Download answers2004_05_BC - Particle Physics and Particle Astrophysics

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
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 Way’s 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?
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?
 Sun’s 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?
 it’s clearly a red giant → hydrogen to helium in shell
around helium core
 (helium-burning giant phase is much shorter, so unlikely)
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
star’s 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?
B2 Answer
 Colour index is determined by
surface temperature
 Presence of both upper mainsequence stars and a long redgiant 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
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, α-process,
neutron-rich, neutron-poor.
B3 Answer
Heavy elements are produced either directly by
fusion or indirectly by the addition of neutrons to
fusion products.
Fusion products include the α-process elements
such as carbon-12, oxygen-16, neon-20 etc.,
which can be produced by successive addition
of α 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.
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 neutronrich unstable nuclei which subsequently β-decay
to neutron-rich stable nuclei.
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
Question B4
Draw a labelled diagram of the “Hubble tuning
fork” system of galaxy classification.
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: elliptical
S: spiral
SB: barred spiral
Irr: irregular
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.
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.
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; there’s 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 it’s space
that’s expanding, not the galaxies that are moving)
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
extrasolar planets
 Note that you know this is coming, so more
detail expected in answers!
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.
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
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.
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
<10% of the star’s lifetime.
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.
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.
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).
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 (< 3AU)
 some planets are in orbits which are very small
indeed (<<1 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
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 Sun’s
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.
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).
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 “asteroidbelt” 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.
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