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

• Arcturus is a red giant star which is
  approximately 100 times as bright as the Sun in
  visible light.
• We call stars like Arcturus giants because they
  have radii which are much larger than those of
  main sequence stars like the Sun. Explain how we
  know that this is so.
• As measured relative to the Sun, Arcturus is
  moving at about 120 km/s. Do you think that
  Arcturus is part of the Milky Ways stellar disc?
  If you do, explain why if not, explain why not.
  
• What fusion reaction is most probably powering
  Arcturus luminosity, and where in the star is
  fusion taking place?

5
B1 Answer

• Size of Arcturus
• red star ? cooler than Sun
• cooler than Sun ? less light per square metre
• but much brighter overall ? much larger
• Is it part of the disc?
• Suns orbital velocity 200 km/s (given)
• 120 km/s comparable to 200 km/s (not 10x
  smaller, as typical for disc stars
• so, probably not part of disc
• Power source?
• its clearly a red giant ? hydrogen to helium in
  shell around helium core
• (helium-burning giant phase is much shorter, so
  unlikely)

6
Question B2

• The diagram shows the Hertzsprung-Russell diagram
  for nearby stars whose parallaxes were accurately
  measured by the HIPPARCOS satellite.
• The colour index B V of the star measures, as
  the name suggests, the stars apparent colour.
  What physical property of the star determines its
  colour?
• What features of this diagram show that the solar
  neighbourhood contains stars of different ages,
  including stars which are younger than the Sun?

7
B2 Answer

• Colour index is determined by surface temperature
  
• Presence of both upper main-sequence stars and a
  long red-giant branch
• HIPPARCOS had a relatively small telescope. What
  differences would you expect to see in this
  diagram if HIPPARCOS had been equipped with a
  larger telescope?
• more lower-main-sequence stars
• more white dwarfs

8
Question B3

• Briefly describe the various processes by which
  elements heavier than helium are made in stars.
  Include in your description an explanation of the
  type of star in which the process in question
  might take place, e.g. main sequence stars, red
  giants, supernovae, etc.
• Key terms to be included in your account
  p-process, r-process, s-process, a-process,
  neutron-rich, neutron-poor.

9
B3 Answer

• Heavy elements are produced either directly by
  fusion or indirectly by the addition of neutrons
  to fusion products.
• Fusion products include the a-process elements
  such as carbon-12, oxygen-16, neon-20 etc., which
  can be produced by successive addition of a
  particles (helium nuclei) during the helium
  fusion stage of stellar evolution, as well as
  elements such as Si, S and Fe produced during
  fusion of heavy elements in pre-supernova stars.

10
B3 Answer

• Neutrons are produced in helium-fusing stars and
  can easily combine with nuclei because of the
  lack of any electrostatic repulsion. If neutrons
  are rare and therefore are added to nuclei
  slowly, any unstable nucleus formed will decay
  before another neutron hits it. This s-process
  produces nuclei close to the line of maximum
  stability.
• In supernovae, neutrons can be added to nuclei
  very rapidly (r-process), producing highly
  neutron-rich unstable nuclei which subsequently
  ß-decay to neutron-rich stable nuclei.

11
B3 Answer

• Neutron-poor nuclei are formed by the p-process,
  which is now believed to be, not proton addition,
  but knocking out of neutrons by high energy
  photons.
• Note need all 5 points, in about this much
  detail, for 5 marks

12
Question B4

• Draw a labelled diagram of the Hubble tuning
  fork system of galaxy classification.

13
Question B4

• Explain briefly how galaxies are classified
  according to this scheme.

ellipticals by shape E0 circular E6 - elongated

• S(B)a?c by
• decreasing size and brightness of bulge
• increasingly loosely wound arms
• S(B)0 no spiral arms

E ellipticalS spiralSB barred spiralIrr
irregular
14
Question B5

• Explain what is meant by the term cosmic
  microwave background.
• Cosmic microwave background blackbody (3K)
  radiation observed to come equally from all
  directions in the universe (isotropic).
• How do we believe the cosmic microwave background
  is generated?
• Believed to be generated when early universe
  comprises a hot dense plasma in thermal
  equilibrium, and then fossilised when electrons
  and protons combine to form neutral hydrogen,
  rendering universe much more transparent to
  radiation.

15
Question B5

• Why does this explanation support the Hot Big
  Bang model of the early Universe?
• Supports Hot Big Bang theory of universe
  because this theory naturally expects the early
  universe to be a hot dense plasma other
  theories, especially the Steady State, have no
  such expectation.
• What property of the CMB is best explained by the
  idea of inflation?
• Extreme isotropy of early universe is difficult
  to explain in Big Bang because radiation does not
  have time to traverse whole of presently
  observable universe before emission of CMB, hence
  hard to explain why regions on opposite sides of
  the sky are at the same temperature.

16
Question B5

• Why is inflation needed to explain this property?
  
• Inflation explains this by postulating early
  period of extremely rapid expansion, which means
  that whole currently visible universe originates
  from a single causally connected region of the
  pre-inflation universe theres no other way to
  ensure that the early universe reaches thermal
  equilibrium (exchanges photons)
• (Note that during inflation universe expands
  faster than light this is perfectly OK because
  its space thats expanding, not the galaxies
  that are moving)

17
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
• black holes
• the search for dark matter
• extrasolar planets
• Note that you know this is coming, so more detail
  expected in answers!

18
Question C1

• The supergiant star Sanduleak -69 202, about 160
  thousand light years away in the Large Magellanic
  Cloud, became famous in February 1987 when it was
  seen to explode as a supernova the first
  visible to the naked eye since 1604.

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C1(a)

• Do you think that when it exploded Sanduleak -69
  202 was (i) much older than the Sun, (ii) of a
  similar age to the Sun, or (iii) much younger
  than the Sun? Explain your reasoning clearly.
• Much younger
• Only stars much more massive than the Sun go
  supernova, so Sk -69 202 was massive
• Massive stars have short lifetimes, because they
  exhaust their fuel supply much faster
• The Sun is halfway through its main-sequence
  life, so Sk -69 202 was much younger than the
  Sun when it exploded

20
C1(b)

• What would Sanduleak -69 202 have looked like
  when it was on the main sequence?
• Very bright and very blue (top left of HRD)
• Describe the evolution of Sanduleak -69 202 from
  its arrival on the main sequence to its eventual
  demise. Your account should include an
  explanation of the nuclear reactions taking place
  in the star at each stage in its life (and where
  they are taking place), its likely location on
  the Hertzsprung-Russell diagram, and the
  approximate fraction of its lifetime spent in
  that stage. Include a brief account of the
  supernova explosion itself.

21
C1(b) answer

• Evolution
• Main sequence, fusing hydrogen in core, at top
  left of HR diagram. Star spends 80 of its life
  here. When hydrogen exhausted in core, star
  shrinks until
• hydrogen fusion starts outside helium core. Star
  will become a red (super)giant, at top right of
  HR diagram. This will last 10 of the main
  sequence lifetime. Hydrogen fusion outside heats
  and enlarges the core until
• helium fusion begins in the core. Star will get
  bluer again, moving left on the HRD. When helium
  exhausted in core, fusion moves outside core and
  star will return to the red. This whole period
  lasts lt10 of the stars lifetime.

22
C1(b) answer

• Evolution continued
• Because the star is massive, it will go on to
  fuse elements up to iron. This lasts a
  comparatively short time and the star may move
  back and forth on the HRD. An onion-like
  structure develops.
• Eventually an iron core forms. Iron is stable
  against fusion, so collapse of iron core under
  gravity is not stopped by onset of fusion.
  Eventually a neutron star forms, and the
  collapsing stellar envelope bounces off the
  neutron star surface, creating a shockwave which
  powers the supernova explosion.

23
C1(c)

• How might we detect the post-explosion remnant of
  Sanduleak -69 202? Suggest a reason why we might
  not detect it.
• If remnant is a neutron star, it might be
  detected as a pulsar rapid, very regular pulses
  of radio emission.
• If remnant not detected, lighthouse beam from
  pulsar might not be pointing our way, or core
  might have become a black hole rather than a
  neutron star.

24
C1(d)

• Briefly explain why the Sun is not destined to
  end its days in the spectacular fashion adopted
  by Sanduleak -69 202. What will the Sun
  eventually evolve into?
• Sun is not massive enough to fuse elements
  heavier than helium (core never gets that hot),
  therefore it will not form an iron core (it is
  also not a close binary, so it will not produce a
  Type Ia supernova).
• A white dwarf (surrounded initially by a
  planetary nebula).

25
C2(a)

• Well over 100 planets have now been discovered
  orbiting other stars.
• Explain how the typical properties of these
  extrasolar planets differ from the properties of
  the planets in the solar system.
• most discovered planets are gas-giant-sized, but
  in orbits typical of our terrestrial planets (lt
  3AU)
• some planets are in orbits which are very small
  indeed (ltlt1 AU), where our solar system has no
  planets at all
• many have very eccentric (elliptical) orbits,
  whereas all planets in our solar system are in
  nearly circular orbits

26
C2(a)

• In what respects are the discovered planets
  similar to those of the solar system?
• almost all systems have only one giant planet,
  and very few indeed have more than 2 (cf. Jupiter
  and much smaller Saturn in solar system)
• planets are discovered around stars with heavy
  element content similar to or higher than the Sun
• spectral class is also similar to the Suns

27
C2(b)

• Most of these planets have been discovered by the
  Doppler shift method. Explain how this technique
  works and discuss which planets it might most
  easily detect.
• Doppler shift method measures velocity of star
  (in line of sight) around system centre of mass.
  Expect periodic motion corresponding to
  elliptical orbit. Size of shift gives (lower
  limit to) mass of planet.
• It is easiest to detect massive planets in close
  orbits edge-on to line of sight, because these
  produce the largest shifts.

28
C2(b)

• How does this relate to the typical properties of
  extrasolar planets you described in part (a)?
• Properties in part (a) are definitely biased
  Earth-sized planets not detectable with current
  technology. Jupiter and Saturn are within range
  of masses detected. Our system has one (barely)
  detectable gas giant, so one or two planets per
  system is reasonable (Saturn not detectable with
  current technology).

29
C2(b) continued

• Close orbits are also favoured by the technique,
  and also by the fact that measurements have only
  been going on for 10 years (so even Jupiter
  would have completed not quite one orbit).
  Therefore finding gas giants in asteroid-belt
  sized orbits but not farther out is likely due to
  bias. However, finding gas giants in orbit with
  periods of a few days, though efficiency is
  biased, does demonstrate that such objects
  (unexpectedly) exist.
• High eccentricities not obviously a biased
  result, though for larger orbits it may be -
  large eccentricity gives higher peak velocity,
  hence is easier to detect.
• High heavy element content of stars is not biased
  by technique (people have looked around low
  metallicity stars), and is expected given theory
  that planets form from coagulated dust. Spectral
  class is biased M class stars are hard objects
  for high resolution spectroscopy.

30
C2(c)

• What are the properties we think a planet needs
  to have if life is to evolve on it? Briefly
  describe how future astronomers might find
  evidence for life, not necessarily intelligent,
  in other planetary systems.
• Orbiting in reasonably circular orbit, and not
  tidally locked to star (no great extremes of
  temperature) around stable star (not close
  binary and not flare star) with lifetime in
  excess of 2 Gyr with liquid water (surface water
  implies location within habitable zone, but
  subsurface water may not, cf. Europa).
• Use space-based interferometer working in
  infra-red to get necessary resolution look for
  ozone IR spectral features (terrestrial oxygen is
  biogenic). Assumes that photosynthesis is
  universal, and that not enough oxygen is produced
  abiogenically to make ozone layer