Neutron Stars (NS):

• Understand the origin of neutron stars - the compact remnant of a massive star supernova (Type II) event. Review the seqence of events which lead to this type of supernova.
• Know the basic physical attributes of a neutron size: typical size, surface temperature, spin rate, mass (in solar units), density.
• Review our understanding of the interior structure of a neutron star. Are they solid, liquid, or gaseous ?
• Neutron stars, like all stars, can occur in binary systems. Be aware of the possible observational consequences of NS binary systems, especially the phenomenon of X-ray bursters.

• Understand the nature of pulsars: rotating, neutron stars.
• How fast do pulsars rotate ? Does their rotation rate stay constant during their lifetime, slow down or speed up ?
• Review the observational aspects of pulsars: we observe a "pulse" of electromagnetic radiation at some rate. These pulses are observed from one end of the spectrum to the other. Be aware of what is meant by a "pulse".
• How and when were pulsars first discovered ?
• Understand how the pulsar phenomenon was explained as originating in a rotating neutron star.
• What is the Crab Nebula pulsar, and why is this particular pulsar important in understanding the connection between pulsars, neutron stars, and supernova explosions ?
• Millisecond pulsars, discovered in the 1980s, appear to be a weird variant of pulsar, neutron stars that have been "spun up" by accreting matter from a companion. Review astronomer's ideas on how these pulsars might have formed.

• What is meant by a gamma-ray "burst" ?
• Why were GRBs not discovered until the 1960s ?
• Review the locations on the sky from which these bursts originate. GRBs are observed to occur in all directions of the sky. This piece of evidence has convinced astronomers that GRBs are cosmic phenomenon -- objects at great distances from Earth, occuring in all corners of the universe.
• Q?   Why must we use space-based observatories to locate and study GRBs ?
• What action did NASA recently take to further study this enigmatic phenomenon ?

• Be aware of the basic definition of a "black hole". BHs are ultra-compact regions of space in which mass is crushed under its own gravity to "infinite" density.
• Radiation cannot escape a BH: the escape speed is greater than the speed of light.
• Know the definition of the event horizon and the Schwarzschild radius of a black hole. How does the Schwarzschild radius depend on the mass of the hole ?
• Understand the fate of matter which falls into a BH - lost forever from the universe.

• Review how astronomers might detect a black hole - through X-ray radiation from heated, colliding matter as it swirls into a BH.
• Q?   What is Cygnus X-1 ? What is its signficance to the hunt of black holes ?
• Why must astronomers use space-based observations to study black holes ?

• Understand how the MW appears to us on Earth, when viewed without optical aid. How does that appearance relate to the Milky Way's actual structure ?
• Be aware of how the MW appears when viewed through a telescope. The faint band of diffuse light seen with the eye is resolved into millions of stars, dark (dust) clouds, bright emission nebulae, and numerous multiple star systems and star clusters.
• Review the basic picture of our Galaxy: a disk galaxy, about 100,000 light years(30,000 parsecs) in diameter, containing about 200 billion stars.
• The Sun is located about 1/2 of the way out from the center.
• Be cognizant of the 3-D structure of our home Galaxy:
  • A flattened disk, 30 thousand parsecs in diameter, about 300 parsecs thick, 100 times larger in diameter than in thickness. The Sun is located within the disk.
  • Within the disk there are spiral-shaped concentrations of young, blue stars, termed spiral arms.
  • The halo: a sparsely-populated spherical region containing old stars. Globular clusters, dense collections of up to a million stars also populate the halo. The Milky Way has about 150 globular clusters.
  • The central bulge: a region within 1 kiloparsec of the center, populated by old, red stars, which is "thicker" than the disk.
  • The Galactic Center: located in the direction of the constellation Saggitarius. Not visible optically, due to the extinction of light produced by intervening dust clouds. Motions of gas clouds around the center indicate that the nucleus is very small and very massive, with a few million solar masses of gas within a region not much bigger than the solar system. Probably contains a massive black hole.
• Review the pattern of motions of the stars and gas within the Galaxy.
  Q?   How do the stars in the disk move ?   How do stars in the bulge move ?
• Understand how observations of the motions of stars in the MW, obtained from proper motion and radial velocity measurements of stars and gas clouds, indicates the presence of unseen matter.
• The total mass of the MW appears to be about 6 × 10¹¹ solar masses. How is this determined ?
• Review the techniques used by astronomers to probe galactic structure. Understand how these bits of information are obtained and how they fit together to build a picture of our Galaxy. Be cognizant of how each of the following techniques is applied and what information each provides.
  • Star counts (in different directions around the sky)
  • Proper motions of stars
  • Radial motions of stars and gas via measurment of the Doppler effect on their spectra.
  • Radio observations of neutral hydrogen
  • Distance estimates to various components of the galaxy using Cepheid variables, and spectroscopic parallax.
• Review the principal evidence for dark matter within the Milky Way. How was this evidence uncovered ?
• Q?   What are spiral arms ?   What happens when gas in the disk of a spiral galaxy flows into an arm ?

• Understand the galaxy classification system used by astronomers, originally developed by Hubble. A galaxy's classification depends on its shape, optical color, gas content, and star populations. Review the types of galaxies, and what their defining properties.
  • Elliptical galaxies:   Spheroidal systems, containing negligible amounts of cold gas and dust. Contains mostly old (10+ billion years) stars and appear red in color. Stars orbit in random directions; there is no net rotation.
  • Spiral galaxies:   Flattened systems containing a disk, with gas, dust and young stars, which surrounds a bulge, containing mostly old, red stars. Stars in the disk orbit in roughly circular direction around the center; stars in the bulge have more random orbits.
  • Irregular galaxies:   Relatively young, unevolved stellar systems, rich in gas, dust, and young stars. Show no well-defined overall structure.
• Be cognizant of the "neighborhood" of our home galaxy, the Milky Way.
  • The Galaxy exists in a collection of galaxies, known at the Local Group.
  • The Local Group consists of 3 spiral systems: the Milky Way, Andromeda, and M33, the Large and Small Magellanic Clouds (both irregulars), in orbit around the Milky Way, and about 30 small, faint, dwarf galaxies.
  • Galaxies in the Local Group interact through their gravity.
• Be aware of the typical sizes of galaxies and their average separations, both measured in thousands of parsecs (kiloparsecs).

• Standard candles are objects or phenomena whose intrinsic (true) brightnesses are known to good accuracy. Review how astronomers utilize standard candles to determine the distance to objects in space.
• One example of a standard candle are Type I supernova-- the detonation of an "overloaded" white dwarf located within a binary star system. Type I supernova appear to reach a fixed maximum brightness, then fade in a predictable way. They are extremely bright-- permitting their detection at great distances.
• Another extremely useful standard candle are Cepheid variables, stars that pulsate in a predictable way. If you observe a Cepheid, you can determine its period of variability, known as a light curve. This period is directly related to the star's true brightness: longer periods imply more luminous stars.
• Know which standard candles are useful at which distances, and why. Use the cosmic distance "pyramid" diagrams presented in your text.

• Understand the observational basis for "Hubble's Law": Spectra of almost all galaxies show a redshift: a shift of spectral lines (emission or absorption) towards the red end of the spectrum.
• Be aware of the meaning of Hubble'a law: the rate at which a galaxy recedes from us is directly proportional to its distance.
• Hubble's law can be used as a distance indicator-- objects with larger redshifts are more distant.
• Review the interpretation of Hubble's Law: that the universe as a whole is expanding, from an origin in time about 14 billion years ago.

• Astronomers have identified two types or mechanisms for supernova explosions. Understand the basic physical difference between these two types of supernova. What type of remnant is left behind ? Describe the overall sequence of events before, during, and after a supernova explosion.
• Know the difference between a white dwarf star, a neutron star, and a pulsar. Compare their sizes, masses, and structures.
• What keeps a NS from collapsing under its own gravity ?
• Understand the lighthouse model for a pulsar.   Why does the pulsar's spin produce "pulses" ?
• Pulsars do not always maintain a constant pulse rate. This rate can both slow down (the natural order of things) and in odd cicumstances, speed up. Why does either phenomenon happen ?

• Two different models for gamma-ray bursts have been proposed - what are they ?
• Review the fundamental mystery of GRBs - why do astronomers infer that their intrinsic (true) luminosity is so huge ?

• Review the specifics of a fall into a black hole. What does an external observer (the fortunate one) see ? What does an infalling observer (the UNforunate one) see and feel ??
• If radiation cannot escape from inside a black hole, how do we identify a black hole, or in other words, how do we "see" one ?
• Be aware of a prediction of Einstein's theory of relativity concerning light in strong gravitational fields, the phenomenon termed a gravitational redshift. Why is this relevant to black holes ?
• How might a black hole form ?

• Consider the conplexity of galaxies: collections of 10⁷ to 10¹² stars, gas and dust clouds with a range of densities, temperatures, and motions, plus the dark matter. Review how these different components are organized in a typical galaxy.
• Be cognizant of the reasons astronomers give for the existence of dark matter in the universe. What does "dark" mean in this context ? How do we infer the presence of dark matter ?
• Understand the basic scenario of galaxy formation: from diffuse gas clouds, collapsing under gravity, into a flattened disk in which stars form.
• Understand why galaxies interact more frequently than individual stars. What forces are operating when galaxies collide ?
• Review the concept of a standard candle. How is an object or phenomenon serving as a standard candle used to find distances ?   Why is this important ?
• Hubble's "law" implies that the universe is expanding. Be aware of the the properties of a galaxy which are related through Hubble's law, and the implications of Hubble's law for the origin of the universe.