Question 1

What fraction of the mass of the Milky Way Galaxy appears to be in the form of "dark matter," which we cannot see but can detect through its gravitational influence?

Accepted Answer

A)  about 50% 
B)  about 10% 
C)  0%-who ever heard of matter that can't be seen? 
D)  about 90%

Question 2

Which statement about galactic motions and interactions is MOST likely to be true?

Accepted Answer

A)  Galaxies occasionally collide with one another, particularly galaxies in clusters. 
B)  Galaxies are so closely packed in the universe that they are always interacting with one another. 
C)  The universe is composed of one giant galaxy of which all observed stars are members; thus, the question of interaction between galaxies is irrelevant. 
D)  Galaxies are so widely separated that they never interact or collide.

Question 3

What is a probable mechanism for the initial triggering,and the subsequent maintenance,of the density wave that forms the visible arms in spiral galaxies?

Accepted Answer

A)  the equivalent of a massive "sound" wave generated at the center of the galaxy by the gravitational energy release at the initial formation of the galaxy, producing progressive compression of material that is shaped by galaxy rotation 
B)  the gravitational influence from the rotating central supermassive black hole at the galactic center of the arm material 
C)  the gravitational interaction from the close passage of another galaxy 
D)  the shock waves from supernova explosions of massive stars within the spiral arms

Question 4

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

Accepted Answer

A)  it is relatively easily absorbed by hydrogen gas in the Milky Way, so measurements are not confused by emission of this radiation from other galaxies beyond the Milky Way. It originates only from cold hydrogen gas and can be used to map this important component. 
B)  radio waves easily penetrate the Milky Way dust and gas, and they are a very narrow-wavelength line emission; thus, its Doppler shift can be used to measure gas motions. 
C)  Doppler shift of this narrow-wavelength line emission is caused by the temperature of the hot hydrogen gas and therefore can be used to measure the distribution and temperature of this important component of the Milky Way. 
D)  this emission can easily penetrate the Milky Way gas and dust and comes only from hot gas, hence its ability to be used to map the distribution of hot hydrogen gas.

Question 5

When we 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,we can tell the distances to different hydrogen clouds because:

Accepted Answer

A)  absorption of extragalactic radiation at this wavelength is greater the farther away the absorber is from the Sun. 
B)  clouds that are farther away have smaller angular sizes. 
C)  the emission is weaker from clouds that are farther away. 
D)  clouds at different distances have different Doppler shifts because of the rotation of the Galaxy.

Question 6

Faint,starlike objects producing intense radio energy that eventually were called quasars were found to have an unusual feature in their optical spectra.This feature was:

Accepted Answer

A)  an extreme redshift of emission lines that indicated high recessional velocities and hence great distances, requiring extremely high energy output to be detected. 
B)  a periodic variation of the Doppler shift from red to blue, indicating a light source oscillating back and forth over a few weeks. 
C)  two sets of spectral lines that indicated simultaneous motion of sources toward and away from the Sun, possibly from a rapidly expanding shell of material around the radio source. 
D)  an extreme blueshift, meaning that these stars in the Milky Way Galaxy were coming toward Earth at very high velocities.

Question 7

Which observation of quasars convinced astronomers that quasars were very distant objects?

Accepted Answer

A)  an extremely red spectrum, reddened by extreme interstellar absorption of the blue part of the spectrum, meaning that the source must be a very long way away 
B)  an extreme redshift of visible Balmer and UV Lyman hydrogen emission lines, indicating high recessional velocities and hence, by the Hubble law, very large distances 
C)  extreme faintness at visible wavelengths; the inverse square law for visible light showed that they must be very distant 
D)  appearing as pointlike star images under the highest magnification, meaning that they must be very far away

Question 8

What is the significance of the object Sagittarius A* ("Sagittarius A-star")in the Milky Way Galaxy?

Accepted Answer

A)  Sagittarius A* appears to be a jet of material ejected from an accretion disk around a supermassive black hole in the galactic nucleus. 
B)  Sagittarius A* is a globular cluster passing close to the galactic nucleus, and the RR Lyrae stars in it allow the distance to the galactic center to be calculated. 
C)  Sagittarius A* is a bright, high-speed cloud of gas close to the galactic nucleus that allows the mass of the nucleus to be calculated. 
D)  Sagittarius A* appears to be the actual nucleus of the Galaxy.

Question 9

What is the present range of measured values of Hubble's constant from various techniques?

Accepted Answer

A)  somewhere between 200 and 1000 km/s Mpc 
B)  somewhere between 67 and 75 km/s per Mpc 
C)  somewhere between 80 and 300 km/s per Mpc 
D)  between 28 and 32 km/s per Mpc

Question 10

Why do the spiral arms show up so clearly in spiral galaxies?

Accepted Answer

A)  Stars are spread almost uniformly over the galaxy (outside the nuclear bulge), but the brightest stars occur only in the spiral arms, where their brightness and the fact that they illuminate gas clouds make the arms stand out. 
B)  Stars are spread uniformly over the galaxy, but the dust forms a spiral pattern, absorbing starlight; the spiral arms are the dust-free regions between the dust lanes. 
C)  The number of stars in the arms is several times larger than in the regions between, so these arms are reasonably distinct. 
D)  Stars occur only in the spiral arms (and the nuclear bulge), with essentially none between the arms, so the arms show up very distinctly.

Question 11

Which speeds are described by Hubble's law?

Accepted Answer

A)  speeds of individual galaxies in the Local Group and other nearby galaxy clusters 
B)  speeds of superclusters of galaxies 
C)  speeds of stars in the Milky Way Galaxy 
D)  speeds of individual clusters of galaxies in the Milky Way's local supercluster

Question 12

The emission lines in quasar spectra were difficult to identify initially because:

Accepted Answer

A)  no one expected violet and ultraviolet spectral lines to be shifted so far toward the red. 
B)  the observed emission lines were so broad because of internal motions in the quasar that they were difficult to identify. 
C)  they appeared to be created by elements that did not exist on Earth. 
D)  they were very faint and could not be measured accurately.

Question 13

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

Accepted Answer

A)  globular clusters in the halo of the Galaxy. 
B)  Balmer emission lines of visible radiation from hydrogen. 
C)  21-cm radiation from interstellar hydrogen and the distribution of young stars. 
D)  Lyman UV radiation from hot hydrogen gas.

Question 14

The discovery of the peculiar galaxy Cygnus A was a surprise to astronomers because:

Accepted Answer

A)  the redshift of its radio wavelength signal was as high as anything measured up to that time. 
B)  it was first discovered at X-ray wavelengths and detected at optical and radio wavelengths only some time later. 
C)  it was so bright at optical wavelengths that no one expected it to be a galaxy. 
D)  it was very faint at visible wavelengths but extremely bright at radio wavelengths.

Question 15

What is the Tully-Fisher relation?

Accepted Answer

A)  The more distant the galaxy, the greater is the recessional velocity. 
B)  The brighter the Cepheid, the longer is the pulsation period. 
C)  The more distant the galaxy, the fainter it appears. 
D)  The brighter the galaxy, the wider is the 21-cm radio emission line.

Question 16

Quasars,double-radio sources,and BL Lacertae objects may well be the same kind of object.The difference in their appearance is simply that:

Accepted Answer

A)  their position in the universe is different: Quasars are in the Milky Way Galaxy, double-radio sources are associated with other galaxies, and BL Lacertae objects are in the vast voids of space between galaxies. 
B)  the orientation of our line of sight to the axis of their ejected jets of matter is different. 
C)  their ages are different: BL Lacertae objects evolving through the double-radio source phase end as quasars. 
D)  their sizes are different: Quasars are star-sized, double-radio sources are larger, and BL Lacertae objects are galaxy-sized.

Question 17

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

A)  1.3 × 10⁶ M. 
B)  2.6 × 10⁶ M. 
C)  4.3 × 10⁶ M. 
D)  9.4 × 10⁹ M.

Question 18

How is galactic evolution described by the Hubble tuning fork diagram?

Accepted Answer

A)  Galaxies begin as type E0 and evolve to lenticular and then continue as either normal or barred spirals. 
B)  Galaxies are created as either normal or barred spirals, but either eventually evolves to lenticular. These then evolve through the various types of ellipticals. 
C)  Everything starts as a lenticular galaxy. Then, for reasons not well understood, the subsequent evolution is either as a spiral (normal or barred) or as an elliptical. 
D)  We do not believe galactic evolution is described by the Hubble tuning fork diagram.

Question 19

Globular clusters:

Accepted Answer

A)  are young formations and always contain many hot O and B type stars. 
B)  are old formations and always contain only old, red stars. 
C)  can be formed in collisions of galaxies and thus can be young if the collision occurred recently. 
D)  are continually forming in spiral galaxies, like the Milky Way, and thus they show a great range of ages in any one galaxy.

Question 20

When,by whom,and by what means was the first quasar detected?

Accepted Answer

A)  in 1944 by Reber with a small radio telescope 
B)  in 1951 by Baade and Minkowski with the 200-inch Hale telescope on Mount Palomar 
C)  in 1992 by Zeeman with the Hubble Space Telescope 
D)  in 1955 by a British team using the Cambridge radio telescope

Exam 13: The Galaxies