astro ch 16

Question 1

Star A is identical to Star B, but Star A is twice as far from us as Star B. Therefore

Answer 1

both stars have the same luminosity, but the apparent brightness of Star B is four times that of Star A

Question 2

All stars are born with the same basic composition, yet stars can differ greatly in appearance. Which two factors are most important in determining the current appearance of a star?

Answer 2

mass and stage of life

Question 3

Why do some of the bright stars (such as the one indicated by the arrow) in this photo have cross-shaped spikes over them?

Answer 3

The spikes are an artifact of photography through a telescope.

Question 4

Which of the following terms is given to a pair of stars that we can determine are orbiting each other only by measuring their periodic Doppler shifts?

Answer 4

spectroscopic binary

Question 5

If the star Alpha Centauri were moved to a distance 10 times farther than it is now, its parallax angle would

Answer 5

get smaller

Question 6

How did astronomers discover the relationship between spectral type and mass for main sequence stars?

Answer 6

By measuring the masses and spectral types of main-sequence stars in binary systems.

Question 7

The total amount of power (in watts, for example) that a star radiates into space is called its

Answer 7

luminosity

Question 8

According to the inverse square law of light, how will the apparent brightness of an object change if its distance to us triples?

Answer 8

Its apparent brightness will decrease by a factor of 9.

Question 9

Suppose our Sun were suddenly replaced by a supergiant star. Which of the following would be true?

Answer 9

Earth would be inside the supergiant.

Question 10

Sirius is a star with spectral type A star and Rigel is a star with spectral type B star. What can we conclude?

Answer 10

Rigel has a higher surface temperature than Sirius.

Question 11

What is Rigel’s surface temperature?

Answer 11

10,000 K

Question 12

Rigel’s luminosity is about _____ times the Sun’s luminosity

Answer 12

100,000

Question 13

Rigel’s radius is about _____ times the Sun’s radius.

Answer 13

80

Question 14

Based on its location on the HR diagram, what can we say about Rigel’s mass and lifetime?

Answer 14

Nothing, because it is not on the main sequence.

Question 15

Which is brighter in our sky, a star with apparent magnitude 2 or a star with apparent magnitude 7?

Answer 15

A star with apparent magnitude 2.

Question 16

Which units are appropriate for measurement of apparent brightness?

Answer 16

watts per square meter

Question 17

Compared to a main-sequence star with a short lifetime, a main-sequence star with a long lifetime is

Answer 17

less luminous, cooler, smaller, and less massive

Question 18

Compared to a high-luminosity main-sequence star, stars in the upper right of the H-R diagram are

Answer 18

cooler and larger in radius

Question 19

Compared to a low-luminosity main-sequence star, stars in the lower left of the H-R diagram are

Answer 19

hotter and smaller in radius

Question 20

If star A is closer to us than star B, then Star A’s parallax angle is

Answer 20

larger than that of Star B

Question 21

What can we infer, at least roughly, from a star’s luminosity class?

Answer 21

its size (radius)

Question 22

A star with a parallax angle of 1/20 arcsecond is

Answer 22

20 parsecs (~ 65.2 light-years) away

Question 23

You observe a star and you want to plot it on an H-R diagram. You will need to measure all of the following, except the star’s

Answer 23

mass

Question 24

Which of these stars has the coolest surface temperature?

Answer 24

a K star

Question 25

What is the common trait of all main-sequence stars?

Answer 25

They generate energy through hydrogen fusion in their core

Question 26

Our Sun is a star of spectral type

Answer 26

G

Question 27

Star A has an apparent magnitude = 3 and star B has an apparent magnitude = 5. Which star is brighter in our sky?

Answer 27

Star A

Question 28

Which of the following must be true if we are to infer (calculate) a star’s luminosity directly from the inverse square law for light?

Answer 28

No interstellar gas or dust absorbs or scatters light between us and the star.
We have measured the star’s apparent brightness.
We have measured the star’s distance.

Question 29

What do we mean by a star’s spectral type, and how is spectral type related to surface temperature and color.

Answer 29

Spectral types are a way of classifying stars according to their color or what spectral lines we see in their light. The spectral types run OBAFGKM, where O stars are the hottest and M are the coolest. Hotter stars look bluer to us, and cooler stars look redder.

Question 30

Before we can use parallax to measure the distance to a nearby star, we first need to know

Answer 30

the Earth-Sun distance

Question 31

Which of the following is a valid way of demonstrating parallax for yourself?

Answer 31

Hold up your hand in front of your face, and alternately close your left and right eyes.

Question 32

What is the cause of stellar parallax?

Answer 32

Earth’s orbit around the Sun.

Question 33

The more distant a star, the

Answer 33

smaller its parallax angle

Question 34

Approximately what is the parallax angle of a star that is 20 light-years away?

Answer 34

0.16 arcsecond

Question 35

Suppose that a star had a parallax angle of exactly 1 arcsecond. Approximately how far away would it be, in light-years?

Answer 35

3.3 light-years

Question 36

Describe what would happen to the surface temperature of a star if its radius increased by 4 times, but there was no change in luminosity.

Answer 36

Luminosity increases both with increasing radius and increasing temperature. Therefore, a lower temperature would be required to maintain the star’s constant luminosity.