# Solutions to Assignment on Most of Chapter 20

Read Chapter 20. We have not covered Section 20.3 in lecture yet, but will revisit that section by the time that we finish discussing Chapter 22

1.)
a.) (1 point) The stars in elliptical galaxies are (pick one) young, old, some are young and some are old. Answer: old

b.) (1 point) The stars in irregular galaxies are (pick one) young, old, some are young and some are old. Answer: some are young and some are old

c.) (1 point) The stars in spiral galaxies are (pick one) young, old, some are young and some are old. Answer: some are young and some are old

d.) (1 point) The stars in elliptical galaxies are (pick one) red, blue, various colors including red and blue. Answer: red

e.) (1 point) The stars in irregular galaxies are (pick one) red, blue, various colors including red and blue. Answer: various colors including red and blue

f.) (1 point) The stars in spiral galaxies are (pick one) red, blue, various colors including red and blue. Answer: various colors including red and blue

g.) (1 point) Which type of galaxy has more gas (compared with the mass of its stars), a spiral galaxy or an elliptical galaxy? Answer: a spiral galaxy

h.) (1 point) Which type of galaxy has more gas (compared with the mass of its stars), an irregular galaxy or an elliptical galaxy? Answer: an irregular galaxy

i.) (1 point) Which type of galaxy has had recent star formation, a spiral galaxy or an elliptical galaxy? Answer: a spiral galaxy

j.) (1 point) Which type of galaxy has had recent star formation, an irregular galaxy or an elliptical galaxy? Answer: an irregular galaxy

k.) (1 point) If you had a spiral galaxy and removed the disk, the result would be most like a (pick one) elliptical galaxy, irregular galaxy, or spiral galaxy. Answer: elliptical galaxy

2.) By "random orbit", I mean that the stars can orbit clockwise or counterclockwise and vertically or horizontally, etc. True or false:
a.) (1 point) the vast majority of stars in the bulges of spiral galaxies are on random orbits
b.) (1 point) the vast majority of stars in the disks of spiral galaxies are on random orbits
c.) (1 point) the vast majority of stars in the halos of spiral galaxies are on random orbits
d.) (1 point) the vast majority of stars in elliptical galaxies are on random orbits

3.) (1 point) Is our galaxy in a group of galaxies or a cluster of galaxies?
Answer: our galaxy is in a group of galaxies

4.)
a.) (2 points) What does the term "standard candle" mean in astronomy?
Answer: A "standard candle" refers to a type of object whose luminosity is already known. Some standard candles are types of objects in which all of the objects have the same luminosity. But, a detail that you need not have noted is that in some cases, the object's luminosity is merely calculatable from an observable characteristic of the object (such as the period of brightening and dimming of Cepheid Variable stars) enabling astronomers to determine the object's luminosity.
b.) (2 points) List 2 characteristics that make white dwarf supernovae explosions good standard candles in astronomy.
Answer: Every white dwarf supernova explosion has the same luminosity (at the peak of the explosion) and white dwarf supernova are extremely bright, so can be seen from very far distances

5.) Let us use white dwarf supernova explosions as standard candles. Be aware that immediately after a supernova explosion, it grows in brightness. Then, it reaches a peak brightness, then it dims.
a.) (4 points) First, we need to calibrate our standard candle, i.e. to determine how bright it is. So, astronomers observe a white dwarf supernova explosion that is close enough for its distance to be measured by another method. Suppose that a white dwarf supernova explosion had been observed in the Andromeda galaxy (whose distance is 2.5 million light years from us). Suppose that when the supernova explosion was brightest, observers on Earth observed an apparent brightness of 5.4 x 10-10 Watts/meter2. What is the luminosity of the white dwarf supernova explosion when its brightness is at its maximum?
Answer: L = 4 pi d2 apparent brightness
= 4 pi (2.5 x 106 light years x 9.46 x 1015 meters/light year)2 x 5.4 x 10-10 Watts/meter2
= 3.80 x 1036 Watts

b.) (4 points) Later, astronomers observed another white dwarf supernova explosion, but it was in a galaxy that is much further away. They observed an apparent brightness of 3.4 x 10-13 Watts/meter2 when the brightness was at its maximum. Assuming that the new supernova explosion had the same peak luminosity as the explosion in the Andromeda galaxy, what is the distance between the Earth and the new supernova explosion (and the galaxy in which that star exploded)?
Answer: given that L = 4 pi d2 x apparent brightness,
d2 = L / (4 pi x apparent brightness), so
d = square root of {L / (4 pi x apparent brightness)}
d = square root of {3.80 x 1036 Watts / (4 pi x 3.4 x 10-13 Watts/meter2)}
d = square root of {8.89 x 1047 meters
d = 9.43 x 1023 meters
d = 1.0 x 108 light years, i.e., 100 million light years

6.) (3 points) Excluding white dwarf supernova explosions, list 3 other types of standard candles that are used in astronomy to find distances and that are listed in our textbook.
Answer: any three of the following set would be acceptable:
Individual G2 main sequence stars (i.e. stars like the Sun),
clusters of stars from which the main sequence can be identified (the technique is called main sequence fitting),
Cepheid variable stars,
spiral galaxies (the technique uses the rotation speed and is called the Tully-Fisher relation).
Note that parallax is NOT a standard candle.