Question 1

Where do black holes occur in the Milky Way Galaxy?

Accepted Answer

Black holes occur in the galactic center and within some globular clusters in the Milky Way Galaxy. While they may also occur between the spiral arms, this is not as well established as their presence in the galactic center and within some globular clusters.

Question 2

The halo of the Milky Way Galaxy consists of a number of objects that orbit the galactic center and periodically pass through the galactic plane.Which objects are NOT among the galactic halo objects?

Accepted Answer

The galactic halo objects consist of objects that orbit the galactic center, such as globular clusters and field stars. Supermassive black holes are not typically found in the halo, and galaxies are separate entities outside of the Milky Way and do not orbit it.

Question 3

The supermassive black hole at the center of our Milky Way Galaxy has a diameter of approximately 8 × 10-⁷ pc and is approximately 8000 pc away from us.Figure 13-13b of Discovering the Essential Universe,6th ed.,shows a drawing of the center of the galaxy with a size indicator showing 0.2 arcseconds.How does the angular size of the block hole compare to this?

Accepted Answer

The diameter of the black hole is 8 × 10-⁷ pc and it is 8000 pc away from us. To find the angular size of the black hole, we can use the formula:

Angular size = (diameter / distance) × (180/π) × 3600 arcseconds

Plugging in the values, we get:

Angular size = (8 × 10-⁷ / 8000) × (180/π) × 3600 ≈ 2 × 10-⁵ arcseconds

This is too small to show up on the scale of the figure, which has an indicator of 0.2 arcseconds. Therefore, the answer is D, it is about 2 × 10-⁵ arcseconds-too small to show up on the scale of this figure.

Question 4

In which site in the universe has a supermassive black hole been proposed to account for recent observations?

Accepted Answer

Recent observations have revealed that there is a radio source, called Sagittarius A*, located at the center of the Milky Way. The properties of this radio source can only be explained by the presence of a supermassive black hole with a mass of about 4 million times that of the Sun.

Question 5

The possible presence of a supermassive black hole at the center of the Milky Way Galaxy has been deduced from:

Accepted Answer

The very high orbital speed of ionized gas clouds close to the galactic center indicates that there is a massive object in the center of the Milky Way that is gravitationally attracting the clouds, and that object is likely a black hole.

Question 6

The vast majority of the stars in the galactic halo are isolated field stars.What is the origin of these stars?

Accepted Answer

The answer of The vast majority of the stars in...

Question 7

What evidence now exists for a supermassive black hole at the center of the Milky Way Galaxy?

Accepted Answer

The very rapid motion of matter close to the nucleus of the Galaxy indicates the presence of a very massive body to hold it in orbit, which is believed to be a supermassive black hole. A and B are also evidence for a central black hole, but C is the strongest piece of evidence. D is incorrect, as black holes themselves do not emit any light, and so a "dark void" would not necessarily indicate their presence.

Question 8

What appear to be the characteristics of the object at the center of the Milky Way Galaxy?

Accepted Answer

The object at the center of the Milky Way Galaxy is believed to be a supermassive black hole with a mass of approximately 4 million times that of the Sun, located in a region about the same size as the solar system. Option A is too small to explain the observed effects on stars orbiting the center, Option B is too large to fit within the observed region, and Option D is much smaller in volume than the estimated size of the object.

Question 9

Which objects do we NOT find in orbit around the galactic center?

Accepted Answer

Large spiral galaxies do not orbit around the galactic center. They are usually found in the outer regions of galaxies or as satellites of larger galaxies.

Question 10

The star S0-2 orbits Sagittarius A* (the galactic center of the Milky Way)with a period of 15.2 years and a semimajor axis of about 1000 AU.What does Kepler's third law suggest as the total amount of mass inside the orbital radius of S0-2?

Accepted Answer

Kepler's third law states that for any orbiting body, the square of its orbital period is proportional to the cube of the semi-major axis of its orbit. Using this law, we can calculate the total mass enclosed within the orbit of S0-2 using the equation M = (4π²a³)/(G T²), where a is the semi-major axis, T is the orbital period, and G is the gravitational constant. Plugging in the values, we get M = (4π² x (1000 AU)³)/(G x (15.2 years)²) ≈ 4.3 x 10⁶ M, where M is the total mass enclosed within the orbit of S0-2. Therefore, the best choice is C.

Question 11

If the black hole in the center of the Milky Way Galaxy were 4 × 10⁶ solar masses,what would be the orbital period of Earth around this hole in an orbit of radius 1 AU (assuming that Newtonian physics and Kepler's third law apply)?

Accepted Answer

Using Kepler's third law, the orbital period T is proportional to the square root of the cube of the semi-major axis divided by the mass of the central object. For a black hole of 4 million solar masses and an orbit of 1 AU, the period is approximately 4 hours.

Question 12

The supermassive black hole at the center of our Milky Way Galaxy is believed to be 4.3 × 10⁶ solar masses.The nearest of the stars orbiting it has a period of 11.5 years.What is the semimajor axis of the orbit of this star (in AU)?

Accepted Answer

We can use Kepler’s Third Law to relate the period and semimajor axis of the orbit:
(T/2π)² = (a³/GM)
where T is the period of the orbit, a is the semimajor axis, G is the gravitational constant, and M is the mass of the central object (the black hole in this case).

Rearranging for a, we get:
a = [(GM)(T²)/(4π²)]^(1/3)

Substituting the values given:
a = [(6.67 × 10^-11 m³ kg^-1 s^-2)(4.3 × 10^6 M☉)(11.5 years x 3.15 x 10^7 s/year)²/(4π²)]^(1/3)

a ≈ 1.2 x 10¹³ m

Converting to AU:
a ≈ 828 AU

Question 13

It was recently discovered that the galactic halo contains an outer layer.How does this outer layer compare to the inner layer?

Accepted Answer

The answer of It was recently discovered that the galactic...

Question 14

Which property is NOT characteristic of the center of the Milky Way Galaxy?

Accepted Answer

The center of the Milky Way Galaxy does not have a gigantic millisecond pulsar with rapidly fluctuating emissions; instead, it is known for its supermassive black hole, Sagittarius A*.

Question 15

Where in space would you look for a globular cluster?

Accepted Answer

Globular clusters are typically found in the galactic halo, which surrounds the disk of a spiral galaxy like the Milky Way. They are composed of very old stars that were some of the first to form in the early universe, and they orbit the galactic center in long elliptical orbits. Therefore, choice C is the correct option. The other options are not likely places to find globular clusters.

Question 16

Does the halo of the Milky Way Galaxy include other galaxies?

Accepted Answer

The halo region of the Milky Way includes several small galaxies, including the Magellanic Clouds, which are two small irregular galaxies. Therefore, choice D is correct. Choices A and B are incorrect because they are not true statements. While it is not common for two galaxies to orbit each other, it is not impossible, and the Milky Way does have several small galaxies in its halo. Choice C is incorrect because the Andromeda Galaxy is not part of the Milky Way's halo.

Question 17

If the galactic center of the Milky Way is now thought to contain a supermassive black hole,why is the Sun not falling into it under the black hole's extreme gravity?

Accepted Answer

The Sun has sufficient velocity that it can orbit the galactic center in a circular path, which means that the gravitational force from the supermassive black hole is not strong enough to pull the Sun out of its orbit and towards the galactic center. This is similar to how the Earth orbits the Sun despite the Sun's strong gravitational pull.

Question 18

What is the evidence that indicates to some astronomers that a supermassive black hole exists at the center of the Milky Way Galaxy?

Accepted Answer

The measurement of gas clouds orbiting the galactic center at very high speeds indicates that they would rapidly move out of the Galaxy unless some very massive object were holding them in orbit. This is evidence that there is a supermassive object at the center of the Milky Way, likely a black hole.

Question 19

What is a galactic halo?

Accepted Answer

A galactic halo refers to a spherical distribution of stars and globular clusters that surrounds a galaxy, including the Milky Way. This halo is typically centered on a nuclear bulge, which is a dense region of stars at the center of the galaxy. The halo can also contain dark matter, which makes up the majority of the mass of a galaxy. The Halo of the Milky Way is believed to contain about 90% of all of the mass of the galaxy.

Question 20

The broadening of spectral lines due to the collective Doppler shifts of objects orbiting the center of the Milky Way Galaxy suggests that the mass at the center of the Galaxy is about 4 million solar masses.What is believed to be the diameter of this supermassive object?

Accepted Answer

The supermassive object at the center of the Milky Way, believed to be a black hole, has a diameter roughly comparable to the size of the solar system, despite its massive mass of about 4 million solar masses.

Quiz 13: The Galaxies