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ASTR1001
Astrophysics
Examination Duration: 4 hours
Instructions to Students:
Complete all six questions. They are all worth equal marks.
A formula sheet and a list of useful constants and definitions is at the end of the exam paper.
You may write or word-process your exam. You should save it as a single pdf file and upload that
to Wattle before the 4-hour deadline is over. Please leave enough time to complete the upload
before the deadline.
You must not communicate with other people during the exam: what you write must be entirely
your own work.
You are allowed to access any notes and the internet during the exam, and use any software you
wish (except communication software).
The exam is randomised – other students in the class are doing different questions.
Question 1
Imagine that you have discovered a pair of stars, 79.3 parsecs away from the
earth, and only 2.1 astronomical units apart from each other.
A) What would be the apparent separation of the stars, as viewed from the
Earth, in arcseconds? (2 marks)
B) Imagine that you want to use an optical telescope to get an image of
the pair, clearly resolving the two stars. What sort of telescope would
you need, and how large must it be? (4 marks)
C) Using your telescope, you discover that the two stars look very similar
to each other, both having fluxes of around 1.3x10-15 W m-2, and having
spectra that peak at around 340 nm. Deduce as much as you can about
these stars. (4 marks)
Question 2
A) For the pair of stars in the previous question, derive an equation for
their orbital period, assuming that they have equal masses and move in
circular orbits. Clearly show the steps in your derivation. (5 marks)
B) If they are white-dwarf stars, roughly how long would you have to
observe them to see one complete orbit? (2 marks)
C) Do you think that current or future gravitational wave detectors will be
able to detect gravitational waves from this pair of stars? Carefully
explain your reasoning. (3 marks)
Question 3
Imagine that you have just discovered a strange object out in space:
It consists of a fuzzy reddish object in the centre, surrounded by a blue ring of radius 3.5
arcseconds. The flux from the reddish object is 5.5 x 10-15 W m-2.
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You take a spectrum of the fuzzy reddish object in the centre: it looks like the spectrum
of a fairly old and massive galaxy, with the Hydrogen H-alpha absorption line (lab
wavelength 656.3 nm) shifted to 717.5 nm. You also take a spectrum of the blue ring: it
looks like the spectrum of a young galaxy which is actively forming stars. It has an H-
alpha emission line at 778.8 nm.
You conclude that this is an Einstein Ring, where the fuzzy red object is a galaxy
gravitationally lensing a blue background galaxy.
Deduce as much as you can about the red galaxy.
Question 4
Imagine that you have been studying a Sun-like star that is roughly 25 light-years form
the Earth.
You have made repeated radial-velocity measurements, using an iodine cell to get high
precision:
You also monitored the visible light flux from this star, using the TESS satellite:
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and here is a close-up of the top of the TESS graph:
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You believe that there is a planet orbiting this star.
Deduce as much as you can about this planet and its orbit. Clearly explain your
reasoning.
Question 5
A) What is the cosmic microwave background radiation? What does it look like?
Where does it come from and how was it produced? (3 marks)
B) What are the acoustic peaks of the cosmic microwave background? How are they
measured? Sketch their spectrum and carefully explain its features and how they
are created. (4 marks)
C) How can measurements of the acoustic peaks be used to measure the curvature
of space-time and the average density of matter in the universe? (3 marks)
Question 6
List three different situations in astronomy where you get accretion disks.
For each of the three situations, draw a labelled sketch showing where the disks are
found and marking their key features. Explain carefully the observational evidence for
the existence of all three types of disks. (10 marks)