Astronomy Ch 15 Practice Test

Flashcard maker : Niamh Mitchell
What is the approximate chemical composition (by mass) with which all stars are born?
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.
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
How is the lifetime of a star related to its mass?
More massive stars live much shorter lives than less massive stars.
Each choice below 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 basic factors are most important in determining the current appearance of a star?
its mass and its stage of life
Based on the definition of apparent brightness, which units are appropriate for its measurement?
watts per square meter
Star A is identical to Star B, except that 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 its 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.
The choices below each describe the appearance of an H-R diagram for a different star cluster. Which cluster is the youngest?
The diagram shows main-sequence stars of every spectral type except O, along with a few giants and supergiants.
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?
The diagram shows main-sequence stars of spectral types G, K, and M, along with numerous giants and white dwarfs.

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