Physics 20083 - Study Guide #3

Updated through Friday, March 27. Current study questions can be found here.

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

(93)
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 Ursa Major and Cassiopeia:

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)

(94)
Who is Jocelyn Bell? Briefly summarize and explain her famous discovery. (TQ)

(95)
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.

(96)
What is a black hole? Explain how we use the concept of escape velocity to define the boundary (event horizon) of a black hole.

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.

(97)
What is a magnetar? What is a SGR, and how did it get this name? (TQ)

(98)
What was the source of the famous (to Astronomers, anyway) "March 5th" event of 1979. How was the location of this source pinpointed? (TQ)

(99)
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)

(100)
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)

(101)
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."

(102)
What are tidal forces? Why can black holes exert much stronger tidal forces on some objects compared to, say, stars?

(103)
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?

(104)
Explain why direct observational evidence of black holes is not practically possible.

(105)
Explain how astronomers use observations of binary star systems to indirectly "prove" the existence of black holes.

(106)
Explain how and why the age of a star is related to its metallicity? Does the metallicity of a specific main sequence star change over time? Explain your answers.

(107)
Be able to answer (with explanations) questions comparing the metallicity of two stars, such as "Star X is the same size, mass and temperature as the sun, but it is about 3 billion years older. Which has a higher metallicity, X or the sun?", "Star X is a main sequence star with an unknown age that has a mass ten times that of the Sun. Which has a higher metallicity, X or the sun (or can you not tell)?"

(108)
Explain the relationship between age and color for individual stars. Are blue main sequence stars young? Explain. Are red main sequence stars old? Explain.

(109)
More examples of age/metallicity questions: "Star X is a blue main sequence star, and star Y is a red main sequence star. Which has a higher metallicity, X or Y (or can you not tell)?" and "Star X is a main sequence star with an unknown age that has a mass one half that of the Sun. Which has a higher metallicity, X or the sun (or can you not tell)?"

(110)
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.

(111)
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)?

(112)
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.

For the next four 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.

(113)
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)

(114)
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)

(115)
What are two possible effects described in the article that a neutralino might have on ordinary atoms if they interact with the atoms? (TQ)

(116)
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)

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!

(117)
How do we know that Gamma Ray Bursts do not originate within our own galaxy, the Milky Way? (TQ)

(118)
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)

(119)
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. It might help if you go back to early in the semester in your notes when I explained where the inverse square law comes from. (TQ)

(120)
What are "ghost" GRB's, and what is the accepted explanation for their differences compared to normal GRB's, according to recent observations? (TQ)

(121)
Explain how we use gravitational lensing to discover dark matter in the form of planet-sized MACHO's.

(122)
Describe what our studies of gravitational lensing have led us to conclude about MACHO's and dark matter.

(123)
There are three differences between stellar brightness variations caused by lensing events vs non-lensing events. Name all three, then explain why non-lensing variations usually involve color changes and/or repetition.

(124)
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).

(125)
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?"

(126)
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.

(127)
Explain how the standard ruler method of distance determination works. Why is this method unreliable? Name and briefly explain two benefits of using the SR method in addition to some other distance determination technique.

(128)
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 Stanrdard Ruler technique.

(129)
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.

(130)
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?

(131)
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?

(132)
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?

(133)
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?

(134)
Explain why we expected the period and absolute luminosity to be correlated for Cepheid variables. Also, explain why we expected the rotation velocity and absolute luminosity to be correlated for spiral galaxies.