Chapter 15
Conceptual Quiz
Part A
All stars are born with the same basic composition, yet stars can look quite different from one another. Which two factors primarily determine the innate characteristics of a star?
Hint A.1
Study Section 15.1
ANSWER:

Its apparent brightness and its distance. Its mass and its surface temperature. Its mass and its stage of life. Its age and its location in the galaxy. Its apparent brightness and its luminosity.
Part B
Based on the definition of apparent brightness, which units are appropriate for its measurement?
Hint B.1
Study Section 15.1
ANSWER:

joules Newtons watts per square meter watts
Part C
Suppose two stars are identical except that one is twice as far away from us as the other. Which statement is true?
Hint C.1
Study Section 15.1
ANSWER:

Both stars have the same luminosity, but the apparent brightness of the more distant star is four times as great as that of the closer star. Both stars have the same luminosity, but the apparent brightness of the closer star is twice as great as that of the more distant star. Both stars have the same luminosity, but the apparent brightness of the closer star is four times as great as that of the more distant star. Both stars have the same apparent brightness, but the luminosity of the closer star is four times as great as that of the more distant star.
Part D
Suppose that a star has a parallax angle of 1/20 arcsecond. If so, it is ______.
Hint D.1
Study Section 15.1
ANSWER:

20 light-years away. 10 parsecs away.. 20 parsecs away. 1/20 parsec away.
Part E
The star Vega has an absolute magnitude of about 4 and an apparent magnitude of about 0. Based on the definitions of absolute and apparent magnitude, we can conclude that ______.
Hint E.1
Study Section 15.1.
ANSWER:

Vega has a parallax angle of 1/10 arcsecond. Vega's surface temperature is cooler than the Sun. Vega's luminosity is less than that of our Sun. Vega is nearer than 10 parsecs from Earth.
Part F
Which of the following statements about spectral types of stars is NOT generally true?
Hint F.1
Study Section 15.1
ANSWER:

The spectral type of a star can be determined by identifying lines in its spectrum. The spectral type of a star can be used to determine its surface temperature. The spectral type of a star can be used to determine its distance. The spectral type of a star can be used to determine its color.
Part G
Sirius is a spectral type A star whereas Rigel is a spectral type B star. What can we conclude?
Hint G.1
Study Section 15.1.
ANSWER:

Sirius has a higher core temperature than Rigel. Rigel has a higher surface temperature than Sirius. Sirius has a higher surface temperature than Rigel. Rigel has a higher core temperature than Sirius.
Part H
Astronomers generally can measure a star's mass if only it is a member of a binary star system. What characteristics of the stars must we know to measure the masses of the stars in a binary system?
Hint H.1
Study Section 15.1.
ANSWER:

Their orbital period and average orbital distance. Their absolute magnitudes and their luminosities. Their spectral types and their distance from Earth. Their luminosities and distance from Earth.
Part I
Careful measurements reveal that a star maintains a steady apparent brightness at most times, except that at precise intervals of 73 hours the star becomes dimmer for about 2 hours. The most likely explanation is that:
Hint I.1
Study Section 15.1
ANSWER:

The star is a Cepheid variable. The star is periodically ejecting gas into space, every 73 hours. The star is a member of an eclipsing binary star system. The star is a white dwarf.
Refer to this sketch of an H--R diagram for the following questions.







Part J
To answer this question, refer to the labels (a) through (e) on the H--R diagram sketch. [Note that choice (a) refers to the entire main sequence, while (c) and (d) refer to only small parts of the main sequence. If choices (c) or (d) offer better answers to the question than (a), use the best choice.] Which group represents stars that are cool and dim?


Hint J.1
Study Section 15.2
ANSWER:

D C A B E
Part K
To answer this question, refer to the labels (a) through (e) on the H--R diagram sketch. [Note that choice (a) refers to the entire main sequence, while (c) and (d) refer to only small parts of the main sequence. If choices (c) or (d) offer better answers to the question than (a), use the best choice.]Which group represents stars of the largest radii?


Hint K.1
Study Section 15.2
ANSWER:

E D A C B
Part L
To answer this question, refer to the labels (a) through (e) on the H--R diagram sketch. [Note that choice (a) refers to the entire main sequence, while (c) and (d) refer to only small parts of the main sequence. If choices (c) or (d) offer better answers to the question than (a), use the best choice.]Which group represents the most common type of stars?


Hint L.1
Study Section 15.2
ANSWER:

E D A C B
Part M
To answer this question, refer to the labels (a) through (e) on the H--R diagram sketch. [Note that choice (a) refers to the entire main sequence, while (c) and (d) refer to only small parts of the main sequence. If choices (c) or (d) offer better answers to the question than (a), use the best choice.]Which group represents stars that are extremely bright and emit most of their radiation as ultraviolet light?


Hint M.1
Study Section 15.2
ANSWER:

E D B A C
Part N
To answer this question, refer to the labels (a) through (e) on the H--R diagram sketch. [Note that choice (a) refers to the entire main sequence, while (c) and (d) refer to only small parts of the main sequence. If choices (c) or (d) offer better answers to the question than (a), use the best choice.]Which group represents stars with the longest main-sequence lifetimes?


Hint N.1
Study Section 15.2
ANSWER:

C B A E D
Part O
To answer this question, refer to the labels (a) through (e) on the H--R diagram sketch. [Note that choice (a) refers to the entire main sequence, while (c) and (d) refer to only small parts of the main sequence. If choices (c) or (d) offer better answers to the question than (a), use the best choice.]Which group represents stars fusing hydrogen in their cores?


Hint O.1
Study Section 15.2
ANSWER:

B A D C E
Part P
To answer this question, refer to the labels (a) through (e) on the H--R diagram sketch. [Note that choice (a) refers to the entire main sequence, while (c) and (d) refer to only small parts of the main sequence. If choices (c) or (d) offer better answers to the question than (a), use the best choice.]Which group represents stars that have no nuclear fusion in their cores?


Hint P.1
Study Section 15.2
ANSWER:

A B D C E
Part Q
You observe a star in the disk of the Milky Way, and you want to plot the star on an H-R diagram. You will need to determine all of the following, except:
Hint Q.1
Study Section 15.2
ANSWER:

spectral type of the star apparent brightness of the star in our sky mass of the star distance to the star
Part R
The approximate main-sequence lifetime of a star of spectral type O is ______.
Hint R.1
Study Section 15.2
ANSWER:

3 million years 10,000 years 300 million years 10 billion years
Part S
What happens to a pulsating variable star when its atmosphere is too opaque for radiation to easily escape?
Hint S.1
Study Section 15.2
ANSWER:

The atmosphere puffs out and nuclear fusion slows donw. The star explodes in a supernova explosion. The atmosphere puffs out and becomes more transparent to radiation. The atmosphere cools, allowing the star to shrink.
Part T
How did astronomers discover the relationship between spectral type and mass for main sequence stars?
Hint T.1
Study Section 15.1.
ANSWER:

Astronomers have measured stellar radii of main sequence stars in eclipsing binary systems when one star eclipses its companion; from the radii, astronomers can then calculate the mass of the main sequence star. Astronomers discovered the relationship by measuring the masses of main sequence stars in binary systems, and assume that the same relationship holds for single stars as well. Astronomers understand the process of hydrogen fusion and stellar structure so well that they can predict the masses of main sequence stars. The luminosity of any star, main sequence or not, tells us its mass.
Part U
The choices below each describe the appearance of an H--R diagram for a different star cluster. Which cluster is the youngest?
Hint U.1
Study Section 15.3
ANSWER:

The diagram shows main-sequence stars of spectral types G, K, and M, along with numerous giants and white dwarfs. The diagram shows main-sequence stars of every spectral type except O, along with a few giants and supergiants. The diagram shows main-sequence stars of all the spectral types except O and B, along with a few giants and supergiants. The diagram shows no main-sequence stars at all, but it has numerous supergiants and white dwarfs.
Part V
The choices below each describe the appearance of an H--R diagram for a different star cluster. Which cluster is most likely to be located in the halo of our galaxy?
Hint V.1
Study Section 15.3
ANSWER:

The diagram shows main-sequence stars of every spectral type except O, along with a few giants and supergiants. The diagram shows main-sequence stars of spectral types G, K, and M, along with numerous giants and white dwarfs. The diagram shows no main-sequence stars at all, but it has numerous supergiants and white dwarfs. The diagram shows main-sequence stars of all the spectral types except O and B, along with a few giants and supergiants.