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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
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%)
Last Year’s Exam, Section B
Answer any 3 of 5 short questions
5 marks each
 exam is out of 50
i.e. 120/50=2.4 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!
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 star’s 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
[1]
after correcting for temperature
[1]
Under what circumstances could you determine the
star’s mass?
[1]
 if it is a member of a suitable binary system
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]
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]
Question B3
The table below shows the isotopes of indium,
cadmium, silver, palladium and rhenium. Those
marked ε decay by converting a proton to a
neutron; those marked β 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.
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 rprocess.
[2]
 r-process: rapid addition of neutrons (forms unstable neutron-rich
isotopes which then β-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]
Question B3
number of neutrons
53
54
55
56
57
58
59
60
49In
61
62
63
64
65
66
67
ε
ε
ε
4
β
96
β
29
β
P
ε
48Cd
ε
1
ε
P
47Ag
46Pd
ε
ε
ε
ε
ε
ε
ε
R
12
13
24
12
R
ε
ε
ε
ε
52
β
48
β
β
β
1
ε
11
22
27
β
27
β
12
β
S
S
S
100
β
β
P
45Rh
1
ε
ε
R
β
R
β
direction of
β decay
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]
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]
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]
You can get full marks
for describing ONE
property in DETAIL or
two/three properties in
outline.
Question B4
 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 other’s 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
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
(Einstein’s cosmological constant, Λ), 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]
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]
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 (<0.1 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
Last Year’s 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!
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.
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]
Question C1(b)
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 (<< 4.6 Gyr old
Sun!)
[1]
 Faint giants evolve from relatively faint
(hence long-lived) MS stars
[1]
these are HB
stars, so are
also older
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]
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]
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!
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 star’s life, which ex-6. When core He runs
out, star starts fusing He
plains why most stars are on MS
2. When core H exhausted, starin shell around core,
becoming a giant again
shrinks under gravity, heating up
until H outside core starts to fuse.
4. H fusion in shell
causes
7. During stage 6, star sheds most of its outer envelope owing
to star to
become
brighter,
instability. This exposes the extremely hot carbon core, whose
ultraviolet
ascending
red giant
radiation causes the expanding
shell of and
expelled gas to glow
– a planetary
3. Star expands
branch
as seen in
nebula. When the gas cools,
shell has
dissipated,
is revealed
moving
right onthe cooling core
HIPPARCOS diag.
as a white dwarf at the subgiant
bottom left
of theofHR diagram.
branch
HR diagram
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.
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
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 Galaxy’s centre of mass on
timescales of a few years. Newton’s 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]