Chapter 15

Basic Skills/Definitions Quiz
Part A
What is the approximate chemical composition (by mass) with which all stars are born?
Hint A.1
Study Section 15.1
ANSWER:

three quarters hydrogen, one quarter helium, no more than 2% heavier elements 98% hydrogen, 2% helium half hydrogen and half helium 95% hydrogen, 4% helium, no more than 1% heavier elements
Part B
The total amount of power (in watts, for example) that a star radiates into space is called its ______.
Hint B.1
Study Section 15.1.
ANSWER:

absolute magnitude. flux. luminosity. apparent brightness.
Part C
Which of the following correctly states the inverse square law for light?
Hint C.1
Study Section 15.1
ANSWER:

\rm apparent brightness = \frac{luminosity}{4\pi\times (distance)^2} \rm luminosity = \frac{apparent brightness}{4\pi\times (distance)^2} \rm distance = \frac{luminosity}{4\pi\times (apparent brightness)^2} \rm apparent brightness =luminosity\times4\pi\times (distance)^2
Part D
Assuming that we can always measure the apparent brightness of a star, what does the inverse square law for light allow us to do?
Hint D.1
Study Section 15.1
ANSWER:

It allows us to calculate a star's surface temperature if we know either its luminosity or its distance. It allows us to determine the distance to a star from its apparent brightness. It allows us to calculate a star's luminosity if we know its distance, or calculate its distance if we know its luminosity. It allows us to determine both distance and luminosity from the apparent brightness.
Part E
If star A has an apparent magnitude of 3 and star B has an apparent magnitude of 5, which star is brighter in our sky?
Hint E.1
Study Section 15.1.
ANSWER:

Star B The two stars have the same brightness in our sky, but Star A is closer to us than Star B. Star A There is not enough information to answer the question.
Part F
Ten parsecs is about ______.
Hint F.1
Study Section 15.1.
ANSWER:

150 million kilometers. 10,000 seconds. 10 parallax seconds of angle. 32.6 light-years.
Part G
If star A is closer to us than star B, then Star A's parallax angle is ______.
Hint G.1
Study Section 15.1.
ANSWER:

fewer parsecs than that of Star B larger than that of Star B. hotter than that of Star B. smaller than that of Star B.
Part H
From hottest to coolest, the order of the spectral types of stars is:
Hint H.1
Study Section 15.1
ANSWER:

OBAFGKM ABCDEFG OMKGFBA OBAGFKM. ABFGKMO.
Part I
Our Sun is a star of spectral type _______.
Hint I.1
Study Section 15.1
ANSWER:

G F M S
Part J
Astronomers can measure a star's mass in only certain cases. Which case below might allow astronomers to measure a star's mass?
Hint J.1
Study Section 15.1.
ANSWER:

The star is of spectral type G. We know the star's luminosity and distance. The star is a member of a binary star system. The star is of spectral type A.
Part K
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?
Hint K.1
Study Section 15.1
ANSWER:

spectroscopic binary visual binary eclipsing binary double star
Part L
The axes on a Hertzsprung-Russell (H-R) diagram represent ______.
Hint L.1
Study Section 15.2
ANSWER:

luminosity and apparent brightness luminosity and surface temperature mass and luminosity mass and radius.
Part M
What can we infer, at least roughly, from a star's luminosity class?
Hint M.1
Study Section 15.2
ANSWER:

Its surface temperature. Its radius. Its mass. Its age in years.
Part N
On an H--R diagram, stellar radii ______.
Hint N.1
Study Section 15.2
ANSWER:

increase diagonally from the lower left to the upper right. are impossible to determine. decrease from left to right. are greatest in the lower left and least in the upper right.
Part O
On an H--R diagram, stellar masses ______.
Hint O.1
Study Section 15.2
ANSWER:

decrease from left to right. can be determined from a star's position in the diagram only for main sequence stars, and decrease from upper left to lower right. are impossible to determine. are greatest in the lower left and least in the upper right.
Part P
How is the lifetime of a star related to its mass?
Hint P.1
Study Section 15.2
ANSWER:

More massive stars live slightly shorter lives than less massive stars. More massive stars live considerably longer lives than less massive stars. More massive stars live considerably shorter lives than less massive stars. More massive stars live slightly longer lives than less massive stars.
Part Q
Each choice below lists a spectral type and luminosity class for a star. Which one is a red supergiant?
Hint Q.1
Study Section 15.2
ANSWER:

Spectral type M2, luminosity class I Spectral type G2, luminosity class V Spectral type O9, luminosity class I Spectral type M1, luminosity class V
Part R
What is the common trait of all main sequence stars?
Hint R.1
Study Section 15.2
ANSWER:

They generate energy through hydrogen fusion in their core. They are all spectral type G. They are in the final stage of their lives. They all have approximately the same mass.
Part S
Suppose our Sun were suddenly replaced by a supergiant star. Which of the following would be true?
Hint S.1
Study Section 15.2
ANSWER:

Earth would be inside the supergiant. Earth would fly off into interstellar space. The supergiant's surface temperature would be much hotter than as the surface temperature of our Sun. The supergiant would appear as large as the full Moon in our sky?
Part T
What is a white dwarf?
Hint T.1
Study Section 15.2
ANSWER:

It is a star that follows a period-luminosity relation. It is a main sequence star of spectral type F, which tends to look white in color. It is the remains of a star, comprised mostly of carbon, that no longer produces energy by nuclear fusion. It is a type of star that produces energy by gravitational contraction.
Part U
What is a light curve?
Hint U.1
Study Section 15.2
ANSWER:

A plot of how brightness changes with time. A plot of how brightness changes with size. The curve of a light ray as it is emitted by a star. The trajectory of a photon around a very dense object.
Part V
Which of the following statements comparing open and globular star clusters is NOT true?
Hint V.1
Study Section 15.3
ANSWER:

For both open and globular clusters, we can assume that all the stars in a particular cluster are about the same age. Open and globular clusters each typically contain a few hundred stars. Open clusters are found only in the disk of the galaxy while globular clusters may be found both in the disk and the halo of the galaxy. Stars in open clusters are relatively young while stars in globular clusters are very old.
Part W
What do we mean by the main-sequence turnoff point of a star cluster, and what does it tell us?
Hint W.1
Study Section 15.3
ANSWER:

It is the point in a star cluster beyond which main sequence stars are not found, and it tells us the cluster's distance. It is the mass of the most massive star in the star cluster, and it tells us the cluster's size. It is the spectral type of the hottest main sequence star in a star cluster, and it tells us the cluster's age. It is the luminosity class of the largest star in a star cluster, and it tells us the cluster's age.