Question 1

Inside of galaxy clusters, which of these components contains about twice as much mass as the stars inside the cluster's galaxies?

Accepted Answer

The very hot intergalactic gas emitting X-rays in clusters contains about twice as much mass as the stars inside the cluster's galaxies. This gas is a major component of the total mass of galaxy clusters, significantly outweighing the mass in stars.

Question 2

One of the big puzzles about the properties and behavior of large clusters of galaxies is that

Accepted Answer

Clusters of galaxies exhibit gravitational effects that suggest there is more mass present than what we can see directly through luminous matter like stars and galaxies. This discrepancy has led to the hypothesis of dark matter, which does not emit light but exerts gravitational forces.

Question 3

Globular clusters

Accepted Answer

Globular clusters are generally old formations, consisting mostly of old stars, but they can indeed be formed in the collisions of galaxies, which means they can be young if the collision occurred recently. This is an exception to their typical characterization as ancient structures.

Question 4

Which one of these statements about clusters of galaxies is true?

Accepted Answer

The observed mass of galaxies in a cluster, based on visible matter, is significantly less than what would be needed to hold the cluster together gravitationally. This discrepancy suggests the presence of a significant amount of dark matter, which does not emit light but exerts gravitational force.

Question 5

Most of the light from a galaxy comes from the inner parts. If this means that most of the galaxy's mass is also in the inner region, then how would the galaxy's speed of rotation behave in its outer region?

Accepted Answer

If most of a galaxy's mass is concentrated in its inner region, the gravitational pull on outer stars decreases with distance. According to Kepler's laws, objects in orbit around a mass will move slower if they are further away from the mass. Thus, the galaxy's rotation speed should decrease smoothly with increasing distance from the center.

Question 6

The mass of the presently observable matter in a typical rich cluster of galaxies appears to be approximately what fraction of the mass required to hold the cluster together by gravity?

Accepted Answer

The mass of the observable matter in a typical rich cluster of galaxies is only about 10% of the mass required to hold the cluster together by gravity. This discrepancy is evidence for the existence of dark matter, which does not emit light or radiation that we can currently detect.

Question 7

If most of the mass of a galaxy is located near the center of the galaxy, then in the outer part of this galaxy the orbital speeds of stars must decrease with increasing distance from the center. This behavior would be equivalent to

Accepted Answer

Kepler's third law describes the relationship between the distance of planets from the Sun and their orbital periods, implying that objects farther from the center of mass (like the Sun or, by analogy, the center of a galaxy) should move slower if most of the mass is concentrated at the center. This principle can be applied to the orbital speeds of stars in a galaxy, where speeds would decrease with increasing distance from the center if the mass distribution followed a similar pattern.

Question 8

Which of these techniques has NOT provided a means by which astronomers can infer the presence of dark matter in the universe?

Accepted Answer

The measurement of the orbital speeds of stars near possible supermassive black holes at the centers of galaxies is used to infer the presence of the black holes themselves, not dark matter. Techniques B, C, and D are used to infer the presence of dark matter through its gravitational effects on visible matter and light.

Question 9

Many rich, regular clusters of galaxies contain substantial amounts of very hot, intergalactic gas. What is believed to have produced this hot gas?

Accepted Answer

The hot intergalactic gas in rich galaxy clusters is primarily believed to be produced by collisions between galaxies in the cluster. These collisions can strip gas from the galaxies, heating it to high temperatures as it falls into the cluster's gravitational potential well.

Question 10

As much as 90% of the matter in the universe may be unseen "dark matter." Where is this dark matter in galaxies?

Accepted Answer

Dark matter is theorized to exist in spherical halos around galaxies, extending well beyond the regions occupied by visible matter. This distribution helps explain the observed rotation curves of galaxies, which do not match the expected behavior if only visible matter were present.

Question 11

Dark, unknown forms of matter appear to make up about what fraction of the mass of a typical rich cluster of galaxies?

Accepted Answer

Dark matter is estimated to constitute about 85% to 90% of the total matter in the universe, making it the dominant form of matter in the structure of galaxies and galaxy clusters.

Question 12

Much of the mass in galaxies and clusters of galaxies is dark matter. Which of these techniques is NOT useful for detecting evidence of this dark matter?

Accepted Answer

Gravitational waves are ripples in spacetime caused by some of the most violent and energetic processes in the universe, such as colliding black holes. They are not directly used to detect dark matter. The other options (A, B, C) involve observing the effects of gravity and are methods used to infer the presence of dark matter by its gravitational influence on visible matter or light.

Question 13

Dark matter is matter that gives out no observable radiation and is detected by its gravitational interaction alone. Which of these does NOT provide direct evidence for dark matter?

Accepted Answer

The emission from hot intergalactic gas in galaxy clusters is not direct evidence for dark matter. Instead, it is the gravitational effects, such as the separation of visible and dark matter in collisions, gravitational lensing, and the discrepancy between visible mass and the gravitational binding of galaxy clusters, that provide direct evidence for dark matter. The emission from hot gas is related to the baryonic (normal) matter component and does not directly indicate the presence of dark matter.

Question 14

The rotation curve of a galaxy is a graph showing the galaxy's speed of rotation at different distances from the center. The observed rotation curve in the outer parts of a typical large spiral galaxy

Accepted Answer

The observed rotation curves of large spiral galaxies remain roughly flat with increasing distance from the center, indicating that the speed of stars and gas in the outer parts of the galaxy does not decrease as expected with distance. This observation is one of the main pieces of evidence for the existence of dark matter.

Question 15

Which single major problem perhaps puzzles astronomers MOST as they attempt to interpret the properties and behavior of clusters of galaxies?

Accepted Answer

The missing-mass problem, or "dark matter" problem, is a major puzzle for astronomers because the gravitational effects observed in galaxy clusters suggest there is significantly more mass present than what we can see directly through telescopes. This discrepancy indicates that the majority of the mass in the universe is invisible to current detection methods, challenging our understanding of the universe's composition.

Question 16

Gravitational lensing of distant objects by closer galaxies indicates that the

Accepted Answer

Gravitational lensing observations have shown that the visible matter in galaxies (stars, gas, and dust) accounts for only about 10% of the total mass needed to produce the observed lensing effects. The remaining mass is thought to be dark matter, which does not emit, absorb, or reflect light, making it invisible and detectable only through its gravitational effects.

Question 17

One piece of evidence for the existence of dark matter is the

Accepted Answer

The discrepancy between the observed mass (calculated from visible matter) and the mass required for gravitational binding in rich clusters of galaxies is a key piece of evidence for the existence of dark matter. This unseen mass is thought to account for the additional gravitational force needed to hold these clusters together.

Question 18

The amount of visible matter in a typical cluster of galaxies is

Accepted Answer

The amount of visible matter in a typical galaxy cluster is insufficient by about a factor of 10 to hold the cluster together gravitationally, indicating the presence of dark matter to account for the missing mass.

Question 19

The source of the hot, intergalactic gas in many rich, regular clusters of galaxies appears to be

Accepted Answer

The hot, intergalactic gas in rich, regular clusters of galaxies is primarily due to the gravitational interactions and collisions between galaxies within the cluster, which heat the gas to high temperatures.

Question 20

How has the hot, X-ray-emitting intergalactic gas in galaxy clusters affected the dark matter problem for the clusters?

Accepted Answer

The hot, X-ray-emitting intergalactic gas in galaxy clusters contributes about 2/10 of the dark matter mass. Along with visible matter, this still leaves about 7/10 of the mass required to stabilize the cluster unaccounted for, indicating that the dark matter problem persists.

Quiz 17: Galaxies