Question 1

The 21-cm line is one of the most important wavelengths in radio astronomy. The fact that it is a relatively long wavelength is important because long wavelengths

Accepted Answer

Long wavelengths, like the 21-cm line, are less likely to be absorbed or scattered by interstellar clouds of gas and dust, allowing them to pass through and enabling astronomers to observe regions of space that might otherwise be obscured.

Question 2

Recent observations seem to indicate that, rather than being a spiral galaxy, the Milky Way may be

Accepted Answer

The Milky Way is classified as a barred spiral galaxy, characterized by a central bar-shaped structure composed of stars. Recent astronomical observations and data, particularly from infrared surveys that peer through the dust obscuring the galaxy's core, have provided strong evidence supporting this classification.

Question 3

The major advantages of the 21-cm radio emission from hydrogen gas for investigating the spiral structure of the Milky Way Galaxy are that

Accepted Answer

The 21-cm radio emission from hydrogen is advantageous for studying the Milky Way's spiral structure because radio waves can penetrate the galaxy's dust and gas. The narrow-wavelength nature of this emission allows for precise measurements of gas motions through Doppler shift analysis.

Question 4

Which of these statistics is NOT useful for mapping the locations and shapes of the spiral arms of the Milky Way Galaxy?

Accepted Answer

Globular clusters are old and distributed in a halo around the Milky Way, not associated with the spiral arms, making them not useful for mapping the locations and shapes of the spiral arms.

Question 5

The stellar components of the Milky Way Galaxy that act as tracers for the mapping of the spiral arm structure in the Galaxy are

Accepted Answer

Bright Population I stars and emission nebulae are young and located in the spiral arms, making them effective tracers for mapping the spiral structure of the Milky Way.

Question 6

When a hydrogen atom is excited by some addition of energy and it responds with electron transitions between electron energy levels, the radiation it emits is usually in which regions of the electromagnetic spectrum?

Accepted Answer

When a hydrogen atom is excited, the electron transitions between energy levels often result in the emission of radiation in the visible and ultraviolet regions of the electromagnetic spectrum, such as the Balmer and Lyman series.

Question 7

The major features of the Milky Way Galaxy have been found to include all of these EXCEPT

Accepted Answer

The Milky Way Galaxy does not have a spherical symmetry; instead, it has a disk-like shape with a bulging center and spiral arms, making option C incorrect.

Question 8

What is the distribution of giant molecular clouds in the Milky Way Galaxy and other similar galaxies?

Accepted Answer

Giant molecular clouds are primarily found in the spiral arms of galaxies like the Milky Way. This is because the spiral arms are regions of higher density, which facilitates the conditions necessary for the formation of these large molecular clouds.

Question 9

What atomic transition occurs in the atoms of hydrogen gas in the spiral arms of the Milky Way Galaxy to produce the 21-cm radio emission?

Accepted Answer

The 21-cm radio emission in hydrogen gas is produced by the inversion of the electron spin relative to the proton spin, from parallel to antiparallel. This is a hyperfine transition in atomic hydrogen.

Question 10

Which of these components of the Milky Way Galaxy outlines the spiral arms of the Galaxy?

Accepted Answer

Young O and B stars, dust, and gas are found in the spiral arms of the Milky Way Galaxy, outlining its structure. These components are significant in marking the spiral pattern because young stars, especially the hot and luminous O and B types, form in the dense regions of gas and dust within the arms. Globular clusters, solar-type stars, and white dwarfs are distributed differently within the galaxy and do not specifically outline the spiral arms.

Question 11

A neutral hydrogen atom consists of a proton and an electron. The energy of the atom is

Accepted Answer

The answer of A neutral hydrogen atom consists of a...

Question 12

If astronomers aim a radio telescope at a distant spiral arm of the Milky Way Galaxy, they will probably observe a 21-cm line. If they point a large optical telescope at this same region, they will probably not be able to detect the neutral hydrogen that gives rise to the 21-cm radio signal. Why not?

Accepted Answer

The answer of If astronomers aim a radio telescope at...

Question 13

When astronomers measure the narrow line emissions of hydrogen at 21-cm radio wavelengths along a particular line of sight through the disk of the Milky Way Galaxy, they can tell the distances to different hydrogen clouds because

Accepted Answer

The 21-cm line emissions from hydrogen clouds in the Milky Way are used to determine distances based on the Doppler shifts caused by the rotation of the Galaxy. Clouds at different distances move at different velocities relative to us, leading to variations in the observed wavelength due to the Doppler effect. This allows astronomers to map the structure and motion of the Galaxy.

Question 14

When astronomers look out into the plane of the Milky Way Galaxy, how far can they see?

Accepted Answer

Astronomers can see about 24,000 light-years into the plane of the Milky Way Galaxy due to dust and gas clouds that obscure their view beyond this distance.

Question 15

The dimensions of the disk of the Milky Way Galaxy are diameter _____, thickness _____.

Accepted Answer

The Milky Way Galaxy's disk has a diameter of about 100,000 light-years (ly) and a thickness of about 2,000 light-years, making option A correct.

Question 16

What quantum transition occurs inside a hydrogen atom to produce a 21-cm radio photon?

Accepted Answer

The 21-cm radio photon is produced by the hyperfine transition in which the electron in the hydrogen atom flips its spin from being parallel to the proton's spin to being antiparallel. This is a very low-energy transition and is not related to the electron changing energy levels in terms of its orbit around the nucleus.

Question 17

The 21-cm line is one of the most important wavelengths in radio astronomy. It is produced mainly in

Accepted Answer

The 21-cm line is produced by neutral hydrogen atoms when the spins of the electron and proton flip from being parallel to antiparallel (or vice versa). This specific wavelength is crucial in radio astronomy for studying the structure and dynamics of galaxies, as neutral hydrogen is abundant in the interstellar medium.

Question 18

Where are bright, young O and B stars MOST likely to be found in the Milky Way Galaxy?

Accepted Answer

O and B stars are young, massive stars that are most commonly found in the spiral arms of galaxies like the Milky Way. These regions are sites of active star formation, where the density of gas and dust is higher, providing the necessary materials for the birth of such stars.

Question 19

Astronomers aim a radio telescope at a distant region of the Milky Way Galaxy and detect 21-cm radio waves with no Doppler shift. Which one of these possible explanations is NOT correct?

Accepted Answer

Explanation: The absence of a Doppler shift indicates that the neutral hydrogen is not moving toward or away from Earth along the line of sight. If it were moving away from Earth, a redshift (indicative of a Doppler shift) would be observed. Choices A, B, and C all describe scenarios where no Doppler shift toward or away from Earth would be observed, making them plausible explanations for the lack of observed Doppler shift.

Question 20

How far away from Earth is the Andromeda Galaxy compared with the diameter of the Milky Way Galaxy?

Accepted Answer

The Andromeda Galaxy is approximately 2.537 million light-years from Earth, while the Milky Way's diameter is about 100,000 to 200,000 light-years. This puts Andromeda at roughly 20 times the diameter of the Milky Way away from us, making option B the correct choice.

Quiz 16: The Milky Way Galaxy