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Physics 55
Final Exam
5/3/11
General Instructions
In answering word problems, try to make your answers concise, but complete. This is
best achieved by figuring out what is the main point you want to convey and concentrating
on that point. This works a lot better than writing scattershot about everything that could
conceivably be remotely connected to the issue at hand, in the hope of scoring some hits.
Some of the problems ask more than one specific question about a subject. Check that
your answer addresses all of the questions.
In solving problems, it would help if you explain (minimally) what the logic is that
you are following, and make your calculations and intermediate results clearly visible. Pay
attention to the units in which various quantities are expressed, including your answers!
If you make a calculation error, I am usually generous with partial credit, but if all I have
is the incorrect answer this is hard to do. Also, incorrect answers that are completely
unreasonable (e.g. the distance to a galaxy computed to be 1.27 × 1011 m – can this be
right?) will cost you credit. If you can’t find the error, at least explain if the answer is
unreasonable and why. Make sure you answer all parts of a problem!
I hope you enjoyed PHY55 and learned something. I surely did both. You are invited
to keep in touch in the future, to ask science questions or to tell me where you’ve gone on
from here. You know where to find me. Keep looking up!
Solve 4 of the following problems (15 pts each)
1. The star Menkent in the constellation Centaurus is located at RA 14h 7m Decl.
−36◦ 220 1300 .
(a) At what time of year would we be most likely to see Menkent from Durham at
the time of day we like to observe – around 10 pm?
(b) What would be its maximal altitude above the horizon?
(c) When Menkent is at its maximal altitude, where would you look for Spica, in
Virgo, located at RA 13h 25m Decl. −11◦ 90 4000 ?
2. The two stars that make up the overcontact binary W Ursae Majoris have estimated
masses of 0.99 M and 0.62 M . The light curve of the binary system shows an orbital
period of 8.0 hours.
(a) Find the average separation of the two stars.
(b) The radii of the two stars are estimated to be 1.14 R and 0.83 R respectively.
Draw a picture, roughly to scale, of the system.
(c) If, in the future, W Ursae Majoris produces a nova, which of the two members of
the pair would most likely be the source of this. Why?
3. The bright star Spica in Virgo has a parallax angle of 0.02300 .
(a) What is the distance to Spica?
(b) Spica appears to be 8.4 × 10−12 as bright as the Sun. Find the luminosity of
Spica in terms of L .
(c) Spica is classified as spectral type B1 V. How will it end it stellar life?
4. A planet orbiting the star HD209458 has been discovered by finding a variability in
the star’s apparent brightness when the planet eclipses it. The star has a surface
temperature of 6065 K.
(a) Find the wavelength of maximum emission for this star. What type of light is
this?
(b) The light curve yields a period for the planet’s orbit of 3.52 days and astrometric
measurements lead to an estimated orbital radius of 0.047 AU. Use this data to
estimate the mass of HD209458 in terms of the Solar mass. Does this fit with the
surface temperature given above?
(c) HD209458 has a luminosity of approximately 1.47 L . Find the surface temperature of its planet, assuming an albedo of 0.2 and neglecting possible greenhouse
effects.
(d) Explain how your calculation above explains the discovery of a periodic dip in
the star’s apparent luminosity when observed in the infrared. How long after the
dip in visual brightness would this occur?
5. Consider a type-Ia supernova at a redshift of z = 2.2.
(a) One identifying characteristic of type-Ia supernovae is the presence of a strong
absorption line of ionized Silicon. In the lab, this line is observed at a wavelength
of 35 micrometers. At what wavelength would we find this line in the spectrum
of this high-z supernova?
(b) Estimate the distance to this supernova and the lookback time (how long ago we
are observing it).
(c) Type-Ia supernovae reach peak luminosities of 109 L . Estimate the peak apparent brightness of this supernova. Would it have been visible to the naked eye on
a clear night?
Briefly answer 4 of the following questions (10 pts each)
6. The galaxy NGC1275 is observed to contain bright blue globular clusters. Explain
why these are assumed recent and why this is evidence of a possible recent galactic
collision or other drastic event that triggered their formation.
7. I have often, in class, described close binary systems as stars with “built-in probes.”
Give two examples in which the existence of a binary partner has allowed us to learn
something about a star that would otherwise have been difficult to find. Give one
example of a phenomenon that occurs in close binary systems that would not have
existed without them, and describe something we have learned from this.
8. An important thread in our discussions has been the construction of the three-dimensional
universe from the two-dimensional picture we see in the sky. This construction relies
on the cosmic distance ladder. Describe conceptually what the distance ladder is and
how it is constructed; for two stages of the ladder describe how they are calibrated
and how they are used, as well as the distance range over which they are effective and
the limitations that prevent their use beyond that range.
9. Astronomers tell us most of the matter in the universe is not visible. Give two examples
of ways in which we can observe the presence of “dark matter” and explain what they
tell us about its nature.
10. Many scientists consider Pluto to be not really a planet, but a Kuiper belt object.
What is the Kuiper belt? What evidence do we have that it is out there, and what
do we know of its properties?
11. I have tried to emphasize this term how discoveries in astronomy and our understanding of fundamental physics reinforce each other. Give two examples in which
astronomical observations have led to insights into the fundamental laws of physics
and explain briefly how these insights follow from the observations.