Physics 20083 - Study Guide #4

Updated through Wednesday, April 21. Current study questions can be found here.

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

(111)
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.

(112)
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)?"

(113)
Explain the relationship between age and color for individual stars. Are blue main sequence stars young? Explain. Are red main sequence stars old? Explain.

(114)
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)?"

(115)
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.

(116)
Two of the most easily recognizable constellations that are up in the sky at this time of year are Canis Major and Auriga. Find the following information about these two constellations:

Remember the links: for mythology it is http://www.emufarm.org/~cmbell/myth/myth.html. (alternate site is Star Tales). 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)

(117)
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)?

(118)
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.

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!

(119)
How do we know that Gamma Ray Bursts do not originate within our own galaxy, the Milky Way? (TQ)

(120)
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)

(121)
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)

(122)
What are "ghost" GRB's, and what is the accepted explanation for their differences compared to normal GRB's, according to recent observations? (TQ)

(123)
Explain how we use gravitational lensing to discover dark matter in the form of planet-sized MACHO's.

(124)
There are two major differences between stellar brightness variations caused by lensing events vs non-lensing events. Explain why non-lensing variations usually involve color changes and/or repetition.

(125)
Describe what our studies of gravitational lensing have led us to conclude about MACHO's and dark matter.

(126)
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).

(127)
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?"

(128)
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.

(129)
From Chapter 20 of your book, briefly explain why the rotation speed of a galaxy and the galaxy's luminosity are related (the Tully-Fisher relation). (TQ)

(130)
From Chapter 20 of your book, explain what we measure for galaxies in order to use the Tully-Fisher technique, and how does this measurement lead us to a distance estimate for the galaxy? (TQ)

(131)
Briefly explain why it is not practical to hunt for solitary black holes using the same techniques we use to hunt for MACHO's.

(132)
Explain how the standard ruler method of distance determination works. Why is this method unreliable? Name and briefly explain a benefit of using the SR method in addition to some other distance determination technique.

(133)
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.

(134)
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.

(135)
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?

(136)
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?

(137)
Briefly explain how the merger hypothesis explains the differences between spiral galaxies and elliptical galaxies.

(138)
Explain how the distribution of spirals and ellipticals in the sky (where they are typically) found supports the merger hypothesis.

(139)
Be able to plot a graph of a race at various times as we did in lecture given some basic data. Given a graph of a race, be able to find the slope of the graph and the age of the race.

(140)
What is the Hubble constant? A Hubble constant of 70 implies that the age of the Universe is about 14-15 billion years. What if the Hubble constant were recalculated to be 50...how would our estimate of the age of the Universe change (younger or older)? Explain.

(141)
If Astronomers had been alive back when the Universe was only about 5 billion years old and tried to plot a Hubble diagram, would it have looked the same back then as it does now? Explain, keeping in mind the car race analogy we did in class.

(142)
Why is it that some galaxies do not appear to be moving away from us? Does that mean Hubble's Law is not valid? Why not?

(143)
Why can we not use Hubble's Law to reliably find distances to galaxies in the nearby Virgo Cluster (related to question 142)?

(144)
If the expansion of spacetime is causing the Universe to expand, is the galaxy itself expanding? Is the solar system expanding? The Earth? Explain.

(145)
From Chapter 21 in your book, learn about quasars and answer: How do we know quasars are extremely far away from us? Also, explain how we know that quasars have relatively small sizes (not much larger than our solar system). As part of your answer, explain why the timescale of variability is related to the size of a quasar. (TQ)

(146)
Also from Chapter 21 in your book, explain why the strength of Hydrogen absorption lines in a quasar spectrum increases with increasing redshift. (TQ)

(147)
From Chapter 22 of your book, learn about other evidence for dark matter besides galaxy rotation curves and answer: Name and briefly explain two lines of evidence in galaxy clusters that tell us dark matter is present in these clusters in an abundance roughly 10 times that of visible matter, just like in our own galaxy. (TQ)

(148)
Also from Chapter 22, describe the evidence related to gravitational lensing by galaxy clusters of more distant objects that confirms the existence of dark matter in these clusters. (TQ)

(149)
Explain what conclusion we can reach about the finite nature of the Universe simply by observing that the night sky is dark.

(150)
How does the expansion of the Universe (Hubble's Law) contribute to the darkness of the night sky?

The following study guide questions are based on the Scientific American article, "The First Stars in the Universe", from the September 2004 Special Edition of Scientific American:

(151)
What are "Population III" stars? How and why are these stars linked to the appearance of quasars? (TQ)

(152)
Why was it harder for stars to form during the first billion years or so as opposed to now (another way of asking this is: why was the Jeans mass, the minimum mass needed for a cloud to collapse into a star, larger long ago)? (TQ)

(153)
Why was the second generation of star formation more efficient than the first generation, which took a lot longer to form (what did metals have to do with it)? (TQ)

(154)
Early in the history of the Universe, most of the gas became ionized. What caused this? (TQ)

(155)
How is the cosmic background radiation (CBR) related to the big bang? What does the existence of the CBR prove about the nature of the Universe long ago? Explain.

(156)
Explain why Astronomers expected to find that the CBR is lumpy rather than completely uniform.

This is optional, but if you would like to learn more about the CBR, I recommend you visit the website of NASA's latest mission to explore it, the Wilkinson Microwave Anisotropy Probe (WMAP).

(157)
Explain what we can conclude about the Universe from our study of the ages of globular clusters. In other words, how does this information support the Big Bang theory? Explain the "age crisis" and explain why Astronomers did not respond to this discrepancy by abandoning the Big Bang theory.

(158)
The density of the Universe governs the ultimate fate of the Universe (whether it will expand forever or eventually contract). Explain why.

(159)
From your book's chapter 23, explain what inflation theory says about the Universe. Also, explain the "horizon problem", which is related to the fact that the Universe appears to look the same in all directions on large scales, and explain how inflation solves this problem. (TQ)

(160)
Explain why the density of matter in the Universe is related to how much Helium was present in the early Universe. Explain why the abundance of Helium (and other light elements) is a measure of the density of the Universe, independent of other measures of the density (such as counting galaxies in and dividing by a representative volume).

(161)
Explain how and why Astronomers hoped to use Hubble's Law as an independent method for estimating the density of the Universe. How would a high vs a low density (and a zero density) Universe compare on a Hubble law graph? Explain why they differ.

(162)
Explain how current observations of galaxy radial velocities and distances compare to the expectations of Hubble's Law under the influence of gravity. Explain why these observations lead us to believe that the Universe is accelerating away from us in all directions.

(163)
Explain what the Cosmological Constant is. Why did Einstein originally introduce the idea of a Cosmological Constant? Why was it later abandoned?