ASTR Ch. 11 HW

The patterns on the graph of galaxy colors and luminosities allow scientists to make inferences about galaxy evolution. Which of the following statements accurately reflect current scientific thinking about galaxy evolution based on these data?

-All galaxies start their lives as members of the blue cloud. -Very large galaxies tend to use up the gas available for star formation more rapidly than smaller galaxies. -Some large red galaxies arose from mergers of smaller blue galaxies.

Click the icon "Spiral arm and star motion" in the interactive figure and watch the animation of the galaxy's rotation over several hundred million years. Which of the following statements accurately describe the motion?

-Individual stars move in and out of spiral arms over time. -Individual stars orbit around the center of the galaxy.

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?

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

You discover a new cluster of galaxies, and the brightest galaxy in this new cluster has an infrared color ratio of 0.65. Based on the data in the graph, what is the approximate distance to this new cluster?

1 billion light-years

Suppose a collision strips gas out of a spiral galaxy. Why would this tend to change the spiral galaxy into an elliptical galaxy?

A galaxy cannot have a disk if it does not have gas.

Why do astronomers hypothesize that a massive black hole lies at the center of M87?

A very small region at the center of M87 releases an enormous amount of energy.

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

All of them

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.

Each item below belongs either with the population of disk stars or the population of halo stars of the Milky Way Galaxy. Match each item to the appropriate population.

Disk Stars (Yellow): -Youngest stars -the sun -stars that all orbit in nearly the same plane -high-mass stars Halo Stars (Green): Oldest stars globular clusters stars whose orbit can be inclined at any angle stars with the smallest abundance of heavy elements

Astronomers suspect that a galaxy's type can be determined either by initial conditions in a protogalactic cloud or by later interactions between galaxies. Each item below describes either an initial condition or a later interaction. Match these items to the galaxy type expected to form as a result.

Elliptical - stars form rapidly as the protogalactic cloud shrinks - protogalactic cloud has high density - a galaxy collision strips away gas - protogalactic cloud rotates very slowly Spiral - most protogalactic gas settles into a disk - protogalactic cloud has high angular momentum

Astronomers suspect that a galaxy's type can be affected both by the conditions in the protogalactic cloud from which it forms ("initial conditions") and by later interactions with other galaxies. Each item below describes either a condition in a protogalactic cloud or a later interaction. Match these items to the galaxy type you would expect to form as a result.

Elliptical Galaxies: -stars form rapidly as the protogalactic cloud shrinks. -protogalactic cloud has high density. -A galaxy collision strips away gas. -Protogalactic cloud rotates very slowly. Spiral Galaxies: -most protogalactic gas settles into a disk. -Protogalactic cloud has high angular momentum.

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.

Farthest -a globular cluster in the outskirts of the halo -a cloud of gas and dust in the outskirts of the disk -our solar system -the edge of the central bulge Closest

High angular momentum leads to faster rotation. Why does faster rotation tend to lead to a spiral galaxy, rather than an elliptical galaxy?

Faster rotation leads to collisions among gas particles that cause the gas to settle into a spinning disk, rather than a more spread out cloud.

Consider both the original study (with data shown on the graph) and the follow-up study showing that the exceptions to the general trend are galaxies with unusually bright centers (as described in Part C). Which of the following conclusions are supported by this pair of studies?

Galaxies with unusually bright nuclei are more common at greater distances. Galaxies with unusually bright nuclei were more common when the universe was young than they are today. On average, more distant galaxies have larger infrared color ratios.

Which of the four numbered galaxies likely to be spiral or irregular in shape?

Galaxy 1 Galaxy 2

Rank the four numbered galaxies on the graph at left according to their overall color, from bluest on the left to reddest on the right.

Galaxy 1 Galaxy 2 Galaxy 3 Galaxy 4

Which of the four numbered galaxies have ongoing star formation?

Galaxy 1 and 2

Assuming that the bright core of M87 is powered by a supermassive black hole, which of the following best describes the source of energy that makes the core appear so bright?

Gravitational potential energy is converted to thermal energy as matter from the surrounding gas disk spirals into the central black hole.

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).

Highest Abundance: Yellow main-sequence star in open cluster in disk AND hot, blue main-sequence star in disk (on top of each other) Lowest Abundance: red giant in globular cluster M13 AND red main-sequence star in globular cluster M13 (on top of each other)

In which type of galaxy would you expect to find the most massive black hole: a dwarf irregular galaxy, a large spiral galaxy with a small bulge, or a large elliptical galaxy? Select the correct answer and explanation.

In a large elliptical galaxy, because it has the largest bulge.

What do we mean by the halo of a galaxy?

It is a large, roughly spherical region surrounding the disk.

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.

Longest Time -across the diameter of the galactic halo -across the diameter of the galactic disk -from the sun to the centre of the galaxy -across the diameter of the central bulge -through the disk from top to bottom Shortest Time

Rank the four numbered galaxies on the graph at left according to their total luminosities, from dimmest on the left to most luminous on the right.

Lowest Galaxy 3 Galaxy 1 Galaxy 2 Galaxy 4 Highest

Sort each item into the appropriate bin according to whether it is more common in spiral arms or about equally common within and between the spiral arms in a spiral galaxy's disk.

More common in spiral arms -Ionization nebulae -Dense, dusty gas clouds -Star formation -Young stars Equally common within and between spiral arms -Old stars

Which statement describes the trend among the galaxies shown on this graph?

More distant galaxies tend to have a larger infrared color ratio, but there are some exceptions.

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).

Oldest: Red Giant in globular cluster M13 AND red main-sequence star in globular cluster M13Middle (they overlap) the sun in the middle Youngest: Hot, blue main-sequence star in disk

High density tends to lead to more rapid star formation in a protogalactic cloud. Why does this rapid star formation tend to lead to an elliptical galaxy, rather than a spiral galaxy?

Rapid star formation means that there may not be enough gas left to make a disk.

Watch the video that comes up when you click the icon "Star formation in spiral arms" in the interactive figure. Which of the following best describes what spiral arms are?

Spiral arms are waves of higher density that move outward through a galaxy, triggering star formation as they pass.

Following are a number of distinguishing characteristics of spiral and elliptical galaxies. Match each characteristic to the appropriate galaxy type.

Spirals: -contain abundant clouds of cool gas and dust, -are rare in central regions of galaxy clusters, -contain many bright hot stars, -have significant ongoing star formation, -have a flattened disk of stars Elliptical: -contain primarily old, low-mass stars, -are more reddish in color

The third image in the video (with the most detailed view of the galactic center) is labeled "gas disk." Which of the following best describes what we are seeing in this photo?

The black hole is located deep within the bright central region, and around this region we see gas that is orbiting the central black hole.

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.

Different regions of the galaxy tend to contain stars of different ages. Place labels for the ages of stars in the correct regions of the galaxy painting. Drag the labels to the appropriate blanks on the diagram. You may use labels more than once.

Top left=mostly very old stars Top right=both old and young stars Bottom left=both old and young stars Bottom right=mostly very old stars

This painting represents the Milky Way Galaxy as it would appear edge-on from a distance. Label the indicated features; be sure to pay attention to where the leader lines are pointing. Drag the labels to the appropriate blanks on the diagram. You may use a label more than once.

Top row from left to right globular, bulge, halo Bottom row Both are disks

A follow-up study showed that the galaxies that are exceptions to the general trend on the original graph have unusually brighter centers; that is, these galaxies contain active galactic nuclei. Based on this follow-up study, could you improve your confidence in your distance estimate for the new cluster discussed in Part B, and, if so, how?

Yes, by photographing the new cluster with better angular resolution to see whether its brightest galaxy has an unusually bright center.

What do we think resides in the center of our galaxy (that is, in Sgr A*)?

a 4-million-solar-mass black hole

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

a few hundred

Which of these galaxies would you most likely find at the center of a large cluster of galaxies?

a large elliptical galaxy

This painting represents the Milky Way Galaxy as it might appear from a distance. Identify the indicated features. Drag the labels to the appropriate blanks on the diagram.

from top to bottom -spiral arm -bulge -disk

The primary source of a quasar's energy is

gravitational potential energy

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

Where would you be most expect to find an ionization nebula?

in a spiral arm

Where are most of the Milky Way's globular clusters found?

in the halo