark gas, giving the star-forming clouds of the pillars their distinctive shape

What happens to the rotation of a molecular cloud as it collapses to form a star?

The rotation rate increases and results in a flattened disk of material around a protostar.

Why do some of the bright stars (such as the one indicated by the arrow) in this photo have cross-shaped spikes over them?

The spikes are an artifact of photography through a telescope. That's right; the spikes are not real, just artifacts of the observing process

Astronomers can measure a star's mass in only certain cases. Which one of the following cases might allow astronomers to measure a star's mass?

The star is a member of a binary star system.

Astronomers can measure a star's mass in only certain cases. Which one of the following cases might allow astronomers to measure a star's mass?

The star is a member of a binary star system. If we can measure the orbital properties of the star around its companion, then we can measure the mass with Newton's version of Kepler's third law.

Suppose a particular star has a core that is undergoing several stages of fusion simultaneously, as shown in this diagram. Which of the following statements about this star is most likely to be true?

The star will explode as a supernova within a few days. Once it has an iron core, the star is indeed in its final days (or less).

What can we learn about a star from a life track on an H-R diagram?

The surface temperature and luminosity the star will have at each stage of its life.

Infrared light, because the dust that appears dark in the visible light photo glows in infrared light. Notice how the dark areas in the visible photo match up closely to the bright areas in the infrared photo.

The top panorama shows our view of the Milky Way in all directions as it appears in visible light. The bottom panorama shows the same view, but in a different wavelength of light. What wavelength band are we seeing in the bottom photo, and how do you know?

The following figures show several stars found in the disk and halo of the Milky Way Galaxy. Rank the stars based on their current age, from oldest to youngest. If two (or more) stars have approximately the same age (that is, ages within a few million years), rank them as equal by dragging one on top of the other(s).

The two globular cluster stars have the same age, because all stars in a cluster form at about the same time. These are also the oldest stars, because globular clusters usually contain the oldest stars in the universe. The Sun is next, with its age of about 4 1/2 billion years. The hot, blue main-sequence star must be no more than a few million years old, because these stars do not live much longer than that.

Suppose that a white dwarf gains enough mass to reach the 1.4 solar-mass limit? View Available Hint(s)

The white dwarf will explode completely as a white dwarf supernova.

Based on the diagram, which of the following statements best describes the observed pattern of abundances for elements with an atomic number between 6 and 20?

There is a general trend of decreasing abundance with increasing atomic number, but elements with even atomic numbers tend to be more abundant than those with odd atomic numbers. The even-numbered peaks are higher than their neighboring points, for reasons that we will explore in the remaining questions.

You've now found that the central object has a mass of about 4 million solar masses but is no more than about 70 AU in diameter—which means it cannot be much larger than the size of our planetary system. Why do these facts lead astronomers to conclude that the central object is a black hole?

There is no known way to pack so much mass into such a small volume without it collapsing into a black hole. Even if the mass were not a black hole originally, it would have quickly collapsed into one, which is why we conclude that there must be a black hole there now.

Consider the four features of spiral arms that you identified in Part A. What do they all have in common that explains why they are found together in spiral arms?

They are all associated with star formation. Dense, dusty gas clouds give birth to stars, which is why we find young stars in them. Massive stars (spectral types O and B) have short lifetimes, and therefore are always young. And as you saw in Part B, ionization nebulae are ionized by the ultraviolet light of hot, young stars, which means they must also be in the regions in which star formation is occurring.

why are they called molocular clouds

They are called molecular clouds because they are cold enough to allow their hydrogen atoms to pair up into hydrogen molecules (H2).

How are magnetic fields thought to affect star formation in molecular clouds?

They can help resist gravity, so that more total mass is needed before the cloud can collapse to form stars. The magnetic fields that thread clouds inhibit the movement of gas, thereby adding to the resistance to gravity provided by thermal pressure (and turbulent gas motions).

An H-R diagram for a globular cluster will show a horizontal branch—a line of stars above the main sequence but to the left of the subgiants and red giants. Which of the following statements about these horizontal branch stars is true?

They generate energy through both hydrogen fusion and helium fusion.

Which of the following statements is probably true about the very first stars in the universe?

They were made only from hydrogen and helium.

Which of the following statements is probably true about the first stars in the universe?

They were made only from hydrogen and helium. Most of the heavier elements were created in stars and thus did not exist when the first stars formed.

1. The Sun is located in the disk about halfway from the center of the galaxy to the disk's edge, which corresponds to position 1.

This diagram represents an edge-on view of our Milky Way Galaxy. Of the four labeled stars, which one could represent the Sun's position in the galaxy?

2. The orbits of stars near the galactic center, along with radio and X-ray emissions from surrounding gas, provide strong evidence for the presence of such a massive black hole.

This diagram represents an edge-on view of our Milky Way Galaxy. Of the four labeled stars, which one is located closest to a place where evidence suggests we would find a 4 million solar-mass black hole?

4 The halo is the spherical region surrounding the entire disk of the galaxy.

This diagram represents an edge-on view of our Milky Way Galaxy. Of the four labeled stars, which one is located in what we call the halo of the galaxy?

its mass We can use the orbits to calculate the mass from Newton's version of Kepler's third law (or from the equivalent orbital velocity law).

This figure shows observations over more than a decade that have allowed us to determine the orbits of several stars around Sgr A* (the central object). What can we learn about Sgr A* by analyzing these orbits?

Hot gas expanding outward from the site of a supernova. Much of the gas has temperatures of 10-20 million K, which is why it emits X rays.

This photograph shows X-ray emission from a supernova remnant. What is the source of the X-rays?

The bubble will be slightly larger. This is true because the bubble is essentially a sphere of expanding gas, so it will grow larger with time until it dissipates into the surrounding interstellar medium. In 100 years, it will have grown noticeably but will still be intact.

This photograph shows an interstellar bubble about 10 light-years in diameter. If you could photograph this same region about 100 years from now, how would you expect it to look different?

the law of conservation of angular momentum The angular momentum of the cloud stays the same at all times, so like a spinning skater pulling in her arms, the galaxy must rotate faster as it shrinks in size.

This sequence of paintings represents the formation of the Milky Way Galaxy. What law of nature explains why the galaxy began to rotate rapidly and flatten out as it shrunk in size?

a distant star will have

To summarize: Tthe greater a star's distance, the smaller its parallax angle.

This figure shows the life track of a 1 solar-mass star from its beginnings in a collapsing cloud fragment until it becomes a main-sequence star. At which of the numbered points is the object generating energy primarily from gravitational contraction?

Until it reaches the main-sequence, gravitational contraction is its only source of energy. 1,2,3

Estimate the volume of a piece of molecular cloud that has the same amount of water as your body.

Vcloud= 1.0×1031 cm3

How does this volume compare with the volume of the entire Earth?

Vcloud= 9200 VEarth

Suppose that Cepheids did not exist and there were no other standard candle technique that worked at the same distances. Which statement would be true? View Available Hint(s)

We would not be able to measure the distances of distant galaxies. Correct Cepheids provide a key link in the cosmic distance chain. Cepheids are used to measure the distances of nearby galaxies in which white dwarf supernovae have been detected, which allows us to determine the luminosities of these supernovae. White dwarf supernovae, in turn, allow us to calibrate Hubble's law. Therefore, without Cepheids, we would not be able to use either white dwarf supernovae or Hubble's law, and we would be unable to measure the distances of distant galaxies.

Cold, dense molecular clouds in which stars are forming. They are dark because they are dense and cool and contain interstellar dust that absorbs visible light.

What are the dark blobs (indicated by the red arrows) in this photograph from the Hubble Space Telescope?

What is the most accurate way to determine the distance to a nearby star? stellar parallax

What is the most accurate way to determine the distance to a nearby galaxy? using Cepheid variables

brown dwarf

Which of the following statements about brown dwarfs is not true? Brown dwarfs form like ordinary stars but are too small to sustain nuclear fusion in their cores. All brown dwarfs have masses less than about 8% that of our Sun. Brown dwarfs are supported against gravity by degeneracy pressure, which does not depend on the object's temperature.

About how many galaxies are there in a typical cluster of galaxies? a few dozen

Which of these galaxies is most likely to be oldest? a galaxy in the Local Group

white dwarf vs massive supernovas

White dwarf supernova Spectra always lack strong hydrogen lines.Can occur in a very old star cluster Can only occur in a binary system Star explodes completely, leaving no compact object behind.Has a brighter peak luminosity A white dwarf supernova occurs when the white dwarf gains enough mass for the carbon interior of the star to begin carbon fusion. The fusion begins almost instantly throughout the star, so the entire star ignites and the white dwarf explodes completely. Massive star supernova Black hole or neutron star left behind Can only occur in a galaxy with ongoing star formation

Approximately what is the parallax angle of a star that is 20 light-years away?

You can confirm this answer by using the animation "Parallax and Distance" in the Interactive Figure. Move the distance slider until distance shown is 20 light-years, then read the corresponding parallax angle in arcseconds. Alternatively, you can use the parallax formula d(in light-years)=3.26×1p(in arcseconds) or p(in arcseconds)=3.26×1d(in light-years) Using a distance of d = 20 light-years, this formula gives the parallax angle to be p=3.26/20=0.163 arcsecond.

Which of the following represents key observational evidence that stars are born in clouds of gas and dust?

Young star clusters are always associated with clouds of gas and dust. We can determine star cluster ages from their main-sequence turnoff points, and young clusters are always associated with gas and dust, indicating that this is leftover material from their recent formation.

Study this H-R diagram. What is the spectral type of the star Sirius? (The red arrow helps you locate Sirius on the diagram.)

a Spectral type is shown along the horizontal axis of the H-R diagram, and you can see that Sirius has spectral type A.

What kind of object do we think lies in the center of the Milky Way?

a black hole of about 4 million solar masses The black hole's existence is inferred from the orbits of stars near the galactic center, which tell us that an approximately 4 million solar-mass object lies there but is so small in size that it can only be black hole. X rays from this region presumably come from an accretion disk around the black hole.

What do we mean by a protogalactic cloud?

a cloud of hydrogen and helium that contracts to become a galaxy Just as a protostar is a star that is still in the process of forming, a protogalactic cloud is a galaxy that is still in the process of forming.

What kind of gas cloud is most likely to give birth to stars?

a cold, dense gas cloud This type of cloud has lower thermal pressure (because of the low temperature) and stronger gravity (because of the high density), giving gravity the upper hand

What kind of gas cloud is most likely to give birth to stars?

a cold, dense gas cloud This type of cloud has lower thermal pressure (because of the low temperature) and stronger gravity (because of the high density), giving gravity the upper hand.

What is an ionization nebula?

a colorful cloud of gas that glows because it is heated by light from nearby hot stars

What is an accretion disk? View Available Hint(s)

a disk of hot gas swirling rapidly around a white dwarf, neutron star, or black hole

a standard candle

a light source of known luminosity

What is a carbon star?

a red giant star whose atmosphere becomes carbon-rich through convection from the core

What is a protostar?

a star that is still in the process of forming It generates energy by gravitational contraction because it is not yet hot enough for nuclear fusion it its core.

The dark area stretching from the center of this picture to the upper right is about 50 light-years long and lies in the plane of the Milky Way Galaxy. What is it?

a star-forming cloud It is dark because it contains cool, molecular gas, which is the type of gas cloud in which stars can form.

What is the CNO cycle?

a type of hydrogen fusion that uses carbon, nitrogen, and oxygen atoms as catalysts

Which of these stars has the hottest core?

a white main-sequence star

ch. 18 The maximum mass of a white dwarf is _________. View Available Hint(s)

about 1.4 times the mass of our Sun white dwarf limit

Suppose a particular star has a core that is undergoing several stages of fusion simultaneously, as shown in this diagram. Based on your understanding of stellar lives, the mass of this star is probably ____________.

about 10- (or more) solar masses Only high-mass stars go through these many stages of fusion.

What do astronomers consider heavy elements?

all elements besides hydrogen and helium

According to our modern understanding, what is a nova? View Available Hint(s)

an explosion on the surface of a white dwarf in a close binary system It occurs when fresh hydrogen from the accretion disk piles up on the surface.

Which of the following does not accurately describe what we observe toward the Galactic center?

at optical wavelengths, we see a cluster of old, red stars

What is the most common form of gas in the interstellar medium?

atomic hydrogen

Which of the following statements about the disk of the Milky Way is false? a.The average age of disk stars is less than that of halo stars. b. Disk stars are all younger than 5 billion years. c. Disk stars orbit in the same direction around the Galactic center. d. Disk stars have a higher proportion of heavy elements, on average, than halo stars. e. The length of the disk is about 100 times its thickness.

b. Disk stars are all younger than 5 billion years.

The ultimate fate of our Sun is to _____.

become a white dwarf that will slowly cool with time

According to current understanding, the two most abundant elements in the universe were made __________.

big bang Hydrogen and most of the helium in the universe formed during the first 5 minutes of the universe's birth in the Big Bang.

How do we know the total mass of milky way that is contained within the sun's orbital path?

by applying Newton's version of Kepler's 3rd law to the sun's orbit around the center of the galaxy The orbital velocity formula allows us to calculate mass from orbital velocity and distance; remember that it tells us only the mass contained within the Sun's orbital path. To measure the mass within a larger region of the galaxy, we must apply the formula to the orbit of a more distant star or gas cloud.

How can we see through the interstellar medium?

by observing in high-energy wavelengths such as X rays and long wavelengths of light such as radio waves

On an H-R diagram, stellar masses _________.

can be determined for main sequence stars but not for other types of stars Along the main sequence, mass decreases from upper left to lower right; but there is no clear pattern to masses for stars that are not on the main sequence.

What law explains why a collapsing cloud usually forms a protostellar disk around a protostar?

conservation of angular momentum A protostellar disk forms as a consequence of conservation of angular momentum, which makes the cloud rotate faster as it shrinks in size.

What do we mean by the star-gas-star cycle?

continuous recycling of gas in galactic disk between stars and interstellar medium This cycle is critically important to our existence because it explains how the elements manufactured in past generations of stars. which are the elements of which we and our planet are made, came to be incorporated into star systems that formed later, including our own solar system.

Compared to the star it evolved from, a red giant is

cooler and brighter.

Which of the following statements about brown dwarfs is not true? View Available Hint(s) a. Brown dwarfs are supported against gravity by degeneracy pressure, which does not depend on the object's temperature. b. Brown dwarfs form like ordinary stars but are too small to sustain nuclear fusion in their cores. c. All brown dwarfs have masses less than about 8% that of our Sun. k

d. Brown dwarfs eventually collapse to become white dwarfs. The terms brown dwarf and white dwarf may sound like they should be related, but they are not. Brown dwarfs will never collapse into white dwarfs. Provide Feedback

Which of the following statements about globular clusters is false? a. Globular cluster stars are very metal-poor relative to the Sun. b. Globular cluster stars are more than 12 billion years old. c. Globular clusters contain many thousands of stars. d. Globular cluster ages increase with distance from the Milky Way. e. Globular clusters are distributed spherically around the Milky Way.

d. Globular cluster ages increase with distance from the Milky Way.

The gravitational force in a molecular cloud depends on

density only

How does diameter of disk of milky way galaxy compare to its thickness?

diameter is about 100 times as great as thickness The diameter is about 100,000 light-years and the thickness about 1,000 light-years. That is why the disk appears so thin.

Suppose you look at a star that can be seen through the edge of a dusty interstellar cloud. The star will look _______ than it would if it were outside the cloud.

dimmer and redder It is dimmer because the dust absorbs some of the star's light, and redder because dust scatters blue light more effectively than red light.

Star formation occurs primarily in the galaxy's ________.

disk Gas resides in the disk, so that is where star formation can occur.

Which of the following statements about halo stars is false? a. Halo stars are some of the oldest known objects in the universe. b. Halo stars have random orbits about the Milky Way center. c. All halo stars are less massive than our Sun. d. Halo stars are no longer being formed at the current epoch. e. Halo stars are made entirely of hydrogen and helium with no heavy elements.

e. Halo stars are made entirely of hydrogen and helium with no heavy elements.

Notice that nitrogen is less abundant than either carbon or oxygen. This is an example of the more general observation that __________.

elements with even atomic numbers are more abundant than the elements with odd atomic numbers that come between them As shown in the video, carbon is atomic number 6, nitrogen is atomic number 7, and oxygen is atomic number 8. The graph shows that the abundances of these three elements follow a general pattern in which even-numbered elements are more common than the odd-numbered elements that come between them. This pattern is a result of the way fusion occurs in high-mass stars near the ends of their lives.

low mass star

end life as a planetary nebula the Sun is an example final corpse is a white dwarf have longer lifetimes A long-lived star such as the Sun eventually ejects its outer layers as a planetary nebula, leaving behind a white dwarf.

main sequence

energy generated by gravitational contraction pressure and gravity are not precisely balanced luminosity much greater than the Sun radius much larger than the Sun

All stars that become supernovae will leave behind a neutron star.

false

It is impossible to know whether our Sun will end its life as a white dwarf or neutron star.

false

Jupiter is a failed star.

false

Stars with high masses live longer than stars with lower masses.

false

The Milky Way looks the same in X rays as it does at infrared wavelengths.

false

The most massive stars generate energy at the end of their lives by fusing iron in their cores.

false

The star-gas-star cycle will continue forever because stars are continually recycling gas.

false

There is no limit to the mass with which a star can be born.

false

We can see most of the galaxy with visible light

false

In order to understand star clusters, we need to be able to estimate their ages. What technique do scientists use for this?

finding the main-sequence turnoff point of the stars

What is the range of timescales for star formation?

from 1 million years for the most massive stars up to 100 million years for the least massive stars

time scale star formation

from 1 million years for the most massive stars up to 100 million years for the least massive stars

In this diagram, red balls represent protons and gray balls represent neutrons. What reaction is being shown?

fusion of helium into carbon Three helium nuclei fuse to make one carbon nucleus, releasing energy in the process.

What makes up the interstellar medium?

gas and dust

What kinds of objects lie in the disk of our galaxy?

gas and dust old K and M stars open clusters O and B stars

What do we mean by interstellar medium?

gas and dust that lies in between stars in the milky way In other words, it's the stuff that occupies the spaces between stars.

Which of the following comprise the oldest members of the Milky Way?

global clusters

As the video shows, the star begins its life from a clump of gas that heats up as it contracts. Where does the energy that heats the cloud come from?

gravitational potential energy that the cloud loses as it shrinks in size Gravity causes the cloud to shrink in size, which reduces its gravitational potential energy. This energy is converted to thermal energy of the cloud particles, which is why the cloud heats up.

The basic requirement for a cloud to collapse to form a star is that __________.

gravity must be strong enough to overpower the cloud's internal pressure The cloud will continue to collapse, ultimately forming a star, as long as gravity is strong enough to overcome the cloud's internal pressure.

Based on the association of star formation with spiral arms, we can conclude that the gas in spiral arms __________ than it does in regions between the arms.

has greater density Spiral arms are associated with star formation because they are regions of enhanced gas density, which allows gravity to gain the upper hand and lead to star birth.

What evidence do we have that the halo population of stars are older than other stars in the galaxy? T

hey have a smaller proportion of heavy elements.

Compared to the star it evolved from, a white dwarf is

hotter and dimmer

As a clump of interstellar gas contracts to become a main-sequence star, its changing position on the H-R diagram tells us __________. View Available Hint(s)

how its outward appearance is changing An object's position on the H-R diagram tells us its surface temperature and luminosity, which are essentially its outward appearance.

Which molecule is the most abundant in molecular clouds?

hydrogen

To calculate the dashed orbits from the stellar positions, astronomers had to assume that __________.

if they observed for many more years, the dots would trace out ellipses This is simply a consequence of Kepler's first law, which tells us that bound orbits are always ellipses.

The sun's location in the Milky way is...

in the galactic disk, roughly halfway between center and outer edge of the disk The Sun lies about 27,000 light-years from the center of the galaxy, which is just over half the roughly 50,000 light-year radius of the disk.

Where does most star formation occur in the Milky Way Galaxy?

in the spiral arms

Where does most star formation occur in the Milky Way?

in the spiral arms The enhanced density of gas in the spiral arms leads to star formation.

Which part of the electromagnetic spectrum generally gives us our best views of stars forming in dusty clouds?

infrared

Which part of the electromagnetic spectrum generally gives us our best views of stars forming in dusty clouds?

infrared Infrared light can pass through dusty clouds, unlike visible light.

The image of our galaxy in radio emission from CO, mapping the distribution of molecular clouds, is closest to the image of our galaxy in

infrared emission from interstellar dust grains.

Which of the following phenomena is not commonly associated with the star-formation process?

intense ultraviolet radiation coming from a protostar Protostars are too cool to emit much ultraviolet light.

Star-forming clouds appear dark in visible-light photos because the light of stars behind them is absorbed by _________

interstellar dust Although dust makes up only a small fraction of a star-forming cloud's mass, it is responsible for absorbing the visible light of stars behind the cloud.

Which element has the lowest mass per nuclear particle and therefore cannot release energy by either fusion or fission?

iron

According to the diagram, what is the most abundant element with an atomic number greater than or equal to 20?

iron In this question you were asked to consider only data points at or to the right of atomic number 20 along the horizontal axis. In this region, the data point for iron is the highest on the graph, which means it has the greatest relative abundance. Note that the iron abundance is only a little more than 10−6=1/1,000,000 that of hydrogen. In other words, for every atom of iron in the Milky Way Galaxy, there are nearly 1 million atoms of hydrogen.

On a graph plotting galaxy luminosities against galaxy colors, the red sequence represents galaxies that are _________ than galaxies of the blue cloud.

larger and more ellipitical The red sequence galaxies lack active star formation and therefore are elliptical, and they also tend to be larger than bluer galaxies.

If star A is closer to us than star B, then Star A's parallax angle is _________.

larger than that of Star B A larger parallax angle means the star is closer.

high mass stars

late in life fuse carbon into heavier elements have higher fusion rate during main sequence life end life as a supernova

Compared to a main-sequence star with a short lifetime, a main-sequence star with a long lifetime is _____

less luminous, cooler, smaller, and less massive Note that there are physical reasons why long-lived stars have these properties: They are less luminous because they burn their fuel at a much lower rate than short-lived stars; they burn it at this lower rate because they are less massive (which means less compression and hence a lower fusion rate in their cores); and their lower masses lead to their smaller sizes and lower surface temperatures.

The vast majority of stars in a newly formed star cluster are ______.

less massive than the Sun In general, the lower the mass, the more common the star.

What type of star is our Sun?

low mass star

The total amount of power (in watts, for example) that a star radiates into space is called its _________. View Available Hint(s)

luminsity

Which kinds of stars are most common in a newly formed star cluster?

m star

protostar phase energy generated by gravitational contraction pressure and gravity are not precisely balanced luminosity much greater than the Sun radius much larger than the Sun

main-sequence phase lasts about 10 billion years surface radiates energy at same rate that core generates energy energy generated by nuclear fusion

Which of the following are you more likely to find within (or on the edges of) spiral arms of a spiral galaxy than in between these arms?

massive stars (spectral types O and B) ionization nebulae dense, dusty gas clouds young stars

We can always determine the recession velocity of a galaxy (at least in principle) from its redshift. But before we can use Hubble's law, we must first calibrate it by __________. View Available Hint(s)

measuring the distances to many distant galaxies with a standard candle technique We need distant standard candles, such as white dwarf supernovae, to measure the distances of galaxies so that we can plot them on a graph of recession velocity (which we can always measure from redshift) versus distance. It is from this graph that we learn Hubble's law and find the value of Hubble's constant.

Interstellar dust consists mostly of _____.

microscopic particles of carbon and silicon

Interstellar dust consists mostly of _____.

microscopic particles of carbon and silicon These tiny, solid particles are important, even though their total mass makes up less than 1% of the interstellar medium.

Calculations show that gravity begins to overcome thermal pressure in clouds that are

more massive than a hundred times the Sun.

A main-sequence star twice as massive as the Sun would last __________.

much less than half as long as the Sun Notice (with the interactive figure) that a main-sequence star with twice the mass of the Sun has a luminosity close to ten times that of the Sun — which means it burns through its hydrogen fuel at a rate ten times faster than the Sun's. As a result, even though it starts with twice as much fuel, its lifetime is much less than half the Sun's lifetime.

From Part E you know the mass of the central object. Now consider its size. Based on what you can see in the diagram, you can conclude that the diameter of the central mass is __________.

no more than about 70 AU Notice that some of the star orbits come within about 100 AU of the central object, so the central object must be smaller than this in size.

The Schwarzschild radius of a black hole depends on ________. View Available Hint(s)

only the mass of the black hole The greater the mass, the larger the Schwarzschild radius.

The diagram indicates that the third most abundant element in the Milky Way Galaxy is _____.

oxygen The vertical axis represents relative abundance, so the fact that oxygen has the third-highest point on the diagram (after hydrogen and helium) means that it is the third most abundant element.

According to our modern understanding of the origin of elements, why are hydrogen and helium so much more abundant than any other elements?

produced in big bang A great deal of evidence indicates that hydrogen and helium are much more abundant than other elements because they were produced in the Big Bang, while virtually all other elements were produced later in stars.

Which of the following sequences correctly describes the stages of life for a low-mass star?

protostar, main-sequence, red giant, white dwarf

Which kind of pressure prevents stars of extremely large mass from forming?

radiation pressure

The primary way that we observe the atomic hydrogen that makes up most of the interstellar gas in the Milky Way is with ____.

radio telescopes observing at a wavelength of 21 cm Radio emission in the 21 cm line is the only significant emission from most atomic hydrogen gas.

Astronomers are seeking to obtain an image of the region around the black hole's event horizon with a project called the Event Horizon Telescope. What type of light does this project seek to observe?

radio waves The Event Horizon Telescope links radio telescopes around the world together with the technology of interferometry in order to obtain much higher resolution than could be obtained by any single telescope.

Pulsars are thought to be _________. View Available Hint(s)

rapidly rotating neutron stars The pulses occur as their magnetic fields sweep by us with each rotation.

Which low-mass star does NOT have fusion occurring in its central core?

red giant

Which of these colors of light passes most easily through interstellar clouds?

red light

spiral arms are

regions of active star formation Spiral arms are marked by clusters of young, blue stars, dark molecular clouds, and ionization nebulae — all of which are associated with star formation.

If the distance between us and a star is doubled, with everything else remaining the same, the luminosity

remains the same, but the apparent brightness is decreased by a factor of four.

When the core of a star like the Sun uses up its supply of hydrogen for fusion, the core begins to ________. View Available Hint(s)

shrink and heat As explained in the video, the core starts to shrink because hydrogen fusion can no longer replace the energy that is flowing toward the surface, and it heats up because the shrinkage means gravitational potential energy is transformed into thermal energy.

Which of the following terms is given to a pair of stars that we can determine are orbiting each other only by measuring their periodic Doppler shifts?

spectroscopic binary We measure Doppler shifts by looking for shifts in the wavelengths of spectral lines.

Which types of galaxies have a clearly defined disk component? an most largest galaxy?

spirals and lenticulars

Star A has an apparent magnitude of three and star B has an apparent magnitude of five. Which star is brighter in our sky?

star a The magnitude scale is "backward" so that smaller numbers mean greater apparent brightness.

The dark area stretching from the center of this picture to the upper right is about 50 light-years long and lies in the plane of the Milky Way Galaxy. What is it? The figure shows an image of starry sky with an oblong area with much lesser star density in it, compared to the surrounding space.

star forming cloud. It is dark because it contains cool, molecular gas, which is the type of gas cloud in which stars can form. admitted more infrared light Molecular clouds are too cool to emit visible light

radi of stars

stellar radii increase diagonally from the lower left to the upper right

Degenearcy pressure arises when ________. View Available Hint(s)

subatomic particles are packed as tightly as the laws of quantum mechanics allow That is why, unlike ordinary (thermal) pressure, degeneracy pressure does not depend on temperature.

What are cosmic rays?

subatomic particles that travel close to the speed of light Their precise origin is still mysterious, but they are likely produced by supernovae.

When we see X rays from an accretion disk in a binary system, we can't immediately tell whether the accretion disk surrounds a neutron star or a black hole. Suppose we then observe each of the following phenomena in this system. Which one would force us to immediately rule out the possibility of a black hole?

sudden intense xray burst

Overall, careful study of the patterns revealed in the graph of measured element abundances has allowed scientists to __________.

test and validate models of how elements are produced by stars Scientists use models of fusion in stars (and of supernova explosions) to predict the expected patterns in the abundances of elements. They then test the models by seeing if their predictions agree with observations. The fact that current models do agree with the observations gives scientists confidence in the models.

What do astronomers mean when they say that we are all "star stuff"?

that the carbon, oxygen, and many elements essential to life were created by nucleosynthesis in stellar cores

The interstellar clouds called molecular clouds are _______.

the cool clouds in which stars form

The interstellar clouds called molecular clouds are _______.

the cool clouds in which stars form They are called molecular clouds because they are cold enough to allow their hydrogen atoms to pair up into hydrogen molecules (H2).

Part complete Hubble's law expresses a relationship between __________. View Available Hint(s)

the distance of a galaxy and the speed at which it is moving away from us Hubble's law is a relationship between galaxy distances and the speeds at which they are moving away from us due to the expansion of the universe, also known as their recession velocities. Other factors also affect total galaxy speeds, such as the gravitational influence of neighboring galaxies, so Hubble's law works well only when these other factors are small compared to the speed due to expansion. This is why Hubble's law works well only for very distant galaxies: Their recession speeds are so great that any motions caused by neighboring galaxies are tiny in comparison.

What is a planetary nebula?

the expanding shell of gas that is no longer gravitationally held to the remnant of a low-mass star

What do we call the bright, sphere-shaped region of stars that occupies the central few thousand light-years of the milky way?

the galaxy's bulge The central bulge is even visible to the naked eye because it makes the Milky Way in the night sky wider in the direction of the galactic center (toward Sagittarius).

What do we mean by the interstellar medium?

the gas and dust that lies in between the stars in the Milky Way galaxy In other words, it's the stuff that occupies the spaces between stars.

Which part of the galaxy has gas with the hottest average temperature?

the halo

Among main sequence stars, those with the highest surface temperatures have

the highest masses and shortest lifetimes Higher mass main-sequence stars are hotter and more luminous than lower mass stars. They are so much more luminous that they have shorter lifetimes than lower mass stars, because their high luminosities mean they use up their hydrogen fuel at a very high rate.

Based on observations, what statement about stars in the Milky Way is generally true?

the older the star, the lower its abundance of heavy elements This observed fact helped astronomers realize that heavy elements must be manufactured by stars.

Suppose that we look at a photograph of many galaxies. Assuming that all galaxies formed at about the same time, which galaxy in the picture is the youngest?

the one that is farthest away

We measure the mass of the black hole at the galactic center from:

the orbits of stars in the galactic center.

Which model best explains why our galaxy has spiral arms?

the spiral arms are a wave of star formation caused by wave of density propagating outward through the disk of the galaxy This wave is called a spiral density wave. The pattern that we see as the spiral arms remains essentially stationary relative to the galactic disk, whereas stars and gas clouds orbit through them. The gas and stars slow as they move through the arms (which are regions of enhanced gravity as a result of a higher gas density), so we see a "pile up" of stars and gas outlining the spiral pattern.

The main illustration in the video shows the life track of a one-solar mass star. Each point along this track represents __________.

the star's surface temperature and luminosity at one time in its life

What can we learn about a star from a life track on an H-R diagram?

the surface temperature and luminosity the star will have at each stage of its life A star's particular life track depends only on its mass.

The fundamental lesson that we learn from the life track is that the star's outward appearance at any moment in its life reflects __________.

the way it is generating energy in its core The method of core energy generation changes at different stages of the star's life, and these changes affect the star's surface temperature and luminosity, which determine its outward appearance.

What characteristics of the orbiting stars do we need to measure in order to calculate the mass of the central object, Sgr A*?

their orbital periods and average orbital distances When a central object is much more massive than the objects that orbit it (as is the case for Sgr A* compared to the orbiting stars), we can use Kepler's third law (or Newton's version of Kepler's third law) to calculate the mass of the central object from the orbital periods and distances of orbiting objects.

Most interstellar clouds remain stable in size because the force of gravity is opposed by _______ within the cloud.

thermal pressure

Most interstellar clouds remain stable in size because the force of gravity is opposed by _______ within the cloud.

thermal pressure Thermal pressure is the pressure resulting from the thermal motions of the particles in the cloud.

Why does it make sense for star-forming clouds to be cold and dense?

these conditions help gravity overcome gas pressure As discussed in the video, gravity can make a cloud collapse only when it is stronger than the gas pressure. Dense gas tends to mean stronger gravity while cold gas tends to mean weaker pressure, so cold and dense clouds are more prone to gravitational collapse.

Why does it make sense for star-forming clouds to be cold and dense?

these conditions help gravity overcome gas pressure these conditions help gravity overcome gas pressure

How do disk stars orbit the center of galaxy?

they all orbit in roughly the same plane and same direction They also tend to bob up and down as they orbit, but this motion still keeps them within the disk and hence in roughly the same plane. Note that without this organized orbital pattern, the stars wouldn't actually form a disk.

What do halo stars do differently from disk stars?

they orbit the galactic center with many different inclinations, while disk stars all orbit in nearly the same plane. These differing orbital patterns explain why the stars form two distinct populations.

A star's luminosity is the

total amount of light that the star radiates each second.

Clouds that appear dark in visible light often glow when observed at long infrared wavelengths.

true

If the distance between us and a star is doubled, the apparent brightness is decreased by a factor of four.

true

In any star cluster, stars with lower masses greatly outnumber those with higher masses.

true

Molecular clouds appear more transparent at longer wavelengths.

true

Most stars are born in clusters containing thousands of stars.

true

Our Sun will end its life in a planetary nebula and become a white dwarf.

true

Photographs of many young stars show long jets of material apparently being ejected from their poles.

true

Protostars start off more luminous than the main sequence stars they become.

true

Which of these elements had to be made in a supernova explosion?

uranium

What two quantities did Edwin Hubble plot against each other to discover the expansion of the Universe? hubble law Hubble's "constant" is constant in space.

velocity and distance The recession velocity of a galaxy is directly proportional to its distance from us. The larger the value of Hubble's constant, the more rapid the expansion of the universe and hence the younger the universe.

Most stars in the Milky Way's halo are....

very old There is no recycling of gas in the halo, so halo stars are quite old.

If you were to take a voyage across the Milky Way, what kind of material would you spend most of your time in?

warm, rarefied clouds of atomic hydrogen

When is thermal energy trapped in the dense center of a cloud?

when excited molecules collide with other molecules before they can release a photon

When does a newly forming star have the greatest luminosity? View Available Hint(s)

when it is a shrinking protostar with no internal fusion If you watch the position of the red dot on the H-R diagram as the video plays, you will see that the dot is highest — meaning the object is most luminous— when it is a protostar and therefore does not yet have internal fusion. This fact can be a little surprising, but do not forget that luminosity depends on both surface temperature and size. Protostars are always much larger than the main-sequence stars they will eventually become, which is why they can be so luminous even though they are still cool.

When does a protostar become a true star?

when nuclear fusion begins in the core

pro star becomes main sequence when

when the rate of hydrogen fusion becomes high enough to balance the rate at which the star radiates energy into space

When does a protostar become a main-sequence star?

when the rate of hydrogen fusion becomes high enough to balance the rate at which the star radiates energy into space This event marks what we consider to be the birth of the star.

What would you be most likely to find if you returned to the solar system in 10.0 billion years?

white dwarf

This Hubble Space Telescope photo shows a planetary nebula. What is the white dot in the center (indicated by the arrow)?

white dwarf The stars that eject planetary nebulae always leave a white dwarf behind.