Download PHYSICS 1500 ASTRONOMY Sample Exam Solutions Section B

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PHYSICS 1500 ASTRONOMY
Sample Exam Solutions
Section A
1d, 2c, 3b, 4c, 5c, 6a, 7c, 8d, 9a, 10b, 11c, 12d, 13c, 14b, 15c, 16c, 17e, 18b, 19b, 20c.
Section B
Question 1
(a)
The seas show less cratering than the highlands.
(b) The orbits of the planets lie almost in the same plane, and the planets revolve around the
Sun in the same sense.
(c)
The orbits of long-period comets are random in inclination and shape implying that they
originate from a distant, approximately spherical distribution of bodies.
(d) The spectrum of the Sun peaks in the visible (green light) while that of a white dwarf
peaks in the blue or ultra violet.
(e)
The Main Sequence in an HR diagram based on Hipparcos parallaxes has a spread (i.e.
width) indicating that has of the stars have evolved from the zero age main sequence
(ZAMS) more than others.
Question 2
(a) The absorption lines in their spectra are much narrower than in the Sun, for example.
(b) Hot (young and massive) stars at the edge of molecular clouds; stars with associated
disks of material in molecular clouds.
(c)
The distribution of molecular clouds and HII regions traces Galactic spiral arms.
(d) The spectra of stars and gas in different parts of a spiral galaxy show absorption or
emission lines that are Doppler shifted by different amounts. These imply one side of
the galaxy is coming towards us and the other is going away (i.e. rotation).
(e)
We often see thin jets coming from the centre of the galaxy into the radio lobes.
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Question 3
(a) As a roughly spherical cloud core contracts under the action of gravity, it spins up
because of conservation of angular momentum. This effect is greatest for the gas
farthest from the rotation axis, which spins up sufficiently to resist farther contraction it orbits the centre instead. Material on the rotation axis, on the other hand, can free-fall
into the centre (also see Seeds, Horizons, 8th editon, figure 12-15)
High angular momentum material
Proto-Sun
Orbiting disk of gas
Low angular momentum material
(b) In the outer solar nebula, the temperature was low enough for grains to build up thick
icy coatings of, for example, CO. Thus the cores that formed were more massive than
those in the inner nebula, where grains were bare. The rock-ice cores were massive
enough to capture gas (H2/He) and build very massive planets.
(c)
Venus' atmosphere is relatively unchanged. Earth's is about 100 less dense and is now
~80% N2 and ~20% 02. Most of the original CO2 has been incorporated into rocks via
the oceans; some was converted to 02 via photosynthesis. The N2 just sat there. Mars'
atmosphere is 104 times less thick than Venus, but has a similar composition - it has
been slowly lost over time because of the planet's weaker gravity.
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Question 4
Red giant
(a)
Luminosity
SUN now
Main Sequence
White dwarf
Surface
temperature
(or see Seeds, Horizons, 8th editon, figure 10-5)
(b) A main sequence star is one that is burning hydrogen into helium in its core. This stage
occupies most of the life of a star. It is also important because we can easily identify the
main sequence in the diagram of clusters of stars, and use it to estimate the distance and
age of the cluster.
(c)
Nuclear reactions in the core of the Sun generate large amounts of energy. The heat
produces very high temperatures and hence very high pressures. These high pressures
support the Sun against the force of gravity and stop it collapsing.
Question 5
(a)
Globular cluster
(or see Seeds, Horizons, 8th editon, figure 12-7)
Halo around the galaxy
Bulge
Disk
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(b) Pop I - young, metal-rich, associated with Galactic disc. e.g. O stars
Pop II - old, metal-poor, associated with halo and bulge, e.g. globular clusters.
(c)
The "phases" are distinct density and temperature regimes that exist because of the
action of spiral arms, star formation, and super nova explosions. Most of the mass lies
in cold, dense molecular clouds, most of the volume is occupied by million-degree
rarefied gas which has little total mass.
(d) The orbital period of stars and gas in galactic disks increases with distance from the
centre. If the spiral arms are physically connected structures, the rotation of the disk
would wrap them up in a few hundred million years - the arms must be a pattern
instead.
A
~100 Myr
B
B
B – 1 orbit
A – 1/2 orbit
A
Question 6
(a) It seems that all Type I supernovae have about the same intrinsic luminosity at their
peak. Therefore, if we observe one in a distant galaxy, we can compare its apparent
brightness with its expected intrinsic brightness to estimate its distance. This will also
be the distance to the galaxy in which the supernova occurred.
(b) If the Universe were infinitely big, we would expect every line of sight to eventually
reach the surface of a star. In this case, the night sky should glow as brightly as the
surface of an average star. It does not, which implies the universe is not infinite, or that
it is not infinitely old (so that light from the more distant stars has not yet reached us), or
both of these.
(c)
Ultimately, the universe will either continue expanding forever or stop expanding and
begin to contract. This depends on whether there is enough matter in the universe for
gravity to halt the expansion. Thus, it is the density of the universe that determines its
ultimate fate : expansion or contraction.
(d) A gravitational lens occurs when light from a distant object is bent by the gravity from
an intervening object. We observe that some quasars are gravitationally lensed into
multiple images by galaxies which we know to be distant. This means the quasars must
be even further away than the galaxies causing the lensing.
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Extra ‘challenge’ Questions from 2005
paper onwards
Question 7
(a)
This illustration shows a dust disk, seen close to edge-on, obscuring a newly formed star
at the centre of the disk. The dust disk is very narrow near the stars and broader further
away. Interactions between the infalling material and the spinning stars creates jets of
gas emitted along the rotation axis of the star.
(b)
(Seeds, Horizons, 8th editon, figure 9-7)
Approximate position
of HH30 star –
around the birth line.
It remains obscured
perhaps only because
of the edge-on
viewpoint. Likely to
be ~1 solar mass.
(c)
Time to collapse depends on mass, but is probably say 1 million years (for a 4 solar
mass star) to 30 million years (for a 1 solar mass star) or more – these are the times to
reach the ZAMS (as illustrated above). Any timescale of this order is acceptable, with
mass noted as the determining factor.
(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. Use an IR camera to
penetrate dust.
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Question 8
(a)
Using the imaging instruments, near-IR cuts through the dust to reveal a very thin disk
and bulge while visible light shows the thicker disk with dust obscuration. 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 along the disk, caused by the systematic rotation pattern
in a spiral galaxy. This rotation curve will show a blue shift on one side and red on the
other – as seen in the image below (Seeds, Horizons, 8th editon, figure 13-6)
(c)
The spectrograph observation would be expected to reveal a systematic redshift of the
spectral lines (since the galaxy is ‘much more distant’), plus motions of stars in the
galaxy. IF(!) the galaxy was big enough to allow several positions of the spectrograph
slit across it, we would expect no systematic rotation curve since the stars are not
orbiting in a systematic way. We would also expect broadening of spectral lines caused
by light from stars with many different line-of-sight velocities being captured at once.