This midterm guide is now COMPLETE! (Chs. 17-22).

Chapter 17 (The Sun)

Module 1 (Basic Properties):

How does the diameter of the Sun compare with the diameter of the Earth (for Earth’s diameter, see Ch. 8 (The Earth), Module 1 (Seismic Waves), third section (Inner and Outer Cores))? How does diameter of the Sun compare with its distance from Earth? How can we learn about the composition of the sun? What are the two dominant elements in the Sun? How does this compare with Earth? With the Universe as a whole? Approximately how many Earths would the volume of the Sun hold? How does the Sun’s density compare with that of Earth? Name and briefly describe the three innermost regions of the Sun. Define helioseismology and dopplergram. What does helioseismology tell us about the abundance of helium in the interior of the sun as compared with the surface? About where convection occurs? About rotation in the interior of the sun as compared with the surface? Define solar luminosity and solar constant, and give their numerical values.

Module 2 (Photosphere and Spectrum):

In what part of the electromagnetic spectrum does the Sun emit most light? Roughly what temperature does this correspond to? Surface features with much higher temperatures are seen in what two other regions of the spectrum? What is the photosphere. Is it a hard surface? What causes the dark lines in the solar spectrum? Roughly how many elements have been identified in the Sun? What is granulation? Discuss what causes it, including the size and speed of the material causing it. Discuss the chromosphere, including its thickness and temperature range. When is it most easily visible, and what is it about its appearance that gives it its name?

Module 3 (Magnetic Field):

Who first detected the rotation of the sun, and how did he or she do it? Why are sunspots darker than the rest of the solar surface? Describe how the number of sunspots varies in time. Are sunspot maxima associated with high or low solar activity? Describe direct evidence that sunspots are associated with strong magnetic fields. How does the period of the Sun’s magnetic cycle compare with that of the sunspot cycle? How is this known? How does the strength of the Sun’s magnetic field compare with Earth’s? Why do magnetic fields make sunspots cooler than the rest of the solar surface? Give a brief description of the Babcock dynamo model.

Module 4 (The Active Sun):

Name and define three phenomena (more violent than sunspots) associated with “active regions” on the Sun. Describe the appearance of quiescent prominences; how long can they last, and how big can they be (say, in comparison with Earth’s diameter)? How long do solar flares last? Characterize the amount of energy released in the solar flare.

Module 5 (Corona and Solar Wind):

What is the corona? Characterize its temperature, density, and brightness. What are two ways in which it can be viewed? What is the high temperature of the corona surprising? What is thought to cause it? How does the appearance of the corona change with high and low solar activity? Define coronal holes and solar wind; what is the relationship between them? What is the solar wind composed of? How fast is it traveling when it reaches Earth? What is the rate of solar mass loss to the solar wind? What are coronal mass ejections, and what is their relationship to the solar wind and to solar flares?

Chapter 18 (Properties of Stars)

Module 1 (Energy Production):

Write the equation relating energy to mass, labeling all quantities. Explain the relationship of this equation to fission and fusion, using the binding energy curve. Which of these two types of nuclear reactions is the energy source of stars? Why? Understand the Coulomb barrier. Explain why nuclear reactions involving charged particles typically occur in a narrow range of particle velocity (or energy).

Module 2 (Stellar Burning Stages):

What is hydrostatic equilibrium? What is the origin of a star’s interior pressure? Name the two sequences of reactions that convert hydrogen to helium. In some of these reactions, a proton is converted to a neutron; such reactions are called (fill-in-the-blank) interactions, and they involve the emission of difficult-to-detect particles called (fill-in-the-blank). Which of the two sequences of reactions involves catalysts? Explain what this means. Explain where and why the two different reaction sequences are dominant. Why is carbon the next heavier element after helium to be formed in stars? Name the process by which carbon forms. At what stage of a star’s life does it occur, and why? Why does fusion to heavier elements require higher and higher temperatures? How far can fusion continue?

Module 3 (Energy Transport):

Name and briefly describe four modes of transporting energy from its production site to the surface of a star? Which two modes are only important in special cases? What are these special cases? Draw simple pictures showing where the other two modes of energy production are important in stars of 0.1 solar masses, 1.0 solar masses, and 10.0 solar masses.

Module 4 (Solar Neutrinos):

Name the three sets (flavors) of neutrino/antineutrino pairs. Explain why neutrinos can give a glimpse directly into the core of a star, while light (photons) cannot. Give a one-sentence statement of the solar neutrino problem. Briefly explain how SNO’s ability to detect all three flavors of neutrino resolved the solar neutrino problem.

Module 5 (Stellar Distance and Motion):

What is the parallax angle of a star? What can this angle be used to determine? Define light year and parsec; what is the relationship between them? How close is the nearest star to us (other than the Sun)? Roughly, what is the largest distance to a star that can be determined by parallax, using a telescope above the atmosphere (e.g. the Hipparcos satellite)? Understand the difference between proper motion and space velocity. What are typical space velocities for stars? Space velocities we measure for other stars correspond to deviations from what average motion?

Module 6 (Stellar Magnitudes):

What is the logarithm of a number? Why does a small range in magnitude correspond to a large range in brightness? Do brighter stars have lower or higher magnitudes? What is the brightest star in the sky, and what is its apparent magnitude? Roughly, what is the apparent magnitude of the dimmest stars visible to the naked eye? Explain the difference between apparent and absolute magnitude. Why must wavelength be taken into account when discussing the brightness of stars? What are filters?

Module 7 (Harvard Spectral Sequence):

What characteristic of a star most strongly determines the nature of its spectrum? Starting with the class with the highest values of this characteristic, name the seven main spectral classes of the Harvard Spectral Sequence.

Module 8 (HR Diagram):

What does the HR stand for? What are the two axes in an HR diagram? Name the four main regions in an HR diagram, and know where they are on the diagram. Which region typically has the most stars, and why? What fraction of stars does this region contain? What is the ranges of masses and luminosities of stars on the main sequence? How is it known that the giants and supergiants are large, and that white dwarfs are small? What are luminosity classes? Explain how the HR diagram can be used to determine the distance to a star, if the star’s spectrum and luminosity class are known.

Chapter 19 (Multiple Stars and Star Clusters)

Module 1 (Binary Stars):

Explain the difference between visual binaries and astrometric binaries. For visual binaries, why do we usually not see the actual orbit? Around what point do the two stars in a binary revolve? Why must Kepler’s original laws for orbital motion be modified? Is it always possible to determine the masses of the individual stars in a binary? For stars on the main sequence, how is a star’s luminosity related to its mass? What are extrasolar planets? How have they been detected?

Module 2 (Spectroscopic Binaries):

What are spectroscopic binaries? State two ways a spectrum can be used to infer the presence of a binary system. In particular, describe what the Doppler effect is and how it can reveal the presence of binary. To what extent can stellar masses be determined in spectroscopic binaries?

Module 3 (Eclipsing Binaries):

What is a light curve? How can it be used to determine if a system is an eclipsing binary? Draw a simple light curve for a small star passing behind a larger star, labeling both axes. Indicate the regions on the light curve related to the diameters of the two stars. Write two equations---one for each star---to compute the diameters of the two stars. Carefully label the quantities in these equations. Tell how the quantities (other than the diameters) are measured, so that the diameters can be computed.

Module 4 (Accreting Binaries):

What is an accreting binary? Under what conditions might gas particles in the outer layers of one star be unstable against being transferred to another star? Understand what the inner Lagrange point and Roche lobes are. What is an accretion disk? Why does it form? What role does accretion play in nova outbursts, X-ray bursts, and Type-Ia supernovae? What do accretion disks have to do with observationally inferring the presence of a black hole?

Module 5 (Open Clusters):

What is the difference between clusters and constellations? Between clusters and associations? State three differences between open clusters and globular clusters. What are Messier objects? Briefly describe how an open cluster is formed. Are all open clusters about the same age? Describe how an HR diagram can be used to determine the age of an open cluster.

Module 6 (Globular Clusters):

How does the density of stars (number of stars per unit volume) near the center of a large globular cluster compare with the density of stars near the Sun? Approximately how many globular clusters does our galaxy have? Is this number approximately the same for all galaxies? What did careful observation of globular clusters tell about the size of our galaxy and our position in it? Is the formation of globular clusters well-understood? What are the primary differences of the HR diagram of a globular cluster from the HR diagrams of stars near the sun or of open clusters? Why do HR diagrams of globular clusters not have any supergiants? (the answer to this is in an animation on the evolution of HR diagrams)

Chapter 20 (Star Birth and the Main Sequence)

Module 1 (Recipe for Stars):

What are the birthplaces of stars? Why does the composition and temperature of these clouds make them dark, and difficult to detect? How can they be detected? How can astronomers peer inside them? Understand six main principles that govern the formation of stars. What is the Jeans density? Compare the size (spatial extent) of the cloud which collapsed to form the Sun with the size of (a) the present-day size of the solar system and (b) the size of the Sun. Why is it believed that fragmentation of collapsing clouds must occur? Recognize five sources of density changes that can induce collapse in molecular clouds. Which are most important in normal, non-colliding galaxies?

Module 2 (Protostars):

What is the protostar phase? Explain why T-Tauri stars are thought to be stars in the process of formation that are still contracting to the main sequence. Understand the evolutionary tracks of collapsing protostars on an HR diagram: How do paths for low- and high-mass protostars differ (in both shape of the path and time spent on it)? How strong is the dependence on composition? What causes the change from a vertical path on the HR-diagram to a rather horizontal one? What event marks the arrival of the star on the “main sequence”?

Module 3 (The Main Sequence):

What is the longest and most stable period in a star’s life? What is happening in the star during this time? What is the most important quantity that determines the destiny of a star? In an HR diagram, why is the main sequence a narrow band instead of an infinitesimally thin line? What are red dwarfs? What determines the lowest possible mass of a star? Roughly how small is this smallest mass? What are brown dwarfs? Do they release any energy? If so, what is the source of that energy? Characterize their range of masses. In identifying brown dwarfs, how can one tell it is not a planet? The presence of what two compounds indicate it is not a star? Why? What determines the maximum possible mass of a star? Roughly what is this mass? Can stars ever expel their outer envelopes?

Chapter 21 (Star Death)

Module 1 (End of Main Sequence Life):

Do more massive stars stay shorter or longer on the main sequence? Why? For isolated stars, briefly describe what happens to the lowest mass stars, intermediate mass stars, and the most massive stars after they leave the main sequence. Why can the evolution of binary stars be more complicated? What is the “ash” left behind by hydrogen burning in the core? What changes occur in the core after most of the core’s hydrogen has been consumed? Describe what hydrogen shell burning is, and why it starts. After this, what are the next two burning stages that occur? (One is in the core, and one is an additional shell.) Does hydrogen continue burning anywhere in these later stages?

Module 2 (Red Giants):

Stars in what range of masses will become red giants after leaving the main sequence? What causes the hydrogen shell source to burn hotter? What is the consequence to the outer envelope of the star of this hotter hydrogen shell burning? What event marks the end of the red giant phase? What is a degenerate gas? Why do thermonuclear reactions “self-limiting” in an ideal gas, and tend to “run away” in a degenerate gas? Name four kinds of explosions in astrophysical environments that are caused by thermonuclear runaways in a degenerate gas. What are the two ways helium burning can begin? Which stars do each of these two ways? What burning is going on in the cores of stars on the horizontal branch of the HR diagram? In the asymptotic giant branch? In this stage, what becomes the primary means of getting energy out of the core?

Module 3 (Planetary Nebulae):

What are planetary nebulae? Where does the name come from? When they have a ring-like appearance, how does this correspond to the actual shape? What happens to mass in the outer envelopes of stars in the red giant branch and asymptotic giant branch phases? What are three consequences of mass loss in the asymptotic giant branch phase? While a newly-born white dwarf is very hot, why might one say that technically it is not a star?

Module 4 (White Dwarfs):

How were white dwarfs discovered? Have they ever been directly imaged? Characterize typical masses and diameters of white dwarfs. Who discovered that there is a maximum upper mass for stars composed of a degenerate gas? What happens as mass is added to a white dwarf? What defines this limiting mass? Observationally, what is a nova? Briefly describe what causes it.

Module 5 (Variable Stars):

Define variable star and light curve. The variability (change in brightness) of Cepheid variables is a direct consequence of what? Describe how Cepheid variables can be used to determine distances to other galaxies. Name another kind of variable star most commonly found in globular clusters. Why can they not be used to determine distances as far as one can with Cepheid variables? How did these two kinds of variable stars come to get their names? How is a variable star’s luminosity related to its period of pulsation? What underlies this relationship? What is interstellar dust? What kind of variable star may be a major source of this dust?

Module 6 (Supernovae):

What are supernovae? Characterize their brightness. Roughly how often does a supernova occur in our galaxy? When was the last supernova observed in our galaxy? How can this discrepancy be explained? Compared with this last supernova in our galaxy, when was the next-brightest supernova observed? When and where did SN1987A explode? Describe the standard mechanism of Type Ia supernovae. What is a standard candle? Compare the range of distances that can be determined with Cepheid variables and Type Ia supernovae. How do the luminosity and frequency of Type II supernovae compare with those of Type Ia? Burning up to what element can occur in a massive star? Why does the core eventually collapse catastrophically? What causes the disruption of the outer envelope after core collapse? How long does this process take? What are the two possible fates of the collapsed core? What was detected in SN1987A besides electromagnetic radiation (light)? What did this provide evidence for? What are supernova remnants? How are they related to you and me?

Module 7 (Heavy Element Production):

What is nucleosynthesis? Where was most of the hydrogen and helium in the universe produced? How is some additional helium produced? How are elements up to iron produced? What are the general trends in temperature and timescales of the various burning stages leading up to iron? Why can elements beyond iron not be produced in the same way as elements up to iron? The capture of what particle is the key to production of elements beyond iron? What is the key property of this particle that makes this process work?

Chapter 22 (Neutron Stars and Black Holes)

Module 1 (Neutron Stars):

Under what circumstances will the endpoint of stellar evolution be a neutron star? Are neutron stars made only of neutrons? What are typical masses and diameters of neutron stars? When was the existence of neutron stars predicted on theoretical grounds? When and how were they finally observed? Neutron stars have a density comparable to densities in (fill in two blanks). True or false: Neutron stars have been directly seen in X-ray images. Why are neutron stars sometimes observed to be moving through space at high speed (kick velocity)? Is it common for a binary to remain a binary after a supernova explosion? Under what conditions is it possible? How do the magnetic fields of typical radio pulsars and magnetars (two kinds of neutron stars) compare with Earth’s magnetic field? Compare and contrast X-ray bursts and novae.

Module 2 (Pulsars):

What is a pulsar? In what wavelength bands have pulsars been detected? Which is most common? In the lighthouse mechanism of pulsar emission, is the rotation axis aligned with the beam of radiation? Why might not all neutron stars be seen as pulsars? What is the range of pulsar periods? What happens to the spin rate of a pulsar as it radiates away its energy? If pulsars tend to spin slower as they age, how could it be that some of the fastest-spinning pulsars are very old? In what year was the supernova giving rise to the Crab Nebula observed? Who recorded the supernova’s appearance? How do we know that the pulsar in this nebula is powering the nebula’s energy output? Why did the discovery of a binary pulsar garner a Nobel Prize? What is a magnetar?

Module 3 (Stellar Black Holes):

Understand the following concepts: gravitational redshift, black hole, and event horizon. In principle (if not in practice), could any object of any shape become a black hole? What does it take? What causes the X-ray emission that may indicate the presence of a black hole? Suppose there is a massive star in a binary with a companion that is invisible except for X-ray emission, and that the mass of the unseen companion can be determined; how does one tell if the unseen companion is a black hole? Define the inner horizon and outer horizon of a rotating black hole. What limits the energy that could (in principle) be extracted from a black hole?