Here is some advice and responses to frequently asked questions about study guide emails.

(103)

Two of the most easily recognizable constellations that are up in the sky at this time of year are Ursa Major and Cassiopeia. Find the following information about these two constellations:

• Approximate location in the evening sky this month, based on the star chart on in the book's appendix for Jan/Feb/Mar (rough altitude and azimuth),
• star chart (sketch of the pattern of the 6-8 brightest stars that make up the constellation),
• name of the two brightest stars,
• 2-3 sentence summary of the mythology behind the constellation.

Remember the links: for mythology it is http://www.emufarm.org/~cmbell/myth/myth.html. For simple star charts, it is http://www.dibonsmith.com/constel.htm (but I would prefer you use the back of your book star chart given above). For star names and other information, try http://www.astro.wisc.edu/~dolan/constellations/. (TQ)

(104)

How does mass transfer occur between stars in a binary system? How can a binary system evolve to the point where one star is a white dwarf and the other is a red giant? Why don't they both evolve in the same way simultaneously?

(105)

Describe how nova explosions occur in binary systems. Explain why recurrent nova systems may be candidates to explode as supernovae in the future.

(106)

Who is Jocelyn Bell? Briefly summarize and explain her famous discovery. (TQ)

The following four questions can be answered using http://solomon.as.utexas.edu/~duncan/magnetar.html as a resource. You may skip over the sections "for advanced readers" if you like since none of the questions are answered in those sections.

(107)

What is a magnetar? What is a SGR, and how did it get this name? (TQ)

(108)

What was the source of the famous (to Astronomers, anyway) "March 5th" event of 1979. How was the location of this source pinpointed? (TQ)

(109)

In the August 27 section of the same "Magnetar" web page, there is an interesting story about how a new SGR was detected. Explain how and why the Earth's ionosphere changes in response to a gamma-ray burst (this is one way Astronomers detected the outburst, by monitoring changes in the Earth's ionosphere). (TQ)

(110)

Explain why AM radio signals carry further at night than they do in the daytime. If you are interested in more detail on this August 27 event, you can read about it further here. (TQ)

(111)

What is a pulsar? How is it related to a neutron star? Why do some neutron stars appear to be pulsars on Earth while most do not? Your book can help you with this one.

(112)

What is a black hole? Explain how we use the concept of escape velocity to define the boundary (event horizon) of a black hole.

(113)

Explain why direct observational evidence of black holes is not practically possible.

(114)

Explain how astronomers use observations of binary star systems to indirectly "prove" the existence of black holes.

(115)

Explain how the force of gravity of a black hole is similar to and different from a normal object like the Sun. Be able to answer questions like, "The Sun's radius is 400,000 miles. Who would feel stronger gravity? An astronaut at the surface of the Sun or an astronaut 400,000 miles from a one-solar-mass black hole? Explain. What if the distance from the center of both were 100 miles? Explain."

(116)

What are tidal forces? Why can black holes exert much stronger tidal forces on some objects compared to, say, stars?

(117)

How do we know that there is a supermassive black hole (20 million times the mass of our Sun) at the center of our galaxy? Will this black hole eventually suck up all the material in our galaxy? Why or why not?

(118)

Occasionally, there are both gas clouds and stars in the halo. Explain why the gas clouds eventually became part of the disk while clusters and stars remain in the halo.

(119)

Explain why regions of young stars or regions with lots of star formation (like the disk) tend to have a blue color. Explain why regions with mostly older stars and no recent star formation tend to appear red, orange or yellow.

(120)

Explain why the spiral arms are so much brighter than the rest of the disk.

(121)

Given the equation of orbital velocity, be able to explain the basic shape of the Keplerian rotation curve. Why does our solar system follow the Keplerian curve while Astronomers initially anticipated the rotation curve of the Milky Way galaxy would not follow this curve (but instead have orbital velocities slightly higher than the Keplerian prediction)?

(122)

Explain how the flat rotation curve of our galaxy leads us to believe that the galaxy has a very large amount of dark matter, much more than the visible matter in stars, gas and dust that we can easily see.

(123)

The image located at http://imgsrc.hubblesite.org/hu/db/1994/41/images/b/formats/web_print.jpg tells the story of a search with the Hubble Space Telescope search for very low mass stars in a representative sample of nearby stars, the core of a globular cluster. Use this to help explain the evidence that tells us very low mass stars do not account for a significant portion of the dark matter in our galaxy. (TQ)

As an aside here, if you are interested in finding out about the status of our hunt for WIMP's as dark matter sources, you can read more here, but this is not required for the study guide.

For the next several questions, please read the article "The Search for Dark Matter" from the March 2003 issue of Scientific American. For help in retrieving Scientific American articles from the TCU Library digital archive, see previous study guides.

(124)

Explain two reasons (one having to do with mass, one with temperature) that neutrinos are not believed to make up the bulk of dark matter in galaxies. (TQ)

(125)

Why do expect dark matter detections to vary between Winter and Spring (which would distinguish such detections from random noise caused by other particles)? (TQ)

(126)

What are two possible effects described in the article that a neutralino might have on ordinary atoms if they interact with the atoms? (TQ)

(127)

Explain how using two detectors helps us distinguish from dark matter interactions and other (not so interesting) kinds of interactions, like cosmic rays or radioactive decay? (TQ)

(128)

Explain how we use gravitational lensing to discover dark matter in the form of planet-sized MACHO's. Name and explain two ways that stellar brightness variations due to lensing differ from variations due to other more natural causes, like pulsation or binary system eclipses.

(129)

Describe what our studies of gravitational lensing have led us to conclude about MACHO's and dark matter.

(130)

Why does it look like we will be unlikely to look for solitary black holes (a dark matter candidate) via lensing in the near future? Explain.

(131)

What are Cepheids? Explain how we can use Cepheids to find the distances to other nearby galaxies (like Hubble found the distance to the Andromeda galaxy).

(132)

Be able to answer questions about Cepheid distance determination, such as: "Compare the light curves of two Cepheids (for example, where the period of star A is twice as long as the period of star B). Which Cepheid is more luminous? If both stars have the same apparent luminosity, which is further away?"

(133)

Briefly summarize how the standard candle method of distance determination works. Explain why individual stars (even bright Cepheids) are not useful as standard candles for very distant galaxies.

(134)

Explain how the standard ruler method of distance determination works. Why is this method unreliable? Explain how we can use statistical arguments (e.g. using the largest/brightest galaxy in a cluster as a standard instead of a randomly selected galaxy) to improve the technique.

(135)

Explain why galaxies are not very useful as standard candles. Explain how and why we can use statistical arguments (e.g. using the brightest galaxy in a cluster as the standard) to improve the technique.

(136)

Why are Type Ia supernovae such great standard candles? What do you have to do to find these standard candles (can you just look at any galaxy and find an ongoing supernova)? Why are they so difficult to find?

(137)

Explain the role that parallax and the Cepheid distance determination technique played in figuring out the absolute luminosities of Type Ia supernovae, which were then used to generate Hubble's Law.

(138)

Explain why there is a limit (known as the Eddington limit) on how massive (and therefore how luminous) a star can be. Answer this in terms we're familiar with from stellar structure, such as outward-pushing pressure, gravity, hydrostatic equilibrium, etc. (TQ)

Wendy Freedman was the astronomer who led the Hubble Space Telescope's key project to find the distances to Cepheids in many other galaxies in which type Ia supernovae were located. This process, also called "calibration", established the luminosity of these supernovae so that we could use them as standard candles in more distant galaxies. As such, she played a crucial role in establishing the distance scale of the Universe.

I highly recommend (but do not require) that you read her article "The Expansion Rate and Size of the Universe" from the Scientific American online archives (Spring 1998 special edition on the "Magnificent Cosmos"), as it summarizes many different distance determination techniques (some of which we will cover in lecture) and also puts this all into the context of cosmology. That is, the article helps explain why we care so much about these distances and how important accurate results are to our theory of how the Universe works. It will make very good background reading for topics I will move through fairly quickly in lecture. Your textbook's discussion of the distance scale is also good but not quite as detailed or specific as Freedman's article.

The following four study guide questions are based on the article "The Brightest Explosions in the Universe" from the June 2004 Special Edition of Scientific American, by Neil Gehrels and collaborators. Surprisingly, the title does NOT refer to supernova explosions!

(139)

How do we know that Gamma Ray Bursts do not originate within our own galaxy, the Milky Way? (TQ)

(140)

Explain we are able to deduce (for example, in the case of GRB970508) that Gamma Ray Burst sources expand to a large size very quickly. (TQ)

(141)

Our initial estimates for the luminosity of GRB's was extremely high (over a billion times brighter than a supernova) based on the assumption that they radiate equally in all directions. Ifwe assume the GRB's emit their energy in the form of a jet, then our estimate of their luminosity drops considerably. Explain why. (TQ)

(142)

What are "ghost" GRB's, and what is the accepted explanation for their differences compared to normal GRB's, according to recent observations? (TQ)

(143)

Explain what the Tully-Fisher (TF) relation is. Describe briefly how one would use the TF relation to find the distance to an edge-on galaxy. How is the rotation velocity of a distant galaxy determined?

(144)

Explain why the TF relation is distance limited (describe the role played by the galaxy's tilt in the method). In other words, why doesn't it work as a distance determination technique beyond a certain distance from the Earth?

(145)

What is Hubble's Law? Explain how it can be used to determine the distance to a galaxy. How is the radial velocity of a distant galaxy determined?

(146)

Explain why the Hubble relation, which indicates that all galaxies seem to be moving away from our location, is not a violation of Copernican Principle.