Physics 20083 - Study Guide #2

Updated through Friday, February 29. Current study questions can be found here.

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

(54)
Two of the most easily recognizable constellations that are up in the sky at this time of year are Leo and Orion. Find the following information about Leo and Orion:

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)

(55)
Stars are often classified according to their type with letters from A through O. The Sun, for example, is a G-type star. What was the original use of the ABCDEF... system? In other words, what distinguishes a type A from a type B from a type C star? Today, we usually sort stars into the sequence OBAFGKM. What is this sequence based on? Your book can help you answer this. (TQ)

(56)
The original stellar classification system was developed and later revised by Annie Cannon. One of her colleagues, Cecilia Payne-Gaposhkin, made an important discovery about the composition of stars. Describe the nature of her discovery (which, at the time, was not taken seriously even by her academic advisor). (TQ)

(57)
Explain why atoms like Hydrogen tend to have weaker absorption line strengths at very low temperatures and very high temperatures.

(58)
If a star was very weak (or non-existent) Hydrogen lines, does that means the star has no Hydrogen? Explain. Also, how would we be able to tell whether the weak Hydrogen lines are due to high temperatures, low temperatures or simply a lack of Hydrogen?

(59)
What is the difference between the terms "larger" and "more massive"? Are larger stars necessarily more massive? Explain.

(60)
Given the equation of angular size, briefly explain how you could find the linear size (R) of an object given its angular size and the distance from Earth. Why can we only measure the linear size of a few stars directly this way?

(61)
Explain two reasons why spectral line widths should depend on the density of a gas (and therefore the size of a star).

(62)
If a star has a higher temperature, must it always have a broader spectral line? Explain. If a star has a broad line, how would you determine whether the line width is due to temperature or density?

(63)
Given two spectral emission lines from two stars of the same temperature and mass, be able to state which one probably has a smaller size based on the line width (is the broader-lined star smaller?), and be able to explain your reasoning. Answer questions like, "If star A and B have the same temperature and mass, but star B has broader lines, which star is larger?" or "If star A is cooler than star B , but the two stars have identical spectral line widths and identical masses, what can you say about the relative sizes of the two stars?" Explain your answers in each case.

(64)
Suppose two stars have spectral characteristics identical to the Sun, but star A is fainter (lower apparent luminosity) than star B. Which star is further away? Explain. If we could measure it, which star would have a smaller parallax angle? Explain. (TQ)

(65)
Why is the parallax method of distance measurement limited to only nearby stars?

(66)
Given the period equation and the equation of orbital velocity, explain how we can find the mass of the Sun. For example, we know the Earth's orbital distance (93 million miles) and the period (365 days). How could we use that information to find the Sun's mass? Describe the steps, but you don't need to solve equations specifically with algebra.

(67)
When astronomers observe binary star systems edge-on, the spectral line fingerprints of the system sometimes seem to split, then merge, then split, then merge, etc. If I show you an edge-on view of a tilted or edge-on system, be able to explain at each point in the orbit of the companion whether the spectral lines would be merged or split and why.

(68)
How do we use the spectral line shifts in a binary system to determine the orbital velocity? How does the orbital velocity of the companion relate to the shift of its spectral lines? If the system is tilted, how does the measured radial velocity compare to the true orbital velocity of the system? How does our estimate of the central star's mass compare to the true central star mass in the system? Explain your answer.

(69)
Explain how we determine the period for the orbit of the companion star. Given the equation of orbital velocity and the period equation, explain how we use the period and the spectral line shifts to estimate the mass of the central star in an edge-on binary system. Again, just describe the steps, you don't need to solve equations specifically with algebra.

PDF versions of Scientific American articles can be downloaded to your computer and printed using the following method:

(1) Go to www.lib.tcu.edu.
(2) In the "Online Catalog Search" box, change the selector from "Words Anywhere" to "Journal/Serial Name Begins With..."
(3) Type "Sciencitic American" (without quotes) into the search box and press the "Search" button.
(4) On the search results page, click on the top listing, which reads "*SCIENTIFIC AMERICAN*"
(5) On records page, click on the first "Full text available to the TCU Community from Scientific American Archive Online" link that you see under detail record 1 (4th choice).
(6) Now you are on the database page for Scientific American Online. In the Find box, type the title of the article you want and hit "Search".
(7) You may view the as a PDF by clicking on the appropriate link.

Now answer questions 70-72 about the Scientific American article "The Mystery of Brown Dwarf Origins":

(70)
Why do Astronomers believe that Brown Dwarfs cannot form like ordinary planets form? (TQ)

(71)
Describe the two possible scenarios discussed in the article for the formation of brown dwarfs, the "turbulence" scenario and the "ejection" scendario. (TQ)

(72)
What are two predicted properties that brown dwarfs should have if the "turbulence" scenario is correct? Explain why the brown dwarfs would have these properties while the "ejection" brown dwarfs would not. Which scenario seems to be supported by the observations? (TQ)

(73)
Imagine a sample of the 1,000 stars with the highest apparent luminosity as seen from the Earth. Why would this sample not be representative of the stars in our galaxy? Imagine a sample of the 1,000 stars with the highest surface temperature. Would this sample be representative? Explain.

(74)
Imagine a sample of the 1,000 nearest stars to Earth. Would this sample be representative? Explain. As part of your answer, briefly explain the basis of the Copernican Principle.

(75)
Answer the following questions about "nebulium" (Google can help): What led scientists to think nebulium was a previously unknown element? What is nebulium, and why weren't we able to originally identify it correctly? What are forbidden lines, and why do we not see forbidden lines in typical laboratory spectra while we see them all the time in interstellar gas clouds? (TQ)

(76)
About every six seconds on average, the Earth is hit by a "fastball" from space. Read http://www.gsfc.nasa.gov/scienceques2002/20021213.htm and answer the following: What are these fastballs (what is another name for them)? What is one possible theory for their origin? Why do we think the point of origin must be nearby in terms of galactic distances? (Another good source for this question and the next one is at http://www.srl.caltech.edu/personnel/dick/cos_encyc.html.) (TQ)

(77)
Read a little more about cosmic rays at http://www-spof.gsfc.nasa.gov/Education/wcosray.html and answer: What is the likely origin for the most common (not fastballs) galactic cosmic rays that don't come from the Sun? Why do cosmic rays seem to come evenly from all directions on the sky (this is part of what makes it so difficult to guess their origins)? (TQ)

(78)
What is the name of the brightest star in the nighttime sky (in terms of apparent luminosity)? How does its absolute luminosity and mass compare to our Sun? It turns out that this star is part of a binary system. Describe this bright star's companion. What is the name of the closest star (it is also a binary system) in the nighttime sky (not counting the Sun, obviously)? Though there are many places to look for this information, a fun web site to explore that has the answers to these questions is The Astronomy Picture of the Day. (TQ)

(79)
The Interstellar Medium (ISM) has two effects on stars, extinction and reddening. Explain why stars appear redder when seen through a cloud of gas and dust. Explain the effect that extinction has on our estimate of the apparent luminosity and distance to stars.

(80)
Given the modified form of the inverse square law (with the ISM correction term "X" included), explain how we find X using interstellar absorption lines. How and why do interstellar lines differ from other absorption lines in stellar spectra?

(81)
What is 21-cm radiation? From what kind of atom does it originate? Why is it so important to our study of the ISM?

(82)
Why are cool, dense interstellar clouds called "molecular clouds"? What are two reasons molecules are able to survive there? Explain.

(83)
What is pressure equilibrium? Given the pressure equation, explain why the condition of pressure equilibrium leads to the inverse relationship between density and temperature in the ISM.

(84)
What is pressure equilibrium? Given the pressure equation, explain why the condition of pressure equilibrium leads to the inverse relationship between density and temperature in the ISM.

(85)
During the collapse of a molecular cloud, what is happening to the self-gravity, the density, the temperature and the outward-pushing pressure of the cloud? Briefly explain why the temperature increases.

(86)
Why do stars have a minimum mass? In other words, why is it that objects with masses smaller than about 8% of the mass of the Sun never form into stars?

(87)
What is Hydrostatic Equilibrium (HSE)? If a star's outward-pushing pressure were to increase or decrease, describe what would happen to the star as it returned to an equilibrium state.

(88)
During the main sequence lifetime of the star and especially near the end of this main sequence phase, the size of the star slowly increases due to increased outward-pushing pressure. Explain what is going on in the core that causes this.

(89)
What is the "Faint Sun Paradox" and how has it been "resolved" over time by the Earth?

(90)
Do some research on the World Wide Web regarding the "Anthropic Principle" and define two versions of it (Weak, Strong, etc) in your own words. Name and briefly explain three examples of an "Anthropic Coincidence", the kind of thing that leads people to propose the Anthropic Principle in the first place. As an example, don't just say "The lifetime of Beryllium in a star's core is a hundred millionth of a second." Explain why a deviation from that value would be critical and potentially devastating to the potential for life, etc. (TQ)

(91)
Why do giant stars turn red during the latter part of their lives, even though their cores are burning at a somewhat hotter temperature?

(92)
Explain why Helium fusion requires a higher temperature and a higher density (and therefore a higher mass star) compared to Hydrogen fusion.

(93)
Describe the process that causes a planetary nebula in stars like the Sun. Why do high mass stars not undergo this planetary nebula phase?

(94)
We've talked about blue stars, yellow stars and red stars. Why do we not see green stars in the night sky? Do some research on the web to find out. (TQ)

(95)
What is the Main Sequence on the H-R diagram? Explain why, in a representative sample, about 90% of the stars will fall on the main sequence.

(96)
Describe how and explain why the H-R diagram of a star cluster changes over time. Be able to tell by looking at H-R diagrams of different clusters which one is the youngest.

(97)
Explain how we determine the age of a cluster from its H-R diagram. If a cluster has a sun-like star at the main sequence turnoff, what can we conclude about the age of that cluster (given that the main sequence lifetime of the Sun is 10 billion years)?

(98)
Explain how and why the lifetime of a star is related to its mass.

(99)
If a cluster has a 2 solar-mass star at the main sequence turnoff, what is the approximate age of the cluster? Explain your answer. What if there were a 0.5 solar-mass star at the main sequence turnoff? We don't actually see clusters this old, so this is just a hypothetical.

(100)
Search the web to find out about "blue stragglers". What are they, and why do they evolve differently from ordinary stars (what causes them to "straggle")? (TQ)

(101)
What is different about Iron fusion compared to the fusion of other, lighter elements? Why does the onset of iron fusion signal the end of the lifetime of even the most massive stars? What causes the supernova explosion after iron fusion begins in a massive star's core?

(102)
What role do supernovae play in the existence of elements heavier than Iron on Earth? How do we know that all the heavy elements (heavier than iron) originated in supernova explosions?