Download Due: January 15, 2014 Name

Homework Chapters 14-15
Due: January 15, 2014
ASTRON 311 Introduction to Astronomy
Prof. Menningen
p. 1/3
Name: ______________________________________
1.
At which phase of a star's life will nuclear fusion reactions that convert helium into carbon and oxygen in the central
core of a star occur?
a. during and immediately after the (first) red giant or supergiant stage
b. during the protostar stage, before the main sequence
c. in the red giant stage, before the helium flash
d. after the main-sequence phase, before the star becomes a red giant
2.
A planetary nebula is
a. the spherical cloud of hot gas produced by a supernova explosion.
b. the disk of material in which planets are forming around a star other than the Sun.
c. a shell of ejected gases, glowing by fluorescence caused by ultraviolet light from a hot but dying
central star.
d. a gas cloud surrounding a planet after its formation and before the formation of the planet's moons.
3.
As a white dwarf evolves, the direction of its motion on the Herzsprung-Russell diagram below the main sequence is
upper left to lower right, which means that
a. its size must be increasing; therefore its luminosity is also increasing.
b. the release of gravitational energy heats the star as it shrinks and it will die as a hot but very small star.
c. its size or radius remains constant as it cools and becomes less luminous.
d. it shrinks as it cools, becoming a cold neutron star.
4.
Why do Cepheid stars pulsate? Why are these stars important to astronomers who study galaxies beyond the Milky
Way?
Cepheids pulsate because gravity and internal pressure cannot establish an equilibrium. These stars
are important to astronomers who study galaxies because Cepheids can be seen at extra-galactic
distances, and their period of pulsation reveals their luminosities, which in turn tells astronomers
their distances.
5.
When a supernova explosion results from core collapse in a massive star it appears to leave behind
a. a rapidly expanding shell of gas and a central neutron star or black hole.
b. a rapidly rotating shell of gas, dust, and radiation, but no central object.
c. a rapidly expanding shell of gas and a compact white dwarf star at its center.
d. nothing, the explosion changes all the matter completely into energy, which then radiates into space at
the speed of light.
6.
An astronomer plots the HR diagram of a star cluster and finds that it contains hot B-type stars on the main sequence
and cooler G- and K-type stars noticeably above the main sequence. This cluster is
a. impossible, because one cannot have cool stars above the main sequence when hot stars are on the main
sequence.
b. old, because the G and K stars are already evolving away from the main sequence.
c. of indeterminate age, because one cannot estimate the age of the cluster from the information given.
d. very young, because the G and K stars are still evolving toward the main sequence.
Homework Chapters 14-15
7.
ASTRON 311 Introduction to Astronomy
Prof. Menningen
p. 2/3
In July 1997 a supernova named SN 1997cw exploded in the galaxy
NGC 105 in the constellation Cetus (the Whale). It reached an
apparent magnitude of +16.5 at maximum brilliance, and its spectrum
showed an absorption line of ionized silicon. Use this information to
find the distance to NGC 105. (Hint: Inspect the light curves in the
figure to find the absolute magnitudes of typical supernovae at peak
brightness. The ionized silicon indicates it must have been a Type Ia
supernova. Recall that m  M  5log d  5 .)
From the figure we can therefore assume that the peak
absolute magnitude was M  –19. Because m = 16.5, we
can again turn to the equation
m  M  5log d  5
5log d  m  M  5
 16.5  19  5  40.5
40.5
log d 
 8.1  d  108.1  1.3 108 pc  130 Mpc
5
The galaxy NGC105 is approximately 130 Mpc from Earth.
8.
A pulsar is
a. a pulsating star, in which size, temperature, and light intensity vary regularly.
b. a binary star in which matter from one star is falling onto the second star.
c. a rapidly rotating neutron star, emitting beams of radio radiation and in some cases X rays and
visible light.
d. an object at the center of each galaxy, providing energy from its rapid rotation.
9.
The structure of a neutron star is likely to consist of
a. a corona, chromosphere, photosphere, convective zone, radiative zone, and core.
b. a very thin atmosphere, a rigid crust, and a liquid “sea” of neutrons.
c. an icy crust, a liquid metallic hydrogen “sea,” and a solid core of neutrons.
d. an accretion disk atmosphere, a layer of degenerate carbon atoms, and a solid core of neutrons.
10. Do all supernova remnants contain pulsars? Explain why or why not. Are all pulsars found within supernova remnants?
Explain why or why not. (See p. 410)
A type I supernova leaves no remnant star, unlike a type II supernova, which results from the collapse of
a massive star's iron core and which leaves a neutron star or a black hole. Furthermore, not all neutron
stars are pulsars because the radiation beam of a spinning neutron star might not point toward the Earth.
Not all pulsars are found within supernova remnants because the supernova blast can “kick up” the
neutron star to hundreds of kilometers per second, allowing it to leave the remnant.
11. The apparent expansion rate of the Crab nebula is 0.46 arcsec per year. The apparent size of the nebula is about 4 by 6
arcmin. If the expansion rate remains constant, calculate how long it will be until the long axis of the nebula has the
same apparent size as the full moon (1800 arcsec).
The full Moon has an angular diameter of 0.5o or 1800 arcsec. Both ends of the
nebula are expanding at 0.23 arcsec/yr so the diameter increases at 0.46 arcsec/yr.
t
diameter
1800  6.0 arcmin  60 arcsec/arcmin

 3100 yr
expansion rate
0.46 arcsec/yr
Homework Chapters 14-15
ASTRON 311 Introduction to Astronomy
Prof. Menningen
p. 3/3
12. Which effect has been useful (and successful) in the search for and identification of black holes in the universe?
a. their magnetic fields and their influence on nearby matter.
b. the effect of their angular momentum or spin on nearby matter.
c. the influence of their intense gravitational field on atoms and molecules that are emitting light from
the event horizons of the black holes.
d. their gravitational influence on nearby matter, particularly companion stars.
13. The physical properties of a black hole that allow it to interact with the rest of the universe include
a. its mass, the chemical or atomic structure of the matter within it, and its overall size.
b. its mass, its angular momentum or spin, and its temperature.
c. its mass, its electric charge, and its angular momentum or spin.
d. the size of its event horizon, the strength of its magnetic field, and the size of its solid core.
14. Describe two different predictions of the general theory of relativity and how these predictions were tested and
confirmed experimentally.
Relativity predicts the bending of starlight by the Sun. This was confirmed during an eclipse, and can
also be observed as the gravitational lensing by distant galaxies. Relativity also predicts a loss of energy
for an upward moving beam of light. This was confirmed by measuring the increase of the wavelength.
Orbital precession is also predicted by relativity and was confirmed by careful measurement of
Mercury's orbit. Special and general relativity have made many other predictions that have been tested
and confirmed experimentally.
15. Use Kepler’s third law to find the orbital period of a star moving in a circular orbit of radius 500 AU around the
supermassive black hole in M87, which has a mass of about 3.0×109 solar masses.
Applying Kepler’s third law:
P2 
a3
but if m1 is the mass of the star, it is much smaller than the black hole mass:
m1  m2
P
 500 AU 
a3

m2
3.0 109 M
3
 0.204 yr  74.5 days!