29:50 Modern Astronomy
Third Hour Exam
December 12, 2003
Form A

Read through the question and all the answers before choosing. Budget your time. There are 30 questions. No whining.

Walk with Ursus!

1. What is the observational status of neutron stars? Do they exist, or not exist? If they exist, in what form?
   (a) no: despite extensive searches, nothing which can be interpreted as a neutron star has been found.
   (b) yes: planetary nebulae are now known to represent outflows from neutron stars, even though the small stars themselves have not been detected.
   (c) yes: radio pulsars are neutron stars X
   (d) yes: neutron stars have been found as binary companions to a number of nearby stars such as Sirius and Procyon
   (e) no: in view of their properties (very small size), it would be pointless to look for them
2. Which of the following is a physically correct description of a black hole?
   (a) an object that has a temperature of absolute zero, so it emits no light
   (b) a large concentration of mass in a small volume of space, such that there is infinite curvature of spacetime X
   (c) an object where the magnetic force is so strong that no light is emitted from it, or its nearby surroundings
   (d) any star with a mass greater than 3.25 solar masses
   (e) the region between the Milky Way and M33
3. As your lab project, you make observations of a weird binary star. One of the components is a spectral class G main sequence star and is clearly moving in an orbit. You cannot detect the light from the other component of the binary, but from your measurements you deduce that it has a mass of 5 solar masses. What kind of star or other object is it?
   (a) another spectral class G main sequence star
   (b) spectral class M giant
   (c) white dwarf
   (d) neutron star
   (e) black hole X
4. A basic statement of the physical content of the theory of General Relativity is
   (a) the presence of electrical charge triggers ripples in the cosmic electromagnetic field
   (b) to every action there is an opposite and equal reaction
   (c) gravitational forces correspond to mass being converted to the strong nuclear force
   (d) all measurements in the physical sciences are relative, and without objective basis
   (e) the presence of mass warps or bends spacetime X
5. Which of the following characteristics of the Milky Way galaxy can you deduce on the basis of naked eye observations?
   (a) the fact that the Sun lies very near the center of the Milky Way
   (b) the fact that it is an approximately spherical system of stars
   (c) the distance to the center of the Milky Way
   (d) the fact that magnetic fields are involved in the establishment of the overall helical shape
   (e) the fact that the galactic center lies in the direction of the constellation Sagittarius X
6. The Milky Way is a member of what class of astronomical objects?
   (a) Seyfert galaxies
   (b) quasars
   (c) elliptical galaxies
   (d) irregular galaxies
   (e) spiral galaxies X
7. Approximately how far is it from the Sun to the center of the Milky Way galaxy?
   (a) 8.5 kiloparsecs = 28,000 light years X
   (b) very close, probably less than 500 parsecs (1600 light years)
   (c) 1.2 kiloparsecs = 3,900 light years
   (d) 20 parsecs = 65 light years
   (e) 7.3 Megaparsecs = 23.8 million light years
8. In class I said that stars, in the process of forming, tear up and destroy the molecular cloud out of which they form in about 30 million years. In other words, this is the lifetime of a molecular cloud. What kind of observation could be the basis of such a statement?
   (a) observing open star clusters, finding the most massive main sequence star in each star cluster, then seeing if there was a molecular cloud in the vicinity X
   (b) observing open star clusters, determining their distances, then seeing if the star cluster was in back of a molecular cloud
   (c) observing globular star clusters, counting the number of Cepheids, and determining the distances
   (d) observing molecular clouds, and seeing if there are any pulsars in their vicinities. If there are, these molecular clouds must be very old
   (e) observing globular star clusters, determining their distances from Cepheid variables, and seeing if they have the same distribution in space as molecular clouds
9. The Virgo Cluster is
   (a) the nearest example of a rich cluster of galaxies X
   (b) a cluster similar to the Local Group
   (c) a somewhat distant (5-7 kiloparsecs) open star cluster
   (d) a globular cluster which also has the designation M31
   (e) the largest known collection of objects in the universe
10. A spectrum is taken of a galaxy, and the wavelengths of the spectral lines in its spectrum can be measured. What information can you extract from the spectrum?
    (a) the speed at which the galaxy is receding and its distance X
    (b) the age of the galaxy and its absolute magnitude
    (c) the speed of the galaxy and the existence of a black hole in its center
    (d) the age of the galaxy and whether it formed at the same time as the Milky Way
    (e) the type of galaxy and whether it contains a quasar
11. A direct proof that the Sun is moving on a circular orbit in the Milky Way is provided by
    (a) physics laboratory measurements that the speed of light depends on the direction of propagation
    (b) observed rotation of the system of globular star clusters
    (c) the motion of a compact radio source at the galactic center against background radio sources X
    (d) observation that the Sun is moving very rapidly, at about 220 km/sec, with respect to the nearby stars
    (e) the appearance (shape and size) of other galaxies believed to be similar to the Milky Way
12. Astronomers believe there is a massive black hole at the center of the Milky Way galaxy. This claim is based on observations of
    (a) the motion of stars in a compact star cluster centered on the black hole X
    (b) powerful, radio-emitting jets emerging from the galactic center
    (c) the fact that the galactic center emits nearly no light of any wavelength
    (d) a nearby, supermassive star that is believed to be the companion to the black hole
    (e) Penrose process radiation, which is produced only by black holes
13. Which of the following is a nearby galaxy which is very similar in its properties to the Milky Way?
    (a) the brightest galaxy in the Virgo cluster, M87
    (b) the quasar 3C273
    (c) the Andromeda galaxy, M31 X
    (d) the Orion Nebula, M42
    (e) the Large Magellanic Cloud
14. Most of our knowledge about the velocity of motion of astronomical objects results from measurement of
    (a) the Stern-Gerlach effect
    (b) Wien's Law
    (c) the Doppler effect X
    (d) the Mossbauer effect
    (e) Newton's Laws
15. The mass (as well as the distribution of mass) of a galaxy like the Milky Way is determined by what kind of measurement?
    (a) the brightness of starlight as a function of distance from the galactic center
    (b) the thickness of the galactic bulge
    (c) the wavelength at which the light of the galaxy is brightest
    (d) the direction in which stars and molecular clouds orbit the galactic center, that is, clockwise or counterclockwise
    (e) the orbital speed of stars as a function of distance from the galactic center X
16. Most of the mass of the Milky Way is contained in
    (a) main sequence stars
    (b) white dwarf stars
    (c) molecular gas
    (d) dark matter of an unknown form X
    (e) the massive black hole at the galactic center
17. Hubble's Law is a statement of a simple mathematical relation between
    (a) the absolute magnitude of a galaxy and its age
    (b) the type of galaxy and its distance from the center of the universe
    (c) the speed of recession of a galaxy and its distance X
    (d) the speed of recession of a galaxy and its absolute magnitude
    (e) the radio emission of a galaxy and the speed with which it rotates
18. The term ``extragalactic radio sources'' includes two types of objects. These are
    (a) radio galaxies and pulsars
    (b) pulsars and quasars
    (c) radio galaxies and quasars X
    (d) dark matter and planetary nebulae
    (e) elliptical galaxies and spiral galaxies
19. The distribution of quasar redshifts (symbol z for redshift) shows a peak at z=2, and a sharp dropoff for redshifts larger than 2. This is interpreted as evidence for
    (a) a highest speed at which quasars are receding from us
    (b) absorption of very distant quasar light by dust in the universe
    (c) a ``turn-on time'' for quasars in the early universe X
    (d) a preferred color for quasars
    (e) nonthermal bremstrahlung rather than blackbody radiation as the source of quasar light
20. In class, much of the discussion of cosmology was centered around the ``cosmic scale factor''. A good illustration or example of the cosmic scale factor would be
    (a) the average diameter of a large galaxy
    (b) the size of the solar system
    (c) the average distance between cosmic rays in the galaxy
    (d) the typical distance between stars in the vicinity of the Sun
    (e) the average distance between clusters of galaxies X
21. What is the orbital period for the motion of the Sun in the Milky Way?
    (a) one year
    (b) 4.5 billion years
    (c) 250 million years X
    (d) 13.7 billion years
    (e) 700,000 years
22. Which of the following indicates that the universe is expanding?
    (a) Stefan-Boltzmann Law
    (b) Doppler effect
    (c) Wien's Law
    (d) Hubble's Law X
    (e) the fact that we see galaxies in all directions
23. How old is the universe?
    (a) 4.5 billion years
    (b) 10 million years
    (c) 14 billion years X
    (d) 400,000 years
    (e) 745 billion years
24. The Milky Way is a member of
    (a) the Virgo Cluster
    (b) a collection of approximately 100 luminous elliptical galaxies, termed the "luminous association"
    (c) a triplet collection which includes M87 and the quasar 3C273
    (d) a group of about 18 galaxies termed the ``Local Group'' X
    (e) the National Rifle Association
25. In a Friedmann model of the universe, the evolution and fate of the universe are determined by
    (a) the age
    (b) the mean density X
    (c) the chemical composition
    (d) the strength of the intergalactic magnetic field
    (e) the time when quasars first formed
26. In the past few years (say 5 years or so), a major discovery has been made in cosmology. This is the discovery of
    (a) the fact that all galaxies are receding from us
    (b) the apparent existence of the ``dark energy'' X
    (c) the fact that the universe is open
    (d) the fact that the universe is closed
    (e) a huge collection of galaxies called the Coma cluster
27. The power source of extragalactic radio sources is
    (a) nuclear reactions in the radio jets
    (b) conversion of magnetic energy to heat energy
    (c) a supermassive black hole at the center of the galaxy X
    (d) simultaneous explosions of large numbers of supernovae
    (e) the electromagnetism of dark clouds
28. The physical significance of the Hubble constant is that
    (a) it is the total mass of the universe
    (b) it is proportional to the rotation rate of the universe
    (c) it determines the chemical composition of the universe
    (d) it determines the fraction of galaxies that are spiral, elliptical, irregular, etc
    (e) it is approximately the time since the Big Bang X
29. Some neutron stars are seen as steady sources of visible light. Where would you expect to find them on the Hertzsprung-Russell diagram?
    (a) above and to the right of the white dwarfs
    (b) in the main sequence
    (c) approximately 2 magnitudes brighter than the main sequence
    (d) to the left of the asymptotic giant branch
    (e) below and to the left of the white dwarfs X
30. Extragalactic radio sources shine (produce radio emission) via a process in which
    (a) fast electrons spiral in magnetic fields X
    (b) fast electrons move in straight lines and impact dense clouds of matter
    (c) hot objects in the sources produce blackbody radiation
    (d) huge electrical currents flow on the surfaces of molecular clouds
    (e) electrons and positrons (antimatter electrons) collide and produce intense radio emission