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

(135)

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.

(136)

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:

• Approximate location in the evening sky this month, based on the star chart on page A-31 (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 stars designated alpha and beta (usually the two brightest),
• 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. (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)

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:

(137)

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

(138)

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)

(139)

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)

(140)

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

(141)

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

(142)

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

(143)

Explain (with sample bar graph) how the star formation histories of spirals and ellipticals supports the merger hypothesis.

(144)

Explain how escape velocity can be used to prove the existence of dark matter in galaxies or galaxy clusters, for example with coronal gas.

(145)

Explain how gravitational lensing can be used to prove the existence of dark matter in galaxy clusters.

(146)

How do we know that quasar light does not come from typical stars or galaxies?

(147)

Explain how we know that quasars are much more distant than nearby galaxies. How do we know they are hundreds of times more luminous than a typical galaxy, trillions of times more luminous than a typical star?

(148)

How do we know that quasars, despite being brighter than typical galaxies, have sizes that are more like the size of our solar system (a few light-days across instead of hundreds of thousands of light years). Explain why the timescale of variability is related to an object's size.

(149)

Describe the distribution of quasars in the sky. Does this violate the Copernican Principle? Explain your answer. Why are the quasars distributed in this way?

(150)

Why do we think QSO's are powered by massive black holes (give two supporting pieces of evidence)?

(151)

Why do we think QSO's are an early stage in the evolution of galaxies (two things quasars and galaxies share in common)?

(152)

Explain why stars have a maximum mass (or maximum luminosity).

(153)

How is the cosmic background radiation (CBR) related to the big bang?

(154)

Who predicted the existence of the CBR? Who first discovered/observed the CBR?

(155)

Why must we observe the CBR from above the Earth's atmosphere?

(156)

Why are major experiments like the rocket-borne experiment typically performed largely by graduate students rather than professional engineers and scientists?

(157)

How do Astronomers hope to use the CBR to study the early "dark age" of the Universe before galaxies formed? In other words, what puzzle about the early Universe do we hope to solve by observing the CBR?

(158)

What did John Huchra and Margaret Geller learn about galaxies, and how does this relate to the puzzle that Andrew Lang, Paul Richards and their colleagues are trying to solve?

(159)

What did the COBE experiment learn about the CBR, and how did this finding compare with Toshio Matsumoto's and Andrew Lang's previous rocket-borne experiment to measure the CBR?

This is optional, but if you would like to read more about the CBR without opening a Scientific American PDF, the web site from NASA's latest CBR explorer, the Wilkinson Microwave Anisotropy Probe (WMAP) is a good place to start.

(160)

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

(161)

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

(162)

What does the existence of the CBR prove about the nature of the Universe long ago? Explain.

(163)

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.

(164)

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.

(165)

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.

(166)

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?

(167)

Why can we not use Hubble's Law to reliably find distances to galaxies in the nearby Virgo Cluster?

(168)

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

(169)

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.

(170)

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.

(171)

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

The following questions are from the Scientific American article "Does Dark Energy Really Exist?" published in the April 2009 issue.

(172)

In 2-3 sentences and in your own words, summarize the article's alternative hypothesis that explains why supernovae near the edge of the visible Universe seem to be accelerating away from us. (TQ)

(173)

Some Astronomers say that galaxies may not be distributed uniformly on the largest scales (hundreds of millions of light years). They say that previous galaxy surveys are unable to conclude this one way or another. Explain why, in your own words. (TQ)

(174)

What is the purpose of the European Space Agency's planned Planck Surveyor satellite? (TQ)

(175)

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?

(176)

Explain the "age crisis" and explain why Astronomers did not respond to this discrepancy by abandoning the Big Bang theory.

(177)

Explain why the density of matter in the Universe is related to how much Helium was present in the early Universe.

(178)

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).

(179)

Explain the horizon problem and how the theory of inflation resolves the problem.

(180)

Explain the flatness problem and how the theory of inflation resolves the problem.