Astronomy Chapter 12

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hat is the approximate chemical composition (by mass) with which all stars are b

Three-quarters hydrogen, one-quarter helium, no more than about 2% heavier elements

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


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

Its apparent brightness will decrease by a factor of 9.

Assuming that we can measure the apparent brightness of a star, what does the inverse square law for light allow us to do?

Calculate the star’s luminosity if we know its distance, or calculate its distance if we know its luminosity.

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

larger than that of Star B

Ten parsecs is about _________.

32.6 light-years

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

Star A

From hottest to coolest, the order of the spectral types of stars is _________.


Our Sun is a star of spectral type _________.


Astronomers can measure a star’s mass in only certain cases. Which one of the following cases might allow astronomers to measure a star’s mass?

The star is a member of a binary star system.The star is a member of a binary star system.

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?

Spectroscopic binary

The axes on a Hertzsprung-Russell (H-R) diagram represent _________.

luminosity and surface temperature

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

Its size (radius)

On an H-R diagram, stellar radii _________.

increase diagonally from the lower left to the upper right

On an H-R diagram, stellar masses _________.

can be determined for main-sequence stars but not for other types of stars

More massive stars have __________ lives than less massive stars.

much shorter

Each choice that follows lists a spectral type and luminosity class for a star. Which one is a red supergiant?

Spectral type M2, luminosity class I

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

They generate energy through hydrogen fusion in their core.

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

Earth would be inside the supergiant.

What is a white dwarf?

The remains of a star that ran out of fuel for nuclear fusion

Which of the following statements comparing open and globular star clusters is not true?

Open and globular clusters each typically contain a few hundred stars.

What do we mean by the main-sequence turnoff point of a star cluster, and what does it tell us?

It is the spectral type of the hottest main-sequence star in a star cluster, and it tells us the cluster’s age.

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?

Mass and stage of life

Which units are appropriate for measurement of apparent brightness?

Watts per square meter

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

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

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

20 parsecs away

A star’s color is related to it’s surface temperature because _________.

stars emit thermal radiation

Which of the following statements about spectral types of stars is not generally true?

The spectral type of a star can be used to determine its distance.

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

Rigel has a higher surface temperature than Sirius.

To calculate the masses of stars in a binary system, we must measure their _________.

orbital period and average orbital distance

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 significantly dimmer for about 2 hours. The most likely explanation is that _________.

the star is a member of an eclipsing binary star system

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 _________.


The approximate main-sequence lifetime of a star of spectral type O is _________.

3 million years

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

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

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