Astro20 / Harpell Topics for Quiz 4 The quiz will have 20 multiple choice questions; several "fill in the blanks" about five short essay questions that may require sketches.. If you can answer everything below, you will do well! The Sun (chapter 14) some of this is review from quiz 3, but you should still know it well! Sun s Energy Source 1. Gravitation, contraction, and heating 2. Early Ideas Kelvin Helmholtz Contraction what were the problems with this idea (why was it rejected?) 3. Hydrostatic (gravitational) equilibrium? What is it? Is Balance a good word to use here? What would happen to the sun if the Core got slightly cooler or hotter? How do the ideas above describe the point at which A Star is Born 4. What are the different forces involved in keeping the sun in Balance? What forces are involved in producing energy? Which are the strongest? Which are the weakest? Does it matter how far apart the particles are? You should know about four forces! 5. Be able to describe in general and in specific terms the mechanism for the sun s energy production (today!) In the Fusion process, what goes in and what comes out? What happens to the Helium nuclei, positrons, gamma rays, and neutrinos that are produced? In Which step of the reaction are photons generated? How long does it take these photons to emerge from the sun? What happens to them along the way? Why are high temperatures necessary for Fusion to occur? What temperatures are required to do this? Why do you need even higher temperatures to create fusion in a laboratory? 6. The Rate of Fusion will increase over the sun s lifetime. Why will this happen and what effect will it have on us? The sun Basic properties 1. Composition which elements is it made of and in what percentage? 2. What is the Sun s Luminosity? What are appropriate units for this?
3. What are the basic layers of the sun? What processes are taking place in each layer? What is Granulation? What are the temperatures at the core, surface, and outmost atmosphere? What do we actually see when we look at the sun (i.e. which layer)? Why? 4. Why does the sun have the color it has? What does this have to do with temperature? 5. Why do Sunspots appear so dark even though they are quite hot? 6. How do Scientists know what is going on inside the sun? 7. What is Helioseismology? 8. How do astrophysicists know if their computer models are correct? The sun magnetic field phenomena and current research 1. What are neutrinos and why are they so difficult to detect? Is there more than one type? What is the Solar Neutrino Problem? What are possible solutions to the Solar Neutrino Problem? 2. Is there a Sunspot Cycle? If so, how long is it? What is happening? 3. When is the sun putting out more energy when there are many spots, or when there are fewer spots? 4. How is the sun s magnetic field responsible for sunspots, solar flares, and Coronal Mass Ejections? 5. Why are sunspots cooler then the surrounding photosphere? 6. What are Coronal Holes? Are they connected in any way to Sunspots? Spectral types and Luminosity classes What are spectral types? What are luminosity classes? What is the luminosity class for a main sequence star? For a supergiant? What are general trends in the spectra of stars of different spectral types (i.e. what kind of absorption lines to you see in one type but not in another..you don t need all the details, just some
general ideas, such as molecular lines show up in type M stars because they are cooler) Know the approximate mass (as compared with the mass of our sun) and surface temperature of each of the following types of stars: OBAFGKM. Especially know what the mass and surface temp of a G2V star is! Why is Temperature the main reason why stars have different spectra? Important: Why is it the mass of a star that determines what its temperature will be? Hint: consider the process of gravitational equilibrium. If we were looking for stars that were good candidates for having planets with intelligent life, which spectral types (and luminosity classes) would be good type? Which would be particularly bad types? Why? One Hint: How long did it take human beings to evolve from single cell organisms? Note that Time is not the only factor here! HR diagrams What is the HR diagram? What are the hottest, largest stars plotted on the HR diagram? What about the coolest, smallest stars? What is the Main Sequence? Why is the main sequence a line that shows stars of increasing mass from lowest at bottom right, to highest at bottom left? What is happening inside a star when it is on the main sequence? What property of a star determines how long it will stay on the main sequence? How long do different types of stars stay on the main sequence? Know how long an O, G, and M star will stay on the main sequence (approximately) in years. Magnitude system What is apparent (visual) magnitude? What is absolute magnitude? What is the naked eye limit for apparent visual magnitude? What is the apparent magnitude for the brightest star in the sky (and which star is it)?
How far away is a star if its apparent magnitude equals its absolute magnitude? How can the apparent magnitude be estimated from a photograph of stars (from class notes) What is the weird equation (from class notes)? If you know apparent magnitude and distance to a star, can you calculate Absolute magnitude? If you know absolute magnitude and apparent magnitude, can you calculate distance? (don t worry, you won t have to use this equation, just know about it) Masses and sizes of stars from Binary Star Studies How are the masses of stars determined from the studies of binary stars? What do we need to know about the binary pair? How are the orbital periods measured for a spectroscopic binary? Newton modified Kepler s third law to relate which three variables (i.e. orbital period, and two other things). If we know the total mass of a binary pair from applying Newton/Kepler s law, how do we know the mass of the individual stars? If we know the total mass of a binary star system, and spectral type and luminosity class of one of the stars, is this enough information to find the mass of the other star? Explain. What is the center of mass of a binary star system? How do the masses and speeds of the two stars about the center of mass relate to one another? Hint: both stars have the same orbital period since they are orbiting each other so does the more massive star move more rapidly or more slowly then the lower mass star about the center of mass? How can the size of stars be determined from their light curves? Be able to define Luminosity Intensity or apparent brightness (they are the same thing) Wein s law Stephan s law Cepheid Variable Stars Stellar parallax
Parsec Spectroscopic parallax (from class notes) What is special about the following stars? Deneb Betelgeuse Sirius b Which is the largest? Most luminous? Smallest? Densest? Compare their values of radius, temperature, Luminosity, and mass with that of the sun How do we know? Of the stars in the summer triangle, which has: Magnitude zero (approximately) Which is the farthest? Which is somewhat flattened due to high rotation (from class notes) General Questions: How does Intensity relate to Luminosity? Is the apparent magnitude of a star a measure of the stars luminosity or apparent brightness? Is the absolute magnitude of a star a measure of the stars Luminosity or apparent brightness? What does the color of a star tell you? Explain how Stephan s law shows how a tiny hot star can be even more luminous that a giant Cool star. In what way are Cepheid variable stars standard candles? (see class notes). How do astronomers judge their Luminosity and then find their distance? The parallax method is only useful out to a certain distance? What is this distance (approximately)? Why does this limit exist? If a star has a parallax angle of.5 arc seconds how far away is it? (Answer: 2 parsecs or about 6.5 light years know the formula for this!) What if the parallax angle was.25 arc seconds? What about.1 arc seconds? Why are parallax angles to stars always less than one arc seconds? In general, how are the distances to stars (such as Deneb) that are too far away to use the parallax method determined?
If two stars have the same luminosity, but one has one forth the apparent brightness of the other, how much farther away is the dimmer star?. Know the general (inverse square law) relationship here! What if one star had one sixteenth the apparent brightness? (answers: twice, four times as the distance doubles, the apparent brightness gets four times smaller..if the distance doubles twice, then the apparent brightness is sixteen times smaller). If two stars have the same apparent brightness, but one star is 100 times as luminous as the other, how much farther away is the dimmer star? Answer: ten times.what is the relationship here? (here is how you do this.note that the more distance star is ten times as far away, so its apparent brightness falls off as one over the distance squared or 1/10 2 so the more luminous star will have its brightness reduced by a factor of a 100. Since it puts out 100 x more light, both stars will appear the same.) Here is some math review that might come in handy 1) Relation between temperature and Color of a star: λ peak = constant (wein's law) Temp for sun, T = 5800K peak = 550nm if T = 11,600 K λ = 225nm 2) Relationship beween Luminosity, size, and temperature λ peak L= D T 2 4 constant (Stephan's law) where D = Diameter, T = temp, and L = luminosity If L = Luminosity of Sun, D = Diameter of Sun, and T = Temp of Sun, then a star with T = 2 T, D = D will have: L = 16L. IF T = T, D= 2 D, then L = 4L. What about a star with D = 100 D, and T = 2 T? 2 4 Answer : L = 100 2 L = 160,000L