Exam Technique

1
Exam Technique

• READ THE QUESTION!!
• make sure you understand what you are being asked
  to do
• make sure you do everything you are asked to do
• make sure you do as much (or as little) as you
  are asked to do implicitly, by the number of
  marks
• Answer the question, the whole question, and
  nothing but the question

2
Exam Technique

• Read the whole paper through before you start
• if you have a choice, choose carefully
• whether or not you have a choice, do the easiest
  bits first
• this makes sure you pick up all the easy marks
• PHY111
• do all of section A (20 questions, 40)
• do 3 from 5 in section B (3 questions, 30)
• do 1 from 3 in section C (1 question, 30)

3
Last Years Exam, Section B

• Answer any 3 of 5 short questions
• 5 marks each
• exam is out of 50
• i.e. 120/502.4 minutes per mark
• hence each question should take 12 minutes to
  answer
• do not let yourself get bogged down, but
• do not write 2 sentences for 5 marks!

4
Question B1

• You have the following pieces of information
  about a nearby star
• the apparent position of the star on the sky,
  measured very accurately over a period of several
  years
• the stars spectrum
• the apparent brightness of the star, measured
  over the whole wavelength range.
• Using this information, explain how you would
  determine
• the surface temperature of the star
• the distance of the star
• the size (radius) of the star.

5
B1 Answer

• Surface temperature
• from spectral line strengths (e.g. molecular
  lines in cool stars)
• (from colour got half a mark)
• Distance
• from parallax (measure stars apparent shift in
  position relative to distant objects, over the
  course of a year or so)
• Size
• Combine brightness and distance to get intrinsic
  luminosity
• Use luminosity and temperature to get size from
  blackbody calculation

6
Question B2

• It is often said that the Sun is an average
  star. Discuss this statement carefully,
  considering properties such as brightness, age,
  position on the HR diagram, location in the
  Galaxy, etc.

7
B2 Answer

• For its being average
• on main sequence, midway through its life
• near middle of brightness/temperature ranges (on
  log scale!)
• located in old disc of galaxy
• Against its being average
• most stars are smaller and fainter (class M and K
  dwarfs)
• its not in a binary (probably more than half of
  all stars are)
• Not clear one way or the other
• its got planets (this is probably more common
  than not, but we dont know yet)
• Conclusion
• the fact that 90 of stars are smaller and
  fainter than the Sun indicates it is not really
  an average star

8
Question B3

• Draw a labelled diagram of the Hubble tuning
  fork system of galaxy classification.
• Explain briefly how galaxies are classified
  according to this scheme.

9
B3 Answer

• En where increasing n indicates increasing
  ellipticity.
• S0 disc galaxies without spiral structure.
• S/SB unbarred/barred
• Sa/b/c bulge size/ brightness decreases, so does
  tightness with which arms are wound.
• Irr amorphous or disrupted.

10
Question B4

• Explain why we might reasonably expect that the
  sky would not be dark at night, carefully listing
  the assumptions that lead to this incorrect
  conclusion.
• How do we explain the darkness of the night sky
  in the Big Bang cosmological model?

11
B4 Answer

• If we assume that the universe
• is infinite in spatial extent
• is infinitely old
• and is not expanding (so, no redshift)
• then every line of sight from any observer will
  eventually intercept a star.
• Therefore, everywhere we look we should see a
  stellar surface, and the average temperature of
  the night sky should be the average stellar
  surface temperature (3000K)

12
B4 Answer

• In the Big Bang model, the universe is not
  infinitely old, and also it is expanding. The
  light from many stars has not yet had time to
  reach the observer, and in any case it would be
  redshifted down to a much lower temperature (and
  so would not be bright).
• Note detailed analysis indicates that finite age
  is much the more important factor for starlight.
  However, you need the expansion to prevent the
  CMB from producing a bright night sky.

13
Question B5

• The Drake equation is a method of estimating the
  number of communicating technological
  civilisations in the Galaxy. It consists of a
  number of different factors multiplied together.
• The first factor in the Drake equation is the
  rate of formation of suitable stars. What is
  meant by a suitable star? Why are other kinds
  of stars unsuitable?
• List, with a brief (one sentence) explanation,
  any three of the remaining factors in the Drake
  equation.

14
B5 Answer

• Suitable star
• not O, B or A class (dont last long enough)
• probably not M class (planets likely to be
  tidally locked also, often flare stars)
• not close binary (unlikely to support stable
  orbits at the right distance)
• not too low in heavy elements (unlikely to form
  planetary system)
• not seriously variable

15
B5 Answer

• Other terms include any three of
• fraction of suitable stars that have planetary
  systems (we assume life requires a planet!)
• number of suitable planets per system
• fraction of suitable planets on which life does
  indeed develop
• fraction of life-bearing planets on which an
  intelligent species evolves
• fraction of intelligent species which develop a
  technological civilisation
• average lifetime of a technological civilisation
  (where technological is here defined as
  capable of sending and receiving radio
  broadcasts the Roman empire does not count)

16
Last Years Exam, Section C

• Answer any 1 of 3 long questions
• 15 marks each, 36 minutes work
• Question C3 is on the seminars
• Write short essays on any three of the following
• binary stars
• the search for dark matter
• neutrinos in astrophysics
• prospects for extraterrestrial intelligence
• Note that you know this is coming, so more detail
  expected in answers!

17
Question C1

• The HR diagram of an old cluster is likely to
  include the following four branches (working from
  the top down)
• A. the red giant branch
• B. the horizontal branch
• C. the main sequence
• D. the white dwarfs.
• Put these branches in the order in which they
  would be visited during the evolution of a
  Sun-like star.
• For each branch, explain what (if anything) is
  being fused to generate energy, and where.
• Use the above information to describe the
  evolution of a Sun-like star.

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C1(a)

• Order CABD
• Fusion processes
• A hydrogen to helium, in shell outside helium
  core
• B helium to carbon, in core
• C hydrogen to helium, in core
• D nothing no fusion occurring

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C1(a)

• A Sun-like star starts off on the main sequence,
  fusing hydrogen to helium in the core.
  Eventually, the core hydrogen is exhausted, and
  the star begins to fuse hydrogen in a shell
  around a the (now pure helium) core at this
  point the star begins to expand and cool, moving
  first to the subgiant branch, then to the red
  giant branch.
• When the star reaches the top of the red giant
  branch, the core is hot enough for helium fusion
  to begin. The resulting reorganisation of the
  stars internal structure causes it to become
  hotter but less luminous it moves to the
  horizontal branch or the red clump, but they
  only know about the horizontal branch.
• When the core helium is exhausted, the star
  begins to fuse helium in a shell around the
  carbon core, moving back toward the red giant
  branch. At this point it is highly unstable and
  begins to lose mass.
• Eventually the star loses its whole outer
  envelope, stopping fusion and revealing the very
  hot, but inert, carbon core. The star has become
  a white dwarf, surrounded by a planetary nebula.

20
C1(b)

• The expanding gas cloud known as the Crab Nebula
  is the remnant of a supernova observed by the
  Chinese in 1054.
• Explain how this type of supernova is produced.
• The Crab Nebula also contains a pulsar. What is
  a pulsar, how do you detect one, and why might
  you expect to find a pulsar associated with a
  supernova remnant such as the Crab?

21
C1(b) answer

• Very massive stars can continue fusing
  successively heavier elements until they
  eventually produce an iron core.
• Iron is the most stable nucleus, so no further
  fusion is possible. Once the iron core can no
  longer support its own weight against gravity, it
  suddenly collapses, producing a neutron star (or
  possibly a black hole).
• The outer layers of the original star, falling
  under gravity, encounter the extremely rigid
  surface of the neutron star and bounce off the
  resulting shock wave causes the supernova
  explosion and produces the remnant gas cloud.

22
C1(b) answer

• A pulsar is a rotating neutron star whose
  rotation axis does not coincide with its magnetic
  axis.
• It is detected (if we lie in the appropriate line
  of sight) by a lighthouse beam of radio
  emission emanating from the magnetic poles of the
  star, and swept around by the stars rotation, so
  that we see the beam as regular pulses of
  emission (hence name).
• Most core collapse supernovae are believed to
  produce neutron stars (rather than black holes),
  and the small size of the neutron star compared
  to the original star concentrates both the spin
  and the magnetic field, so we expect the neutron
  star so produced to be a pulsar. Only question
  is whether we are properly lined up with the
  lighthouse beam in this case we clearly are.

23
C2(a)

• How does each of the following observations help
  to establish the correctness of the Big Bang
  model of the universe (as opposed to the Steady
  State model)?
• the abundances of the light elements helium and
  lithium
• comparison of galaxies observed at very large
  distances with those observed nearby
• the existence and blackbody spectrum of the
  cosmic microwave background.

24
C2(a)

• the abundances of the light elements helium and
  lithium
• Lithium is not produced in stars at all, and
  although helium is produced, its cosmic abundance
  is much higher than we would expect by comparing
  it with elements which are made in stars.
• Therefore the Steady State has no good
  explanation of where the helium and lithium
  present today were made (since they are not being
  made now, and the Steady State assumes that now
  is typical).
• In the Big Bang, the helium and lithium are made
  in the early universe, when the temperature is
  comparable to the interior of stars. The amounts
  made are broadly consistent with what we now
  observe, so this supports the Big Bang theory.

25
C2(a)

• comparison of galaxies observed at very large
  distances with those observed nearby
• Galaxies observed at very large distances look
  rather different from modern galaxies there are
  more active galaxies, more interacting galaxies,
  more small blue galaxies, and fewer
  well-developed spirals.
• Again, this is inconsistent with the Steady
  State, which expects now to be typical
  (therefore distant galaxies should look, on
  average, similar to modern galaxies). In the Big
  Bang model, distant galaxies are seen as they
  were when the universe was a small fraction of
  its present age, and so it is not surprising that
  they might look different. Therefore this
  observation is good evidence against the Steady
  State.

26
C2(a)

• the existence and blackbody spectrum of the
  cosmic microwave background
• The CMB is blackbody radiation, which implies it
  was produced by a hot dense object it comes from
  everywhere, which suggests that the object in
  question is the universe.
• In the Big Bang model, the early universe is hot
  and dense ideal conditions in which to generate
  the CMB. The CMB is therefore strongly
  supportive of the model.
• The Steady State has no good way to generate this
  spectrum, since there is no appropriate hot dense
  era in which to make it.

27
C2(b)

• The basic parameters of the Big Bang model are
  the expansion rate H, the density O, the
  curvature k, and the cosmological constant ?.
• Briefly explain how O, k and the fate of the
  universe are related if ? 0.
• Why do observations of distant supernovae lead us
  to believe that in fact ? ? 0?

28
C2(b)

• O and k
• If O gt 1, k gt 0, and the universe will recollapse
  (Big Crunch).
• If O 1, k 0, and the universe will expand
  forever at an ever-decreasing rate.
• If O lt 1, k lt 0, and the universe will expand
  forever at a rate which remains finite.
• Distant Type Ia supernovae (SNe Ia)
• If we use SNe Ia to construct a Hubble diagram,
  we find that the expansion of the universe is
  accelerating
• This cannot be achieved by any modification of O,
  k or H. The only way to get acceleration is to
  introduce ?

29
C2(c)

• The Big Bang theory has been improved by the
  introduction of inflation. What is inflation,
  and what problems with the Big Bang does it
  solve?
• Inflation is a brief period of very rapid
  expansion in the very early universe, powered by
  ultra-high-energy particle physics. The result
  is that the universe expands by an enormous
  factor.
• This dilutes any pre-existing curvature,
  resulting in a universe which is extremely flat
  (solving the flatness problem).
• It also ensures that the visible universe expands
  from an extremely tiny pre-inflation region,
  which has reached equilibrium (solving the
  horizon problem).