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A series of questions and answers from an astronomy exam held during semester 2, 2010. The questions cover various topics such as observations of stars and planets, the structure of galaxies, and the properties of different types of stars. Students are required to provide good descriptions of observations and discuss the underlying physical principles.
Typology: Exams
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MCQs
1 b
2 e
3 d
4 c
5 b
6 d
7 a
8 c
9 b
10 b
11 d
12 a
13 c
14 c
15 d
16 a
17 b
18 d
19 b
20 d
Question 1
For each part
Some examples are given. Other good examples may be possible.
Consult text or other staff if necessary.
(a)
The Moon has a small core and corresponding low overall density suggesting it was
formed predominantly by the lighter outer layers of the Earth and the impacting
object. It also has similar composition to the Earths upper mantle. The orbit is also
expanding, implying the Moon was much closer long ago. It is difficult to see how it
could have formed so close.
2 marks
(b)
The orbits of the planets lie almost in the same plane, and the planets revolve around
the Sun in the same sense.
2 marks
(c)
Meteorites are large meteors that reach the ground. They occur with different
compositions – e.g. rocky through to iron – rather than uniformly mixed. This
suggests an origin in a body large enough to have been differentiated – i.e. the heavier
elements settled towards the core.
2 marks
(d)
The granulation pattern visible on the surface is the top of the convective cells in the
upper part of the convective zone.
2 marks
(e)
A comprehensive list of the nearest stars shows a dominance of faint red stars.
Assuming this sample of a small portion of space is even approximately typical, it is
clear that they must dominate the numbers of stars.
2 marks
Question 3
(a)
(i)
Terrestrial planets
The force is ultimately Coulomb force/chemical bonds embodied in the strength of
the rocks.
name the force (or a clear indication) – 1 mark
sensible discussion of ‘how it arises’ – 1 mark
(ii)
‘Normal’ stars (like the Sun)
Gas Pressure governed by the temperature of the gas. Also [not required] a
component of radiation pressure that is more significant for more luminous stars.
name the force (or a clear indication) – 1 mark
sensible discussion of ‘how it arises’ – 1 mark
(iii)
White Dwarfs
Electron degeneracy pressure due to Pauli Exclusion, due to the enormous pressure.
name the force (or a clear indication) – 1 mark
sensible discussion of ‘how it arises’ – 1 mark
(iv)
Black Holes
Trick question? No force can compete against gravity here.
name the force (or a clear indication) – 1 mark
sensible discussion of ‘how it arises’ (or doesn’t) – 1 mark
(b)
Gravitational contraction releases gravitational PE, some of which leads to increased
temperature and vastly increased reaction rates because of the extreme sensitivity of
nuclear reactions to temperature.
2 marks
Question 4
(a)
Fusion of Hydrogen into Helium
1 mark
(b)
Mass
1 mark
Intense gravity due to mass requires high outward pressure and hence high
temperature. This leads to high reactions rates in the core and high luminosity.
2 marks
(c)
Helium Flash
1 mark
in which the core of star suddenly and explosively begins to fuse Helium in the triple
alpha process
2 marks
(d)
(i)
1 solar mass
White Dwarf
1/2 mark
red giant wind and planetary nebula phase leads to mass loss and final mass < 1.
solar masses
1/2 mark
(ii)
10 solar masses
Neutron Star
1/2 mark
red giant wind and supernova explosion leads to mass loss and final mass < 3 solar
masses
1/2 mark
(iii)
40 solar masses.
Black Hole
1/2 mark
red giant wind and supernova explosion leads to mass loss, but final mass > 3 solar
masses
1/2 mark
(c)
ISM has different states of gas – different density, temperature and
molecular/atomic/ionization state
1 mark
Specifics – see following table
At least two specific details given - 1 mark
(d)
Stars making up the spiral arms rotate at different velocities, meaning that the arms
should ‘wind up’ with time if the arms behaved like solid objects
1 mark
You don’t see evidence of this because the spiral arms mark the location of a spiral
density wave and stars in the arms at any moment will not continue to be part of the
arms later.
1 mark
Question 6 (a)
The velocities shown in the rotation curves of spiral galaxies do not decline at larger
radii as expected because of the presumed presence of dark matter in the outer
regions of these galaxies.
1 mark
(b)
The basic idea is that if you measure the redshift ( z ) then the Hubble constant ( H )
allows you to calculate a distance.
1 mark
[most should know this, but few will probably get the rest]
In fact, the Hubble law is a velocity-distance relation ( v = Hd ) predicted by theory
(General Relativity).
1 mark
The redshift-distance (z-d) relation is observed.
1 mark
These are only the same if you know how z and v are related – i.e. if you have a
specific cosmological model.
The redshift is a cosmological redshift, NOT a doppler shift.
1 mark
(c)
Type Ia supernova observations are the key observation suggesting acceleration
1 mark
Various other observational evidence (in particualr the microwave background) is
also consistent with an accelerating universe
1 mark
(d)
cosmological principle – any observer sees the same general appearance, regardless of
direction or location (i.e. universe is isotropic and homogeneous)
1 mark
related factors that may be mentioned:
1 mark max. for any of these or another valid comment)
Evidence?
one correct piece of evidence - 1 mark
(d)
Use HST or another space-based telescope (or a telescope using Adaptive Optics if
observing from the ground) for high (spatial) resolution of detail. The image was
taken with HST.
1 mark
Use an IR camera to penetrate dust.
1 mark
Question 8
Note that this is meant to be a more challenging question – reward signs of
intelligence!
(a)
Using the imaging instruments, near-IR cuts through the dust to reveal a very thin
disk and bulge
1 mark
while visible light shows the thicker disk with dust obscuration.
1 mark
For example, see the images (below) of the Milky Way from the inside.
Visible
Near-IR
(b)
Moving the spectrograph observation point (the ‘slit’) along the disk of the galaxy
will reveal the varying Doppler shift of the stars along the disk,
1 mark
caused by the systematic rotation pattern in a spiral galaxy.
1 mark
This rotation curve will show a blue shift on one side and red on the other – as seen in
the image below. The rotation curve will be relatively flat at larger radii.
One correct detail about the rotation curve - 1 mark
