Download PHYS 2410 General Astronomy Homework 8

PHYS 2410
General Astronomy
Homework 8
Due 12/16 before the classes
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Brief Essay (簡答題, 10 分)
1.
Why does the rotation of our galaxy suggest that it is more massive than previously thought?
Ans:
The mass of the galaxy can be found from its rotation curve.
the galaxy contains over 1 trillion solar masses.
only gives us 200 billion solar masses.
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However, the visible light from stellar radiation
To account for the difference, astronomers make a
hypothesis of dark matter that does not interact with the electromagnetic force, but whose
presence can be inferred from gravitational effect on visible matter.
Multiple Choice (單選題, 30 題, 每題 3 分)
1.
*
The density of a neutron star is
a.
about the same as that of a white dwarf.
b.
about the same as that of the sun.
c.
about the same as an atomic nucleus.
d.
about the same as a water molecule.
e.
smaller than expected because the magnetic field is so strong.
2.
A neutron star is expected to spin rapidly because
*
a.
they conserved angular momentum when they collapsed.
b.
they have high orbital velocities.
c.
they have high densities.
d.
they have high temperatures.
e.
the energy from the supernova explosion that formed them made them spin faster.
3.
*
Although neutron stars are very hot, they are not easy to locate because
a.
light does not escape from their event horizon.
b.
most lie beyond dense dust clouds.
c.
solid neutron material cannot radiate photons.
d.
they are only found in other galaxies.
e.
they have small surface areas.
4.
At extremely high densities and temperatures, electrons can be forced to fuse with protons.
This
reaction produces
*
5.
*
6.
a.
hydrogen.
b.
Helium and energy.
c.
degenerate electrons.
d.
neutrons and neutrinos.
e.
large amounts of radio radiation.
The density of a _________ is greater than the density of a ____________.
a.
white dwarf, neutron star
b.
neutron star, black hole
c.
pulsar, neutron star
d.
pulsar, white dwarf
e.
white dwarf, black hole
A pulsar requires that a neutron star
I. rotate rapidly.
II. have a radius of at least 10 km.
III. have a strong magnetic field.
IV. rotate on an axis that is different from the axis of the magnetic field.
*
7.
a.
I & III
b.
I & IV
c.
II, III, & IV
d.
I, III, & IV
e.
I, II, III, & IV
None of the pulsars emit visible light because
a.
pulsars are too hot to emit visible light.
b.
pulsars contain black holes that won't let visible light escape.
c.
the gravitational field of a pulsar is so great that the visible light emitted is red shifted.
d.
pulsars are too far away for the visible light to be bright enough to be detected at Earth.
*
e.
The premise is false.
8.
The event horizon
*
A few pulsars are known to emit visible light pulses.
a.
is believed to be a singularity.
b.
is a crystalline layer.
c.
has a radius equal to the Schwarzschild radius.
d.
marks the inner boundary of a planetary nebula.
e.
is located at the point where synchrotron radiation is created around a pulsar.
9.
The Schwarzschild radius of a 2 M black hole is approximately
*
a.
6 km.
b.
4 km.
c.
2 km.
d.
12 km.
e.
36 km.
10. The escape velocity of an object depends on
I.
the mass of the object.
II. the mass of the particle trying to escape.
III. the radius of the object.
IV. the distance from the center of the object to the particle trying to escape.
V. the speed of light.
*
a.
I, II, III, IV, & V
b.
I & IV
c.
I, II, III, & V
d.
I, III, & V
e.
I, & III
11. An isolated black hole in space would be difficult to detect because
*
a.
there would be no light source nearby.
b.
it would not be rotating rapidly.
c.
it would be stationary.
d.
very little matter would be falling into it.
e.
there would be very few stars behind it whose light the black hole could block out.
12. The orbit of the binary pulsar, PSR 1936+16, studied by Taylor and Hulse
*
a.
is so small that the orbital period is smaller than the pulsar period.
b.
is growing smaller, presumably by emitting gravitational waves.
c.
provides evidence that it is being orbited by at least 6 planets the size of Jupiter.
d.
shows large changes each time an x-ray burst is emitted from the system.
e.
contains a white dwarf and a black hole.
13. The search for black holes involves
*
a.
searching for single stars that emit large amounts of x-rays.
b.
finding x-ray binaries where the compact companion has a mass in excess of 3 M.
c.
searching for large spherical regions from which no light is detected.
d.
looking for pulsars with periods less than one millisecond.
14. The material that accretes onto a neutron star or black hole is expected to emit x-rays because
*
a.
the material will produce synchrotron radiation because of the strong magnetic field.
b.
hydrogen nuclei begin to fuse and emit high energy photons.
c.
the material will become hot enough that it will radiate most strongly at x-ray
wavelengths.
d.
as the material slows down it converts thermal energy to gravitational potential energy.
e.
none of the above
15. As material flows into a black hole
*
a.
the material will experience time dilation.
b.
the material will become hotter.
c.
the material will produce an absorption spectrum.
d.
the material will increase in mass.
e.
a and b
16. The escape velocity at the event horizon around a black hole is
*
a.
smaller than the speed of light.
b.
equal to the speed of light.
c.
larger than the speed of light.
d.
irrelevant since nothing (including light) can escape from a black hole.
17. Which of the following can you never know about a black hole?
*
a.
Mass
b.
Rotation
c.
Electrical charge
d.
The material that has fallen in.
18. The center of our galaxy lies in the direction of the constellation of
*
a.
Ursa Minor.
b.
Ursa Major.
c.
Sagittarius.
d.
Orion.
e.
Monoceros.
19. A type I Cepheid has been located in an open cluster.
-6
The period of the Cepheid variable is 30 days, and
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What is the distance to this open cluster?
*
a.
100 pc
b.
10,000 pc
c.
20 pc
d.
300 pc
e.
2500 pc
-4
Absolute
Magnitude
Type I Cepheids
-2
Type II Cepheids
0
RR Lyrae Stars
0.1
0.3
1
3
10
Period (days)
20. Younger stars have more heavy elements because
*
a.
old stars destroy heavy elements as they age.
b.
young stars burn their nuclear fuels faster.
c.
heavy elements were made in previous generations of stars.
d.
all of the above
e.
heavy elements haven't had time to settle to the core of these younger stars.
21. The age of the Milky Way galaxy has been estimated to be at least 13 billion years based on
*
a.
observations of globular clusters.
b.
observations of open clusters.
c.
21-cm radiation from H I regions.
d.
the rotation curve of the galaxy.
e.
the energy produced by Sagittarius A*
22. The chemical abundance of population I stars
*
a.
indicates that they were formed before the population II stars.
b.
indicates that the material they formed from had been enriched with material from
supernovae.
c.
indicates that they contain very few heavy metals compared to halo stars.
d.
depends on the temperature of the star.
e.
depends on the mass of the star.
23. The orbits of population I stars
I.
are confined to disk of the galaxy.
II. are very elliptical.
III. are nearly circular.
IV. are randomly inclined to the disk of the galaxy.
30
100
*
a.
I
b.
IV
c.
I & IV
d.
II & IV
e.
I & III
24. The traditional theory for the formation of the Milky Way Galaxy suggests that the galaxy formed
*
a.
as material accreted around a massive black hole currently at the center of our galaxy.
b.
from a large cloud of material that formed stars and star clusters and slowly flattened
to a disk.
c.
from material that had been ejected in the violent explosion of a dying galaxy.
d.
as a result of mergers between several smaller groups of gas, dust, and stars.
e.
as two massive galaxies collided.
25. Radio maps of our galaxy show spiral arms because the
*
a.
arms have high Doppler shifts.
b.
gas in the spiral arms is hot enough to emit photons.
c.
dust in spiral arms is more dense than it is between the spiral arms.
d.
gas in spiral arms is more dense than it is between the spiral arms.
e.
stars in the spiral arms emit most of their energy at radio wavelengths.
26. CO observations of the galaxy reveal
*
a.
the location of dense neutral hydrogen clouds.
b.
the location of population II stars.
c.
the location of population I stars.
d.
the location of the galactic corona.
e.
the location of giant molecular clouds.
27. The mass of the black hole at the center of our galaxy can be estimated using the ________ of a
star orbiting it.
*
a.
semi-major axis
b.
orbital period
c.
Both answers a and b.
d.
Neither answer a nor b.
28. A several million solar mass ___________ is thought to be at the center of our galaxy.
a.
spiral arm
*
b.
globular cluster
c.
planet
d.
black hole
29. How does the traditional theory of the formation of the galaxy explain the origin of globular
clusters?
*
a.
They formed in the disk and later were ejected to the halo.
b.
They formed in other galaxies and were captured by close interactions.
c.
They build over time from the collision of stars in the halo.
d.
They formed early on during the free-fall collapse of the proto-galactic material.
30. What behavior do galactic rotation curves exhibit to suggest the existence of dark matter?
*
a.
Small velocities are seen at large distances from the galactic center.
b.
Small velocities are seen at distances close to the galactic center.
c.
Large velocities are seen at large distances from the galactic center.
d.
Large velocities are seen at distances close to the galactic center.