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

• Briefly explain how you would go about
  determining
• the distance of a nearby star
  1
• parallax, i.e. the apparent shift in the stars
  position over the course of a year
• the surface temperature of a star
  1
• either its colour or the strengths of spectral
  lines
• the surface chemical composition of a star.
  2
• the strengths of spectral lines
  1after correcting
  for temperature
  1
• Under what circumstances could you determine the
  stars mass?
  1
• if it is a member of a suitable binary system

5
Question B2

• The Sun is a class G (yellow) main sequence star.
  Vega is a class A (white) main sequence star,
  while Aldebaran is a class K (orange) giant star.
  Both Vega and Aldebaran are considerably more
  luminous than the Sun.
• Explain carefully how you know that Vega is
  younger than the Sun.
  
  2
• From binary stars, white main-sequence stars are
  more massive than yellow main-sequence stars
  ½
• and luminosity increases much faster than mass
  ½
• so Vega must have a much shorter main-sequence
  lifetime than the Sun
  
  ½
• and we know the Sun is halfway through its main
  sequence life, so Vega must be younger than the
  Sun ½

6
Question B2

• Explain how you know that Aldebaran is physically
  larger (i.e. has a greater radius) than the Sun.
  
  1
• From blackbody spectra, red objects are cooler
  than yellow ½
• and cooler objects emit less light per square
  metre, so in order to be brighter Aldebaran must
  have a much larger surface area. ½
• Is Aldebaran (i) necessarily older than the Sun,
  (ii) necessarily younger than the Sun or (iii)
  either older or younger than the Sun it is not
  possible to tell with only this information?
  Carefully explain your answer.
  
  2
• (iii) either older or younger
  ½
• because red giant stars have similar brightnesses
  over a range of masses,
  
  ½
• and therefore we do not know if Aldebaran is a
  massive star which will evolve quickly, or a low
  mass star which evolves slowly 1

7
Question B3

• The table below shows the isotopes of indium,
  cadmium, silver, palladium and rhenium. Those
  marked e decay by converting a proton to a
  neutron those marked ß decay by converting a
  neutron to a proton those marked with a number
  are stable (the number is the percentage of the
  natural metal that is made of that isotope).
  Blank squares indicate nuclei so unstable they
  have never been seen.
• Study this table and answer the questions below.

8
Question B3

• Explain what is meant by the term s-process.
  Which stable isotopes of palladium (46Pd) are
  made by the s-process? 2
• s-process slow addition of neutrons (unstable
  isotopes decay before next neutron is added)
  
  1
• What is meant by the term r-process? Write down
  (i) a stable isotope of palladium which must be
  made by the r-process and (ii) a stable isotope
  of cadmium (48Cd) which cannot be made by the
  r-process.
  
  2
• r-process rapid addition of neutrons (forms
  unstable neutron-rich isotopes which then ß-decay
  to stable isotopes) 1
• Name one stable isotope shown on the table which
  cannot be made by either the s-process or the
  r-process, and name the process by which it is
  made.
  1

9
Question B3
number of neutrons number of neutrons number of neutrons number of neutrons number of neutrons number of neutrons number of neutrons number of neutrons number of neutrons number of neutrons number of neutrons number of neutrons number of neutrons number of neutrons number of neutrons
53 54 55 56 57 58 59 60 61 62 63 64 65 66 67
49In e e e 4 ß 96 ß
48Cd e e 1 e 1 e 12 13 24 12 29 ß
47Ag e e e e 52 ß 48 ß ß ß
46Pd e e e 1 e 11 22 27 ß 27 ß 12 ß ß
45Rh e e e e e 100 ß ß ß
P
R
P
R
S
S
S
R
R
P
direction of ß decay
10
Question B4

• Explain the significance of any TWO of the
  following observations in the context of modern
  cosmology
• the fact that the sky is dark at night
  2.5
• the redshifts of galaxy spectra
  2.5
• the properties of the cosmic microwave
  background
  
  2.5
• the brightness of distant supernovae.
  2.5

11
Question B4

• the fact that the sky is dark at night
  2.5
• In an infinite, eternal, static universe, every
  line of sight must at some point intersect a
  star, which would not give a dark sky.
  1
• Therefore, one or more of the assumptions is
  false
• in the Big Bang model, the universe has finite
  age (light from distant stars may not have
  reached us) and is expanding (light from distant
  stars is redshifted to much cooler temperatures)
  
  1
• in the Steady State model, the universe is
  expanding. 0.5

12
Question B4

• the redshifts of galaxy spectra
  2.5
• Most galaxy spectra are redshifted, and the
  redshift is proportional to the distance of the
  galaxy. 1
• This implies that the universe is expanding (it
  does not imply that we are near the centre).
  0.5
• In the Big Bang model, the universe cools and
  becomes less dense as it expands.
  0.5
• In the Steady State model, new matter is created,
  and the universe looks the same at all times.
  0.5

13
Question B4
You can get full marks for describing ONE
property in DETAIL or two/three properties in
outline.

• the properties of the cosmic microwave
  background
  
  2.5
• The CMB has the following properties
• blackbody spectrum (2.74 K)
• this is important because it says that the
  radiation must have been created in a dense
  environment at one particular time in the past
  (good for Big Bang, bad for Steady State)
• near-uniformity over the whole sky
• this is surprising because different sides of
  the sky should never have exchanged photons, and
  therefore do not know each others temperature
  it is one of the key pieces of evidence for
  inflation
• very small temperature fluctuations (1 in 100000)
  
• by studying these we can show that the universe
  is flat, and also measure many other cosmological
  parameters

14
Question B4

• the brightness of distant supernovae.
  2.5
• Type Ia supernovae all have very similar
  brightness, and are bright enough to be seen out
  to large redshifts, so can be used to check how
  the expansion of the universe changes with time.
  1
• Expected to find that the expansion is slowing
  down owing to gravity in fact found that it was
  accelerating.
  0.5
• This is evidence for the existence of dark energy
  (Einsteins cosmological constant, ?), which we
  now think makes up 70 of the energy density
  needed for the universe to be flat (as we see it
  to be). 1

15
Question B5

• Most of the planets discovered around other stars
  have been detected using the spectroscopic
  (Doppler shift) method.
• Explain how this method works, and what kind of
  planetary systems it is most likely to
  detect. 2.5
• Works by detecting the shift in the spectral
  lines of the parent star as it moves towards and
  away from us in its orbit around the star-planet
  centre of mass. 1
• Requires orbit to be tilted relative to the plane
  of the sky (or no Doppler shift)
  0.5
• Most likely to detect massive planet close to
  star (in edge-on orbit)
  1

16
Question B5

• Briefly describe the properties of the detected
  extrasolar planets. With reference to your
  previous answer, discuss how these properties are
  likely to be biased by the detection method used.
  2.5
• Mostly large planets (at least Uranus/Neptune
  mass, up to several Jupiters one or two only a
  few Earth masses)
• This is clearly biased by the detection method
• Mostly fairly close to star (within around 3 AU)
  some very much closer (lt0.1 AU)
• This is also biased by the detection method
• Many in eccentric (non-circular) orbits
• This is much less biased, though it does make
  detection easier
• Mostly only one planet per system (though up to
  5)
• Not seriously biased

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
• the search for life on Mars
• Note that you know this is coming, so more detail
  expected in answers!

18
Question C1

• The picture shows the Hertzsprung-Russell diagram
  for those nearby stars whose parallaxes were
  accurately measured by the HIPPARCOS satellite.
• Note that the Sun has absolute magnitude 4.8 and
  colour index B V 0.65.

19
Question C1(a)

• The vast majority of the stars are on the main
  sequence. Explain what defines a main sequence
  star, in terms of its energy generation
  mechanism, and why we should expect most stars to
  be on the main sequence. 2
• Fusion of hydrogen to helium in core of star
  1
• Hydrogen is the easiest element to fuse, the most
  abundant, and the most efficient energy generator
  therefore expect this stage to last longest.
  1

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Question C1(b)
these are HB stars, so are also older

• Carefully explain what features of the diagram
  show that the stars included are not all of the
  same age, and, in particular, that they include
  stars which are much younger than the Sun.
  3
• Diagram contains both bright MS stars and faint
  giants 1
• Mass-luminosity relation tells us bright MS stars
  have short lifetimes, hence these must be young
  (ltlt 4.6 Gyr old Sun!)
  1
• Faint giants evolve from relatively faint (hence
  long-lived) MS stars 1

21
Question C1(c)

• What are the stars at the bottom left of the
  diagram, and what can you tell about them purely
  from their position on the diagram? 2
• White dwarfs
  0.5
• They have high surface temperatures (because they
  are on the left-hand side of the diagram)
  0.5
• so, since they are nevertheless faint, they must
  be very small (as hot dense objects emit more
  light per square metre than cooler objects)
  1

22
Question C1(d)

• Bearing in mind that HIPPARCOS had a relatively
  small telescope, do you expect this diagram to be
  a fair sample of the stars in the solar
  neighbourhood? If not, explain which stars will
  be undercounted, and why.
  2
• No
  0.5
• Intrinsically faint stars (lower MS, white
  dwarfs) will be undercounted 1
• because they cannot be seen at large distances
  with a small telescope 0.5

23
Question C1(e)

• Describe the evolution of a star of 2 solar
  masses, from its arrival on the main sequence to
  the end of fusion processes, including an account
  of the remnant left after fusion stops. Include
  a sketch of its trajectory on the HR diagram, and
  where possible relate your description to the
  features of the HIPPARCOS HR diagram shown above.
  6
• Note the key points in this question! Many
  students missed out on marks through NOT
  ANSWERING THE QUESTION!

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Question C1(e)
5. Eventually core He heats up enough to fuse.
Star moves rapidly from tip of RGB to horizontal
branch or red clump
1. On arrival on MS, star is fusing H to He in
core. This stage lasts for 90 of stars life,
which ex-plains why most stars are on MS
6. When core He runs out, star starts fusing He
in shell around core, becoming a giant again
2. When core H exhausted, star shrinks under
gravity, heating up until H outside core starts
to fuse.
4. H fusion in shell causes star to become
brighter, ascending red giant branch as seen in
HIPPARCOS diag.
7. During stage 6, star sheds most of its outer
envelope owing to instability. This exposes the
extremely hot carbon core, whose ultraviolet
radiation causes the expanding shell of expelled
gas to glow a planetary nebula. When the gas
shell has dissipated, the cooling core is
revealed as a white dwarf at the bottom left of
the HR diagram.
3. Star expands and cools, moving right on
subgiant branch of HR diagram
25
Question C2(a)

• Describe, with appropriate diagrams, the Hubble
  tuning fork system for the classification of
  galaxies.
  6

• 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.

26
Question C2(b)

• The Milky Way is a typical large spiral galaxy.
  Explain
• how you can deduce simply from observations of
  the night sky (at a suitably dark site) that the
  Milky Way is a disc galaxy and that the Sun is
  located fairly close to the plane of the disc
  3
• We see a band of stars which cuts the night sky
  in half
• This suggests a flattened distribution like a
  disc (see diagram)
• if MW were a flattened elliptical, band would be
  less well defined
• if we were out of the plane, band would be
  broader and less symmetrical
• why we think that the mass of the Milky Way is
  dominated by dark matter, rather than by stars
  3
• Rotation curve of galaxy is flat out to large
  distances, and value is larger than expected from
  the summed masses of all stars
• therefore most of the mass is not seen as
  luminous stars, and is also more spread out than
  the stars are

27
Question C2(b)

• The Milky Way is a typical large spiral galaxy.
  Explain
• the evidence for the presence of a supermassive
  black hole at the centre of the Milky Way galaxy.
  3
• Stars near the Galactic centre can be seen in
  infra-red light. 0.5
• They are observed to orbit the Galaxys centre of
  mass on timescales of a few years. Newtons laws
  can thus be used to calculate the mass they are
  orbiting, which turns out to be 3 million solar
  masses.
  1.5
• This mass is confined within a volume smaller
  than the solar system (from the orbits and
  evidence of sudden flares in x-ray and radio).
  Therefore it must be a black hole (anything else
  of this mass would be much larger)
  1