Physics 20083 - Introductory Astronomy - Spring 2004
Exam #2B

Answer any 7 of the following 8 questions. All relevant equations have been given on the cover page (not visible in WWW version). Each is worth 14 points.

1) Suppose we wanted to observe some stars in the constellation Auriga. We'd need to know where to point our telescope, and we'd also need to figure out how to determine the properties of the stars there.

a) (8 pts) Draw a simple star chart of Cepheus (and name one of the bright stars in the constellation), then state an altitude and azimuth where it can be located at about 10pm tonight.

b) (6 pts) How would you find the distance to the bright star you named simply be measuring its color and spectral line widths? Assume no intervening ISM along your line of sight.


2) On the H-R diagram, below, point out the approximate locations of two hypothetical stars (4 pts each): Star A has a temperature of 3000K, which is about half the temperature of the Sun, and it has a radius (size) twice that of the Sun. Star B has a temperature of 12,000K, about double the temperature of the Sun, and it has a radius (size) 1/4 that of the Sun. You may show your work if you wish for possible partial credit, but it is not necessary.

Star A and B would have red and blue colors respectively, thanks to their surface temperatures, while the Sun has a yellow color Next to or below the H-R diagram, briefly explain why we do not observe green stars in the night sky (6 pts).


3) A certain kind of cosmic ray called a "fastball" is thought to originate fairly close to us (in terms of galactic distances anyway).

a) (6 pts) Suppose we try to find out more about fastballs by undertaking a study of nearby objects, say the nearest 1000 stars. Would this sample be representative of stars in the galaxy as a whole? Why or why not?

b) (8 pts) Why do we think "fastballs" must be from relatively nearby sources, and why do they seem to come from all directions (even though it may only be a few sources nearby that are responsible for them)? Explain both answers briefly.


4) The radio region of the spectrum is a place Astronomers began to exploit early in the 20th century when the first radio telescopes were constructed. Since then, we've learned quite a bit about stars based on observations at radio wavelengths.

a) (5 pts) Why do we believe that if we ever do successfully communicate with or detect extraterrestrial civilizations, it will probably be in the radio region of the spectrum?

b) (4 pts) Briefly explain the discovery that made radio astronomer Jocelyn Bell famous.

c) (5 pts) We also use radio waves to broadcast signals, not for extraterrestrial communication, but just for information and entertainment (i.e. AM radio). Why do AM radio signals tend to travel further at night to various receivers on the Earth?


5) During the main sequence lifetime of the Sun, it has grown about 30% more luminous.

a) (8 pts) Name and explain the two things that have been occuring in the core that have caused the Sun to burn hotter, leading to this higher luminosity.

b) (6 pts) Some stars change their luminosity as a result of another process unrelated to changes in the core. These stars, called "blue stragglers", are found in globular clusters. What is a blue straggler and why does it evolve differently from ordinary stars?


6) Below is a side view of the situation in which an observer on the Earth is observing a binary star system (with a stationary central star) that is edge-on with respect to the observer's line of sight. The companion star is much smaller than the central star, so even at position A, both stars are still visible to an observer on Earth. The two stars are so far away that they always appear as a single blob of unresolved light. We only know it is a binary by observing its spectrum over time.

a) (8 pts) At point A, the companion star is moving across our line of sight. Would you expect the spectral lines for this system to be merged or split here? What about at point B? Explain both answers with a single sentence each.

b) (6 pts) Describe what measurements you would make and how you would use them in the equations in order to estimate the mass of the central star in this system.


7) A black hole is also known as a "singularity" because the entire mass of the object is thought to be concentrated in a single, infinitestimal point. We can explore the nature of a black hole's gravity with a couple of hypothetical situations...

a) (7 pts) Suppose Adam is standing on the surface of the Earth, about 4,000 miles away from the center of the Earth. He feels a certain gravitational force (his "weight"). Bob the astronaut is located 4,000 miles away from a black hole that has a mass precisely equal to the mass of the Earth. Who feels a stronger gravitational force? Adam, Bob or the same for both? Explain your answer.

b) (7 pts) Now suppose Adam is located 30 feet away from the center of the Earth while Bob is located 30 feet away from the black hole singularity (this is still outside the event horizon of the black hole, by the way). Who feels a stronger gravitational force? Adam, Bob or the same for both? Explain your answer.


8) Astronomers are observing stars in a globular cluster. The H-R diagram for that cluster is shown below with the location of the Sun marked simply as a reference point (the Sun is obviously not a member of the cluster). The mass of a star at the turnoff point on this H-R diagram is about 10 solar masses.

a) (7 pts) Would you expect this cluster to be younger, older or about the same age as the Sun (the Sun is about 5 billion years old, halfway through its main sequence lifetime of 10 billion years)? Justify your answer.

b) (7 pts) Would you expect stars in this cluster to have a higher, lower or about the same metallicity relative to the Sun? As part of your answer, explain why stellar age is related to its metallicity.