• Review the defintion and use of the magnitude scale (see also Part C in this review).
  Magnitudes measure the brightnesses of stars, both their apparent brightness AND their true brightness. The true energy output of a star is known as its absolute magntiude and also its luminosity.
• Review the stellar spectral classification system devised by astronomers.
  • Spectral classes are assigned letters: O,B,A,F,G,K,M
  • How are spectral types assigned ?   What information is used for this purpose ?
  • What property of a star defines its spectral type ?
  • How is spectral type related to the surface temperature of stars ?
  • How is a star's color related to surface temperature ?
• Luminosity class is a second type of stellar classification - defining the range in stellar brightnesses. Know the difference between a dwarf, giant and supergiant star. Which is brighter ? Why ?
• Be familiar with the range of stellar properties:
  • Masses: from 0.08 M(sun), which are the faintest M dwarfs, to 100 M(sun), which are very luminous O stars.
  • Surface Temperature: from 50,000 K (O stars) to 3000 K (M dwarfs).
  • Size (radii): from ~500 times the Sun's radius (supergiant stars) to 0.1 R(sun) (M dwarfs).
  • Luminosities: 10⁵ times L(sun) for O stars, to 10^-4 L(sun) for M dwarfs.
• What is the composition of typical stars ?   Does this vary among stars ?

• Understand the appearance and purpose of an HR diagram. What stellar properties are plotted on the HR diagram ?   What units are used ?
• Know the principal regions within the HR diagram: the main sequence, giants, supergiant region, white dwarfs.
• Know which regions of the HR diagram are heavily populated with stars, and which appear sparse.
• Know that the Sun is a dwarf star, spectral type G2, "located" on the main sequence.

• What is a binary star ?
• Qualitatively describe the orbits of stars in binary systems, and some of the resulting phenomena.
• Define the various types of binaries:
  • Visual binaries
  • Astrometric binaries
  • Spectroscopic binaries
  • Eclipsing binaries
• Understand how binary star systems are vital for measuring the masses of stars, using the orbital properties of a binary system and the law of gravity.

• Review the difference between interstellar gas and dust.
• What is the composition of interstellar gas ?   Of interstellar dust ?
• Dust in interstellar space is found in the form of grains. What is known about these grains ? Be aware of astronomer's ideas about their shape, size, and temperatures.
• Understand how interstellar dust affects starlight:
  • By absorbing passing starlight, a process labeled extinction, which causes stars to appear fainter.
  • By selective absorption (prefering blue photons over red photons) so that stars shining through dust appear redder.
• Review the distinction between:
  • Absorption nebulae, those that block the light of distant stars
  • Emission nebulae, those that glow due to the effects of starlight
• Be familiar with the different types of nebula: those associated with star birth: HII regions, molecular clouds, and those associated with stellar death: planetary nebula, supernova remnants.
• What is 21cm radiation ? In which part of the electromagnetic spectrum is this radiation emitted. Why is the detection of this radiation important ?
• Understand the range of properties of interstellar gas and dust clouds-- especially the dense, dark clouds which contain large quantities of cold gas in molecular form.

• Star formation occurs within the "nursery" of giant molecular clouds (GMC). Be able to describe a GMC, and why it is termed "giant" and "molecular".
• Review the process by which a blob of gas+dust collapses to form a star.
  Q? Which forces are primarily responsible for the formation of stars ?
• A collapsing gas+dust cloud passes through several "phases", as described by current theories. Define these phases.
• Understand how the duration of the star formation process changes with respect to the mass of the newly-formed star. Stars more massive than the Sun collapse faster, stars less massive collapse at a slower rate.
  How long does it take for stars to form ?
• Define a brown dwarf.   Have these objects been spotted ?

• Understand the different types of star clusters, their appearance and stellar content.
  • Open clusters: mostly young stars, loosely arranged, containing 10-1000 stars, most stars located on the main sequence.
  • Globular clusters: old stellar systems; stars tighly packed in an approximately spherical arrangement, containing 10⁴ to 10⁶ stars, no stars more massive than the Sun, many red giant stars.
• Look at the star cluster HR diagrams presented in the text. What do they tell us about the cluster ?

• Understand why stars evolve - exhaustion of the appropriate fuel for nuclear burning in their cores with time.
• Review the basic evolution of a star like the Sun. Know the different phases, why they occur, and the approximate duration (in years) of each phase.
  • Main sequence phase:   The Sun fuses hydrogen into helium in its core, 90% of the Sun's lifetime is spent on the main sequence.
  • Subgiant phase:   The fuses hydrogen in a shell around an inert (non-fusing) helium core. The Sun becomes cooler and slowly expands.
  • Red giant phase:   The core contracts, hydrogen-shell buring continues, the star becomes brighter as it expands to about 100 times the size of the Sun.
  • Horizontal branch phase:   After igniting helium in its compressed core, the Sun evolves to a horizontal branch star, with a hydrogen burning shell surrounding a helium burning core.
  • Asymptotic giant branch:   As the core is depleted of helium, both hydrogen and helium fuse within narrow shells, increasing the Sun's luminosity. The Sun becomes cooler, redder, brighter and larger
  • Planetary nebula phase:   When the core is exhausted, the outer layers are ejected and become a planetary nebula.
  • White dwarf phase: &nbps The dead core of the Sun becomes a white dwarf, a compact stellar remnant, with a mass between 1.4 and 0.5 solar masses, packed into a radius comparable to Earth's.
• Review the fusion processes in stars, which provides a star's energy, and alters its composition, creating new, heavier elements.
  • The proton-proton chain (p-p)
  • The CNO cycle (MP 20-1 in the text)
  • The triple-alpha process, which creates carbon from helium
  • Heavy element fusion, leading to iron
• Understand the evolutionary path of a high-mass star. These stars are able to fuse light elements up to iron, but no further.
• Be familiar with the relative lifetimes of different types of stars. This lifetime depends on the star's initial mass. Solar-mass stars live 10-11 billion years, O stars live just a few million years, M stars may last 100 billion years.
• Review the properties of white dwarfs, their origin, and their evolutionary paths.

• Review how a star in a binary system may evolve differently if its companion is located in a close orbit.
• Review the definition of a Roche lobe.

• Know the difference between a nova and a supernova.
• Know the basic cause of a Type I supernova -- a white dwarf within a binary star system, which has accreted too much additional mass, exceeding the the so-called Chandrasekhar limit.
• Define an accretion disk. Where might astronomers find one ?
• Know the basic cause of a Type II supernova -- a massive star, with an iron core, which collapses onto itself. The star explodes, leaving behind a neutron star.
• Describe the timeline of events as a supernova explosion occurs.
• Q? What is left behind after a Type I SN ?   a Type II ?
• Be able to describe the evolution of a supernova remnant, the expanding shellof gas blasted out into interstellar space by a supernova explosion. The Crab nebula is one example.

• Understand the general pattern of element formation in nature:
  • Hydrogen(1), helium(2), and small amounts of lithium(3), beryillium(4) and boron(5) formed in the Big Bang. [the number refers to the place in the periodic table occupied by that element.]
  • The light elements helium(2), oxygen(8), and carbon(6), are formed in solar type stars, although most of these elements remain "locked up" in their final evolutionary state-- a white dwarf. A fraction of these elements are ejected as planetary nebulae.
  • Middle-weight elements like sulfur, silicon, magnesium, neon, up to iron form in massive stars during normal fusion reactions. These elements are ejected during supernova explosions.
  • Elements heavier than iron (gold, lead, uranium) form during supernova explosions, when atomic nuclei can capture large numbers of neutrons. These elements are ejected back into space to eventually be the ingredients for new stars and planets.
• Develop a rough idea of the relative abundances of elements within the universe.   Which element is most abundant ?

Distances and Motions of Stars:

• Understand why the pattern of stars in the night sky does not change appreciably during human lifetimes, despite the fact that most stars are moving through space at scores to hundreds of kilometers per second.
• Visualize the motions of stars. Contrast motion across the sky with motion along the line-of-sight to a star. How are these measured ? How would these motions change if a star was moved closer/further from Earth, all else being equal ?

• Stars appear as unresolved points of light even in the largest telescopes. How are stellar dimensions (sizes) determined ?
• The intensity of light from a source becomes fainter according to the inverse-square rule. What does this mean, and why does light fade in this manner ?
• The observed brightness of stars depends on their (a) true brightness (luminosity) and (b) their distance to us. Observed brightnesses of stars are measured using telescopes and CCD detectors. How are a star's true brightness and distance determined ?
• What information can be obtained from stellar spectra, that can be used in the classification process ?

• Understand what is meant by a star's "location" in the HR diagram.
• Be cognizant of the significance of the main sequence.
• Understand why the HR diagram of nearby (to the Sun) stars is populated with stars along the lower main sequence, while an HR diagram of bright stars in the sky contains many giant and supergiant stars.
• Classification of stars allows us to estimate their distance via the concept of spectroscopic parallax. How does this work ?
• Interpretation of the HR diagram has resulted in major discoveries related to stars.
  • Which stars are most common in the nearby universe ?
  • Which stars are the least common ?
  • Which stars the hottest ?   coolest ?   most massive ?   least massive ?
  • Which properties of a star defines its location on the HR diagram ?

• Consider how interstellar gas appears.
  What causes interstellar gas to be visible to us ?
  What type of spectrum does an emission nebula have ?
  How is this spectrum different from the spectra of stars ?
• How do astronomer's observe interstellar gas and dust ?   In what portions of the spectrum does the gas and dust radiate ?

• What happens to a collapsing gas cloud as it passes through each evolutionary stage from diffuse interstellar cloud to newborn star ?
• A protostar is the name given to a collapsing interstellar cloud when it has become approximately spherical and radiates mostly in the infrared. Define the properties of a typical protostar, for example one that will eventually become a star like the Sun.
  Is the protostar brighter/fainter, hotter/cooler, bigger/smaller than its final configuration?
• Q? When does a protostar become a "star" ?
• Be aware if the concept of an evolutionary track, the "path" of a star within the HR diagram, which traces its change in temperature and luminosity as it ages. Be able describe these tracks.
• Understand the reason that stars are found to have an upper [~100 times M(sun)] and lower mass limit [0.08 times M(sun)]. Why can't star be more or less massive than these values ?

• Be aware of the usefullness of star clusters in the understanding of stellar evolution.
• Describe the change in stellar content of clusters as the cluster ages. How do their HR diagrams change with time ?

• Stars are stable for most of their lifetime. Review the reasons for this stability.
• When stars begin to run out of nuclear fuel in their cores, the core and the rest of the star respond in specific ways. The Sun, for example will get brighter when this occures.
  Why do stars respond in this manner ?
• Understand the distinction between low-mass and high-mass stars. Stars more massive than about 8 solar masses become supernova, stars less massive than 8 solar masses end up as white dwarfs.
  Why does the Sun cease fusion reactions ?   Why do high-mass stars end their lives differently ?

• Why might binary stars share mass ?
• Define possible evolutionary outcomes for binary star systems.

• Consider the origin of novae.   Why must they be located only in binary star systems ?
• Q? Why can a nova repeat, but not a supernova ?
• Two famous and well-studied supernova remnants have greatly aided our understanding of stellar evolution, the Crab Nebula, and SN 1987A. What is their significance ?

• Why can't stars fuse elements heavier than iron ?
• How are new elements returned to interstellar space ?