name _____________________________________________ Astronomy 203, Spring 2003, Section 1, Dr Olszewski, Midterm 2, Feb 6, 2003 There are 15 multiple-choice questions on xx pages There then follow 10 short-answer questions on pages xx-xx. Please ensure your exam is complete. Exam is closed book and closed note. You may use a calculator if you wish. Fill out the SCANTRON form in pencil. Write neatly and answer the short-answer questions concisely. Drawing a figure often helps. NOTE: There is a context to these questions, namely, our Astronomy course and texts. You are not reading or interpreting these questions as if you did not have this context. Some 2-part multiple choice questions have answers of the form ``first answer, semicolon, second answer.'' 1) About where is the center of mass of the Sun-Jupiter system? About where is the center of mass of the Alpha Cen AB system? a) about halfway between the Sun and Jupiter; very close to Alpha Cen A b) just outside the surface of the Sun; very close to Alpha Cen B c) about halfway between the Sun and Jupiter; near Proxima Cen d) just outside the surface of the Sun; about halfway between Alpha Cen A and B 2) Alpha Cen A is INTRINSICALLY a LITTLE BIT (50%) brighter than the Sun. As seen from Chile (we can't see it from Tucson) it's the third brightest star in our sky. So, if we were observing from a planet orbiting Alpha Cen A, what would the Sun look like in that planet's night sky? a) 50% fainter than Alpha Cen A appears in OUR night sky, so one of the brightest few stars, still. b) Same brightness as Proxima Cen c) We wouldn't be able to see the Sun, it would be too faint. d) No way to tell from the information given. 3) Proxima Cen is INTRINSICALLY 15000 times fainter than our Sun. So, if you replaced the Sun in our solar system with Proxima Cen, what would happen to the brightness of EARTH'S FULL MOON? a) nothing would change, we're talking about the Sun, not the MOON. b) the Full Moon would get 15000 times fainter because it would now shine by reflected "Proxima light". c) the Full Moon would become "the square root of 15000" times fainter because of the inverse-square law d) we don't know how far away Proxima Cen is, so we can't do the problem. 4) Of what is the WARM INTERSTELLAR MEDIUM composed? a) we don't know. b) nothing, there's nothing between the stars. c) 3/4 Hydrogen gas, 1/4 Helium gas, 1-2% every other type of gas. d) It's composed of atoms that have the most emission lines. 5) A photon is emitted from an atom when a) the electron in that atom jumps from a lower energy orbit to a higher energy orbit. b) the electron in that atom jumps from a higher energy orbit to a lower energy orbit. c) only when the spin of the electron flips d) you add neutrons to the nucleus of the atom. 6) The famous 21-cm emission from Hydrogen comes about by a) the electron in that atom jumps from a lower energy orbit to a higher energy orbit. b) the electron in that atom jumps from a higher energy orbit to a lower energy orbit. c) an electron in the lowest energy orbit flips its spin. d) this only happens when there are lots of hydrogen atoms colliding. 7) The red H-alpha line of Hydrogen is at about 650 nm wavelength. If the object emitting this line is moving towards us, what happens to the wavelength of this emission line? a) it gets [slightly] smaller because of the compression of the wave. b) it gets [slightly] larger because of stretching of the wave. c) nothing. waves are unaffected by the motion of the source. d) nothing. electrons are particles, not waves. 8) When a giant cloud of gas starts to collapse under its own gravity, what happens because of the conservation of angular momentum? a) the cloud becomes opaque. b) the cloud starts to form stars. c) the cloud starts to spin faster. d) the cloud starts to give off light. 9) In class, we derived that the lifetime of a 100-solar-mass star is around 1 million years. What two important facts/clues about stars does this calculation immediately tell us? a) stars are being formed even today; stars form far from clouds of gas and dust. b) 100-solar-mass stars live a long time compared to the Sun; solar type stars must be very rare. c) 100-solar-mass stars cannot exist because their lifetime is too short; the Milky Way is composed only of low-mass stars. d) stars are being formed even today; stars form in molecular clouds because that's where we find 100-solar-mass stars. 10) If you look at the spectrum of a star and see Iron absorption lines, what can you immediately conclude? a) there is iron in the star. b) there is no iron, or else we'd see emission lines. c) the star is tremendously hot compared to the Sun. d) the star is moving away from us very quickly. 11) If a visible-light Hydrogen absorption line comes from a transition from the second level to a higher level, how do you get this gas into the correct state to make very dark (strong) absorption lines? a) make the gas very cold. b) make the gas hot enough to put, by collisions, most of the hydrogen electrons in the second level. Then it can absorb the correct wavelength light. c) make the gas so hot that electrons are always in very high levels. d) get rid of one of the electrons in each Hydrogen atom. 12) As a solid body gets hotter, it emits electromagnetic radiation a) that is fainter and whose spectrum peaks at shorter wavelengths b) that is brighter and whose spectrum peaks at shorter wavelengths c) that is brighter and whose spectrum peaks at longer wavelengths d) that is fainter and whose spectrum peaks at longer wavelengths 13) What is a molecular cloud? a) dense, cold gas cloud containing molecular gas. b) hot, thin gas in between the stars in Orion. c) warm gas in between the stars, say, in the anti-Alpha-Cen region. d) clouds in the atmosphere of alpha-Cen-A 14) When we look at the Orion Nebula from Earth (specifically in the "Trapezium" region), what changes do we see as we change wavelengths from optical to infrared? a) we see fewer and fewer stars because infrared light can't penetrate the cloud. b) we see more and more stars because infrared light penetrates the cloud more easily. c) we see no changes. d) we've never observed the Orion Nebula in infrared. 15) When we look at some of the "small, teardrop shaped" dark regions in the Orion Nebula with infrared light, what do we often see? a) compact sources in the center that we think are newly formed stars. b) compact sources in the center that we think are "destroyed" stars. c) lots and lots of planets. d) 21 cm Hydrogen lines. Short Answer: EXPLAIN YOUR WORK IN ALL CASES. 1) Let's imagine, for simplicity, that the Sun and Alpha Cen A are identically the same INTRINSIC BRIGHTNESS. So now we're standing at the Anti-Alpha-Cen point, about the same distance from the Sun as Alpha Cen, but in the opposite direction. We look back and see the Sun and Alpha Cen A in the spacecraft's dark sky. What are the relative APPARENT brightnesses of the Sun and Alpha Cen A, and WHY. 2) Write down the perfect gas law. Give two examples of how you can change the pressure in a car tire that are explained by this law (they have to be real, correct, everyday examples) 3) Imagine that you shine a continuous spectrum (light of all colors) through a tube of hydrogen gas. Explain what you see on the the opposite side of this lightbulb (so the lightbulb, the tube of gas, and you are in a straight line as described and pictured here) and WHY. Your answer should mention a simple model of the atom, and the properties of photons. 4) Let's actually do the math in a Doppler Effect example. The speed of sound on a typical day in Tucson is about 1100 feet per second. An ambulance emitting CONCERT A siren noise (a frequency of 440 cycles per second) is travelling toward you at 68 miles per hour (100 feet per second). What pitch does it have, in cycles per second? The equation is given below. 5) A 2-solar-mass star has 10 times the luminosity of the Sun. Knowing that the Sun lives for 10 billion years, how long can this 2-solar-mass star live? 6) If we think that a gas cloud is rotating, what measurement must we make to show that it's rotating? 7) Describe some of the experiments we did last Thursday with the infrared camera. 8) Describe the conservation of angular momentum demonstration we did in class last Thursday. 9) Last Thursday in class, we looked at actual spectra in one of our demonstrations. Describe what we saw when we looked at the Hydrogen lamp and at the back window. 10) Here are the spectra of two stars covering the identical wavelength range. The left side is "blue to ultraviolet", the right side is "very red." What is the dominant reason that these two spectra look different (a short explanation is needed!)?