Question 1

What useful purpose did variable stars serve for Harlow Shapley in locating the galactic center?

Accepted Answer

The brightness variations of variable stars, particularly Cepheid variables, allowed Harlow Shapley to measure accurate distances to globular clusters in which these stars were located. By determining the distances to these clusters around the Milky Way, Shapley was able to estimate the position of the galactic center.

Question 2

Suppose astronomers could examine a star and determine its apparent magnitude M without any intervening gas and dust. Compared to its apparent magnitude measured with gas and dust between Earth and the star, this "clean" measurement of M would be

Accepted Answer

The apparent magnitude of a star becomes larger (dimmer) when gas and dust are present between the star and the observer because these materials absorb and scatter some of the star's light before it reaches Earth. Without any intervening gas and dust, the star appears brighter, corresponding to a smaller number in the magnitude scale.

Question 3

Which type of radiation has been MOST effective in evaluating the spiral arm structure of the Milky Way Galaxy?

Accepted Answer

21-cm radio emission from electron spin-flip transitions in cool hydrogen gas has been most effective in evaluating the spiral arm structure of the Milky Way Galaxy because it can penetrate the interstellar dust that obscures other forms of radiation, allowing astronomers to map the distribution of hydrogen and thus the spiral arms.

Question 4

In which constellation in the Earth's sky is the center of the Milky Way Galaxy located?

Accepted Answer

The center of the Milky Way Galaxy is located in the direction of the constellation Sagittarius. This is determined by observations of the galaxy's central bulge and the dense concentration of stars and other objects in that direction.

Question 5

Where is the Sun located in the Milky Way Galaxy?

Accepted Answer

The Sun is located in the Orion-Cygnus Arm, also known as the Orion Spur, which is a minor arm located between the Sagittarius and Perseus arms of the Milky Way Galaxy.

Question 6

Which component of the Milky Way Galaxy accounts for interstellar extinction, the dimming of light from distant objects?

Accepted Answer

Interstellar extinction is primarily caused by dust in the Milky Way Galaxy. This dust scatters and absorbs light from distant stars and other celestial objects, making them appear dimmer than they actually are.

Question 7

Radio waves of 21-cm wavelength originate from which component of the interstellar medium?

Accepted Answer

Cool, neutral atomic hydrogen (HI regions) in the interstellar medium is known for emitting radio waves at a wavelength of 21 cm. This emission occurs due to the hyperfine transition of the electron in the hydrogen atom.

Question 8

The plane of the Milky Way Galaxy is filled with clouds of gas and dust. To receive information from objects embedded in these clouds astronomers use receivers sensitive in these ranges EXCEPT

Accepted Answer

Ultraviolet light is strongly absorbed by the interstellar medium, making it difficult to use for observing objects embedded in gas and dust clouds. Infrared, X-ray, and radio wavelengths can penetrate these clouds more effectively, allowing astronomers to receive information from objects within.

Question 9

Trumpler discovered the effect of interstellar dust by comparing two different distance measurements of distant clusters. The first was based on appearance (concentration of stars toward the center, etc.), which was unaffected by dust. The other was spectroscopic parallax (use of the H-R diagram), which was affected by dust. What should such a comparison show?

Accepted Answer

The H-R diagram method would give larger distances because interstellar dust dims the light from stars, making them appear fainter and thus seemingly further away than they actually are when using spectroscopic parallax.

Question 10

If interstellar dust obscures astronomers' view of distant regions of space at optical wavelengths, which of these statements is true?

Accepted Answer

The obscuration by interstellar dust is most severe in the plane of the Milky Way Galaxy because that's where the density of dust and gas is highest. When we look out of the plane, the obscuration decreases because we are looking through less material.

Question 11

Harlow Shapley's original estimate of the distance from the Sun to the galactic center was incorrect because it did not take scattering by interstellar dust into account. Shapley's original estimate was

Accepted Answer

Shapley's original estimate was too large because interstellar dust scatters and absorbs light, making stars appear dimmer and thus seemingly more distant than they actually are.

Question 12

Suppose one could examine a star and determine its absolute magnitude M without any intervening gas and dust. Compared to its absolute magnitude measured with gas and dust between Earth and the star, this "clean" measurement of M would be

Accepted Answer

Absolute magnitude is a measure of the intrinsic brightness of a celestial object, which does not change regardless of the presence of gas and dust or the observer's location. It is defined as the apparent magnitude the object would have if it were located at a standard distance of 10 parsecs from the observer, without any intervening material. Therefore, the presence of gas and dust does not affect the absolute magnitude of a star.

Question 13

When distances were carefully measured from Earth to globular clusters above and below the Milky Way plane (where the view of them is not obscured by interstellar dust and gas), the distribution of the clusters was found to be

Accepted Answer

The distribution of globular clusters is spherically symmetric about a point in the constellation Sagittarius, indicating the center of the Milky Way Galaxy. This concentration towards Sagittarius suggests that the center of the Galaxy is in that direction, not that the clusters are uniformly distributed or aligned with the Sun's position.

Question 14

What happens when the electron in a hydrogen atom flips its direction of spin from parallel to antiparallel to that of the proton?

Accepted Answer

This event describes the hyperfine transition in hydrogen, where the electron flips its spin relative to the proton, resulting in the emission of a 21-cm wavelength photon in the radio spectrum. This is a fundamental process in astronomy for detecting interstellar hydrogen clouds.

Question 15

Why is it that astronomers rely upon the 21-centimeter line associated with the spin flip in hydrogen and not the more familiar hydrogen Balmer lines to gather information from the far side of the galactic plane?

Accepted Answer

The 21-centimeter line, being in the radio part of the electromagnetic spectrum, can penetrate gas and dust clouds more easily than the Balmer lines, which are in the visible and ultraviolet regions and are more readily absorbed or scattered by the interstellar medium.

Question 16

In 1917, Harlow Shapley plotted the positions of almost a hundred globular clusters. But the galactic plane is cloaked by gas and dust, and the best way to "see" through this is with long wavelength radio waves. However, in 1917 radio astronomy did not exist. How did Shapley accomplish his task?

Accepted Answer

Shapley was able to plot the positions of globular clusters because they are mostly located in a halo around the Milky Way, above and below the plane of the galaxy where the dust and gas are less dense, making them less obscured by interstellar dust.

Question 17

The method used by Harlow Shapley in 1917 to estimate the Sun's location in the Milky Way Galaxy was the measurement of the

Accepted Answer

Harlow Shapley used the distribution of globular clusters to estimate the Sun's location in the Milky Way. He observed that globular clusters were not evenly distributed across the sky but were concentrated in the direction of the constellation Sagittarius, leading him to conclude that the center of the Milky Way was in that direction, and thus the Sun was not at the center of the Galaxy.

Question 18

The one component of the material of the Milky Way Galaxy that prevents astronomers from seeing and photographing the galactic center at optical wavelengths is

Accepted Answer

Interstellar dust scatters and absorbs visible light, making it difficult to observe the galactic center at optical wavelengths.

Question 19

The spiral-arm structure of the Milky Way Galaxy has been measured and evaluated MOST effectively by observations of

Accepted Answer

The spiral-arm structure of the Milky Way Galaxy has been most effectively mapped through observations of 21-cm radiation from interstellar hydrogen and the distribution of young stars. This is because 21-cm radiation, which is a form of radio waves, can penetrate the dense dust clouds in the galaxy, allowing astronomers to map the structure of the galaxy, including its spiral arms, with greater accuracy. Additionally, the distribution of young stars, which tend to form in the spiral arms, helps in tracing the shape and location of these arms.

Question 20

Harlow Shapley first located the center of the Milky Way Galaxy in 1917 by

Accepted Answer

Harlow Shapley determined the center of the Milky Way Galaxy by observing the distribution of globular clusters in the galactic halo. He noticed that these clusters were centered around a point in the sky that was not the Earth, leading him to conclude that the solar system was not at the center of the galaxy.

Quiz 16: The Milky Way Galaxy