Test 2 Questions on Introduction to Astronomy and Astrophysics | ASTR 2050, Exams of Astronomy

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Exam #

Introduction to Astronomy and Astrophysics NAME ______________________

Spring 2007 Duration of exam: 2 hours You may use a calculator and the crib sheet provided. Please do your own work. If you collaborate with other students or use materials which are not allowed on this exam, you will receive a substantial penalty.

True/False (2 points each, 10 points total)

______ 1. Material emitted in the jets of quasars travels at speeds faster than the speed of light. ______ 2. Globular clusters are found primarily in the spheroid component of the Milky Way. ______ 3. The bulge stars in the Milky way are 10-15 billion years old, but have metallicities that are about the same as the Sun. ______ 4. The most metal-rich component of the Milky Way is the thick disk. ______ 5. It takes about two minutes for the energy from a photon released in the core of the Sun to reach the surface of the Sun.

Multiple Choice (2 points each, 20 points total). Choose the best answer.

______ 1. Which of the following could be used to estimate the distance to Jupiter? (a) spectroscopic parallax (b) redshift (c) Tully-Fisher method (d) radar ______ 2. It was known in 1918 that spiral “nebulae” are seldom observed near the plane of the Milky Way. We now know that this is because: (a) Dark Energy provides a repulsive force causes these spirals to move away from the Galactic plane. (b) There is a layer of obscuring material in the Galaxy which makes it difficult for us to see spirals in the plane of the Milky Way. (c) The spiral nebulae are falling into the Milky Way and have not had enough time to reach their Virial equilibrium state. (d) Spiral nebulae are made of antimatter which annihilates with gas and dust in the plane of the Milky Way if they get too close. ______ 3. If a galaxy has a distance modulus of 30, how far away is it? (a) 30 kpc (b) 10 Mpc (c) 30 Mpc

(d) 10 Gpc ______ 4. If a quasar is 1000 Mpc away, how long ago was the light that we see from it today emitted? (a) 3.3 Myr (b) 33 Myr (c) 330 Myr (d) 3.3 Gyr ______ 5. A star of absolute magnitude MV = 5.0 is at a distance of 100 pc from the Earth. There are 0.2 magnitudes of extinction from dust between us and the star. What apparent magnitude would we measure? (a) 7. (b) 9. (c) 10. (d) 10. ______ 6. A 1 solar mass and a 3 solar mass star are in a binary system. Which is true? (a) The center of mass is closer to the 1 solar mass star, but the L1 Lagrange point is closer to the 3 solar mass star. (b) The center of mass is closer to the 1 solar mass star, and the L1 Lagrange point is closer to the 1 solar mass star. (c) The center of mass is closer to the 3 solar mass star, but the L1 Lagrange point is closer to the 1 solar mass star. (d) The center of mass is closer to the 3 solar mass star, and the L1 Lagrange point is closer to the 3 solar mass star. ______ 7. Cepheid variables are: (a) pulsating stars that are actually increasing and decreasing in size. (b) binary star systems in which matter from one star is being transferred to the surface of a white dwarf. (c) binary star systems which we view in the plane of their orbits, so that they eclipse each other. (d) the result of matter falling into a black hole from an accretion disk. _____ 8. Which is not a type of active galaxy? (a) Seyfert galaxy (b) Quasi-stellar object (c) BL Lac (d) RR Lyrae ______ 9.Which part of the Milky Way produces most of the visible light? (a) Thin disk (b) Thick disk (c) Spheroid (d) Nucleus ______ 10. In the standard model of the Milky Way galaxy, most of the mass comes from: (a) Thin and thick disks (b) Spheroid (c) Dark Matter

(d) A supermassive black hole

Definitions (2 points each, 6 points total)

1. Roche lobe –

2. radio galaxy –

3. instability strip -

Problems requiring calculation (39 points total)

You must show your work (and units) to receive credit. Partial credit will be assigned.

1. (4 points) (a) A nova outbursts in the Milky Way galaxy, and sends a shell of debris out at a velocity of 2000 km/s. If the nova is 100 pc away, what will the diameter of the debris shell be, as seen from the Earth, after 5 years?

2. (4 points) The mass-to-light ratio of the stars in a 10^6 solar mass globular cluster is 1.5. If there is an equal amount of dark matter in the cluster (an additional 10^6 solar masses), what would the total mass-to-light ratio for this cluster be?
3. The Andromeda galaxy is 1 Mpc away. It has an inclination angle of 77 degrees (an inclination angle of 90 degrees would give a perfectly edge-on view of the disk). Assume a Hubble constant of 80 km/s/Mpc. (a) (4 points) Estimate the recession velocity of Andromeda from the Hubble Law (ignoring peculiar velocity). (b) (4 points) If the stars in Andromeda’s disk, 10 kpc from the center of Andromeda, are orbiting the center of Andromeda in circular orbits at a speed of 200 km/s, what is the largest radial velocity (relative to the velocity of Andromeda) we would measure for these disk stars? (c) (4 points) In the part of the disk that is moving away from us most quickly, what would the observed wavelength of the Ca II K line (λ 0 =393.4 nm) be?

3. (a) (4 points) How far away is a galaxy with a redshift of z=0.1? Assume a Hubble constant of 80 km/s/Mpc
4. The Sun is located 8 kpc from the center of the Galaxy, and has a velocity around the center of the Galaxy of 220 km/s. (a) (4 points) How long does it take the Sun to go around the Galaxy? (b) (4 points) Assuming a flat rotation curve for the Milky Way, what is the orbital period of a star that is 9 kpc from the Galactic center?
5. Suppose you found a Cepheid with a period of 30 days, and you measured the Cepheid to have a mean apparent magnitude of 18.7. (a) (4 points) What is the mean absolute magnitude of the Cepheid? (b) (3 points) Compute the distance to the Cepheid.

Essay (25 points total)

I am looking for a clear and readable explanation in response to this question. Please use complete sentences. Diagrams or equations can be included if they clarify what you are writing (and count for credit). Complex sentences may be judged to contain more than one information unit, and may count for additional credit. I cannot read your mind, so you will have to write what you are thinking on the paper. Two points will be added for correct and relevant pieces of information (sentences, drawings, equations with explanation), and three points will be subtracted for sentences which show misunderstanding of physics or astronomy. Five points are reserved for overall clarity and style of answer. No credit will be assigned for sentences that are illegible, contain repeated information, or are uninformative. A minimum of ten informative sentences (and/or diagrams and/or equations) is required for full credit on this essay.

Explain the Hubble system of classifying galaxies, and the criteria used to make

the classifications. You may also include strengths and weaknesses of the system

if you wish.