Download Chapters 16,17

Homework Chapters 16-17
ASTRON 311 Introduction to Astronomy
Due: January 16, 2014
Prof. Menningen
p. 1/2
Name: ______________________________________
1.
When distances were carefully measured from Earth to globular clusters above and below the Milky Way plane (where
our view of them is not obscured by interstellar dust and gas), their distribution was found to be
a. in a relatively flat disk almost perpendicular to the plane of the galaxy, with relatively higher density of
clusters toward its center.
b. spherically symmetric about a point in the constellation Sagittarius and concentrated in that
direction.
c. concentrated in the plane of the Milky Way and clustered around the Sun's position, indicating that the
Sun is close to the galaxy's center.
d. uniformly distributed throughout space, with no concentration in any area of the Milky Way.
2.
If a spacecraft were to travel outward from the Sun in a direction opposite to the galactic center, how many and which
spiral arms would it have to cross before reaching intergalactic space? (See lecture notes.)
a. None—because this direction takes the spacecraft out of the plane of the galaxy.
b. Six—the Orion-Cygnus, Carina-Sagittarius, Crux-Scutum, Norma, then Norma, Crux-Scutum, and
Carina-Sagittarius arms again.
c. Four—the Orion-Cygnus, Carina-Sagittarius, Perseus, and New Outer arms.
d. Three—the Orion-Cygnus, Perseus, and Outer arms.
3.
Which of the following statements correctly describes the rotation of our galaxy?
a. Objects in the disk have random orbits with no net rotation of the disk about the center of the galaxy,
and the halo rotates differentially (objects further from the center take longer to complete an orbit
than objects closer to the center).
b. The disk rotates differentially (objects further from the center take longer to complete an orbit than
objects closer to the center), and the halo rotates differentially (objects further from the center take longer
to complete an orbit than objects closer to the center).
c. The disk rotates like a solid object (objects at all distances take the same time to complete an orbit), and
the halo objects have random orbits with no net rotation of the halo about the center of the galaxy.
d. The disk rotates differentially (objects further from the center take longer to complete an orbit
than objects closer to the center), and the halo objects have random orbits with no net rotation of
the halo about the center of the galaxy.
4.
In what way are the orbits of stars in the galactic disk different from the orbits of planets in our solar system? (See
lecture notes and Figure 16.24).
The orbits of stars are different from the orbits of planets in that the stars are moving under the influence
of the gravitational force from widely distributed mass rather than that of a single object. The stellar
orbits do not follow Kepler's law, and especially the stars in the outer regions of the galaxy are orbiting
much faster than they would if the galactic mass were concentrated at its center..
5.
Describe the evidence for a supermassive black hole at the center of our Milky Way Galaxy.
The rapid movement of stars surrounding Sagittarius A* suggests that it is a supermassive black hole
whose mass is about 4 million solar masses. The object is less than 10 AU in diameter (about the size of
Jupiter's orbit) and hot gas and stars are in rapid orbit around it
6.
How many times has our solar system has orbited the center of our Galaxy in the past 5.0×109 years?
(a) What is the speed of our solar system as it orbits the galactic center? Our solar system is about 26,000 ly
from the galactic nucleus and the period of its orbit is about 2.2×108 years. There are 3.156×107 s in a year.
v
2 r 2  26000 ly 
ly 9.46  1012 km/ly

 7.426  104

 223 km/s
8
P
2.2  10 y
y
3.156  107 s/y
(b) How many orbits has the solar system made in the past 5.0×109 years?
N
t
5.0  109 y

 23 orbits
T 2.2  108 y/orbit
Homework Chapters 16-17
ASTRON 311 Introduction to Astronomy
Prof. Menningen
p. 2/2
7.
The type or group of galaxies that contains both the largest and smallest galaxies in the universe is ________. (See
lecture notes)
a. the ellipticals.
c. the Sc type galaxies.
b. the barred spirals.
d. the irregulars.
8.
Why are Cepheid variable stars useful for finding the distances to galaxies? Are there any limitations on their use for
this purpose?
Cepheid variable stars are used for finding distances because they are luminous, they are recognizable
due to their variation, and there is a straightforward way to determine their luminosities from their
periods. Their use is limited to the range where they can still be detected by telescopes.
9.
What evidence supports the existence of dark matter in clusters of galaxies?
The motions of galaxies in clusters, together with the universal law of gravity, indicate that there exists
very much more matter than the luminous matter in those regions. Also, in some cases gravitational
lensing by a galactic cluster provides evidence for dark matter.
10. When the results from the Hipparcos mission were released, with new and improved measurements of the parallaxes of
nearby stars within 500 pc, astronomers had to revise the distances to many remote galaxies millions of parsecs away.
Explain why.
Parallax is the bottom rung on the distance ladder. All standard candles are directly or indirectly
calibrated against the parallax measurements of nearby stars.
11. Suppose you discover a Type Ia supernova in a distant galaxy. At maximum brilliance, the supernova reaches an
apparent magnitude of +10.0. The absolute magnitude of a Type Ia supernova is M = −19.9. How far away is the
galaxy? (See lecture notes.)
Apply the formula from Lecture 28 notes:
1 m M 5

d  10 5 
parsecs
d  10 5 
1
10.0   19.9   5 
 9.55  106 pc  9.55 Mpc
12. A certain galaxy is observed to be receding from the Sun at a rate of 7500 km/s. The distance to this galaxy is measured
independently and found to be 1.4×108 pc. From these data, what is the value of the Hubble constant? See pp.470-471.
v  H0 r
H0 
where r  1.4  108 pc 
1 Mpc
 140 Mpc
1  106 pc
v 7500 km/s

 53.6 km/s/Mpc
r 140 Mpc