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Question 1

Stars more massive than the Sun are rare. None of the stars within 30 light-years of the Sun has a mass greater than ………… times that of the Sun.

Answer 1

four

Question 2

Searches at large distances from the Sun have led to the discovery of a few stars with masses up to about …….. times that of the Sun, and a handful of stars (a few out of several billion) may have masses as large as ………. solar masses.

Answer 2

100 / 250

Question 3

According to theoretical calculations, the smallest mass that a true star can have is about ………. that of the …………….. By a “true” star, astronomers mean one that becomes hot enough to fuse protons to form helium

Answer 3

1/12 / Sun

Question 4

Objects with masses between roughly 1/100 and 1/12 that of the Sun may produce energy for a brief time by means of nuclear reactions involving ………….., but they do not become hot enough to fuse protons.

Answer 4

deuterium

Such objects are intermediate in mass between stars and planets and have been given the name brown dwarfs

Question 5

Brown dwarfs are similar to Jupiter in radius but have masses from approximately 13 to 80 times larger than the mass of Jupiter.

Answer 5

R 2

Question 6

The more massive stars are generally also the more luminous. This relationship, known as the ………….. ……..

Answer 6

mass-luminosity relation

Question 7

We can also state the mass-luminosity relation in mathematical terms:

Answer 7

Question 8

It is easy to measure the diameter of the Sun. Its angular diameter—that is, its apparent size on the sky—is about 1/2°. If we know the angle the Sun takes up in the sky and how far away it is, we can calculate its true (linear) diameter, which is 1.39 million kilometers, or about 109 times the diameter of Earth.

Answer 8

Unfortunately, the Sun is the only star whose angular diameter is easily measured. All the other stars are so far away that they look like pinpoints of light through even the largest ground-based telescopes.

Question 9

Accurate sizes for a large number of stars come from measurements of ………… ………… star systems, Some binary stars are lined up in such a way that, when viewed from Earth, each star passes in front of the other during every revolution. When one star blocks the light of the other, preventing it from reaching Earth, the luminosity of the system decreases, and astronomers say that an ………….. has occurred.

Answer 9

eclipsing binary / eclipse

Question 10

The discovery of the first eclipsing binary helped solve a long-standing puzzle in astronomy. The star ……….., in the constellation of Perseus, changes its brightness in an odd but regular way.

Answer 10

Algol

Question 11

During the time interval between the first and second contacts, the smaller star has moved a distance equal to its own diameter. During the time interval from the first to third contacts, the smaller star has moved a distance equal to the diameter of the larger star.

If the spectral lines of both stars are visible in the spectrum of the binary, then the speed of the smaller star with respect to the larger one can be measured from the Doppler shift.

Answer 11

The speed multiplied by the time interval from the first to second contact gives the diameter of the smaller star. We multiply the speed by the time between the first and third contacts to get the diameter of the larger star.

Question 12

Answer 12

Question 13

Answer 13

Question 14

Answer 14

Question 15

Answer 15

Question 16

Answer 16

Question 17

Answer 17

Question 18

Answer 18

Question 19

Answer 19

Question 20

In 1913, American astronomer Henry Norris Russell plotted the luminosities of stars against their spectral classes (a way of denoting their surface temperatures). This investigation, and a similar independent study in 1911 by Danish astronomer Ejnar Hertzsprung, led to the extremely important discovery that the …………….. and ………………. of stars are related

Answer 20

temperature and luminosity

Question 21

Following Hertzsprung and Russell, let us plot the temperature (or spectral class) of a selected group of nearby stars against their luminosity and see what we find . Such a plot is frequently called the Hertzsprung–Russell diagram, abbreviated ………………..

Answer 21

H–R diagram.

Question 22

in the H–R diagram, The great majority are aligned along a narrow sequence running from the upper left (hot, highly luminous) to the lower right (cool, less luminous). This band of points is called the ……….. ……….

Answer 22

main sequence

It represents a relationship between temperature and luminosity that is followed by most stars. We can summarize this relationship by saying that hotter stars are more luminous than cooler ones.

Question 23

A number of stars, however, lie above the main sequence on the H–R diagram, in the upper-right region, where stars have low temperature and high luminosity.

How can a star be at once cool, meaning each square meter on the star does not put out all that much energy, and yet very luminous?

Answer 23

The only way is for the star to be enormous—to have so many square meters on its surface that the total energy output is still large. These stars must be giants or supergiants

Question 24

There are also some stars in the lower-left corner of the diagram, which have high temperature and low luminosity. If they have high surface temperatures, each square meter on that star puts out a lot of energy.

How then can the overall star be dim?

Answer 24

It must be that it has a very small total surface area; such stars are known as white dwarfs

Question 25

Our computer models of how stars evolve over time show us that a typical star will spend about 90% of its life fusing the abundant hydrogen in its core into helium. This then is a good explanation of why 90% of all stars are found on the main sequence in the H–R diagram. But if all the stars on the main sequence are doing the same thing (fusing hydrogen), why do they differ in luminosity and surface temperature (which is what we are plotting on the H–R diagram)?

Answer 25

The model stars with the largest masses are the hottest and most luminous, and they are located at the upper left of the diagram.

The least-massive model stars are the coolest and least luminous, and they are placed at the lower right of the plot. The other model stars all lie along a line running diagonally across the diagram.

In other words, the main sequence turns out to be a sequence of stellar masses.