Question 1

In the Hertzsprung-Russell diagram of Figure 17-15a, which of the following is the correct sequence of stars in order of increasing temperature?

Accepted Answer

The correct sequence in order of increasing temperature is the Sun, Procyon B, Deneb, and Sirius B. The Hertzsprung-Russell diagram plots stars by their luminosity and temperature, with temperature increasing from right to left.

Question 2

Using Figure 17-15, determine which of the following is the correct sequence of stars in order of increasing size or stellar radius.

Accepted Answer

Sirius B is a white dwarf, much smaller than the Sun. The Sun is an average-sized star, smaller than Mira, a red giant. Betelguese, a red supergiant, is the largest among them.

Question 3

Using the Hertzsprung-Russell diagram in Figure 17-15a, determine which type of star has the following characteristics: surface temperature of 40,000 K and luminosity 100,000 times that of the Sun.

Accepted Answer

The given characteristics of the star match those of a blue main-sequence star on the Hertzsprung-Russell diagram.

Question 4

Using the Hertzsprung-Russell diagram in Figure 17-15a. determine which type of star has the following characteristics: surface temperature of 10,000 K and luminosity 1/100 times that of the Sun.

Accepted Answer

White dwarfs are characterized by their high surface temperatures and low luminosities compared to the Sun, fitting the description of a star with a surface temperature of 10,000 K and a luminosity 1/100 times that of the Sun.

Question 5

Betelgeuse (the bright star in the shoulder of the constellation Orion) has a surface temperature of 3500 K and a luminosity 100,000 times that of the Sun. What is its approximate radius in terms of that of the Sun? (See Figure 17-15.(in the text))

Accepted Answer

Betelgeuse's luminosity and temperature can be used with the Stefan-Boltzmann law to find its radius. Given its high luminosity and lower temperature compared to the Sun, Betelgeuse must have a much larger radius, approximately 1000 times that of the Sun.

Question 6

In Figure 17-15, both Regulus and Mira are about 100 times as luminous as the Sun. What is the primary source of their radiant energy?

Accepted Answer

The answer of In Figure 17-15, both Regulus and Mira...

Question 7

What is the luminosity of a white dwarf star with the same surface temperature as the Sun? (See Figure 17-15a.(in the text))

Accepted Answer

According to Figure 17-15a in the text, a white dwarf star with the same surface temperature as the Sun has a luminosity of 10^-4 times that of the Sun. Therefore, the answer is C, which is 10^-4 times the Sun's luminosity, or 10^-4 x 10^1 = 10^-3.

Question 8

How much smaller (in radius) than the Sun is a white dwarf star whose temperature is the same as that of the Sun? (See Figure 17-15b.(in the text))

Accepted Answer

White dwarf stars are significantly smaller in radius than the Sun, typically about 100 times smaller, even if their temperature is the same. This is due to their high density and the state of matter they are composed of, which allows them to be much more compact.

Question 9

Betelgeuse and Barnard's star are both in the red portion of the H-R diagram as shown in Figure 17-15. What does this tell you about their relative sizes?

Accepted Answer

Betelgeuse is classified as a red supergiant, meaning it has expanded to become much larger than a main sequence star like Barnard's star. Therefore, D is the best choice as it correctly describes their relative sizes based on their location in the H-R diagram.

Question 10

All of the statements comparing the Sun to the giant stars are true EXCEPT which of the following? (Refer to Figure 17-15a.)

Accepted Answer

Giant stars are generally larger and more luminous than the Sun, which means they emit more light. However, they are not necessarily hotter; many giant stars have cooler surface temperatures compared to the Sun. Absolute magnitude is a measure of a star's luminosity; since giant stars are more luminous, they indeed have smaller (or more negative) absolute magnitudes, indicating greater brightness.

Question 11

For a main-sequence star with the same radius as the Sun, a surface temperature of 5000 K gives about half the Sun's luminosity. What would a surface temperature of 20,000 K give for the approximate luminosity of a solar mass star?

Accepted Answer

According to the Stefan-Boltzmann Law, luminosity is proportional to the fourth power of temperature. Therefore, if a star with the same radius as the Sun has a surface temperature of 5000 K and a luminosity of 0.5 $L_{\odot}$, then a solar mass star with a surface temperature of 20,000 K would have a luminosity of 0.5 $(\frac{20000}{5000})^4$ $L_{\odot}$, which is approximately 140 $L_{\odot}$.

Question 12

One star has a spectral-luminosity class of B5 V (main sequence) and the other has a spectral-luminosity class of B5 Ia (supergiant). What difference do we see in the spectra of these two stars and why?

Accepted Answer

The main sequence star has broader hydrogen absorption lines due to its denser atmosphere and higher pressure compared to the supergiant. This increased density and pressure in the atmosphere of the main sequence star cause the atoms to be closer together, leading to broader absorption lines in its spectrum.

Question 13

How do the luminosities of main-sequence stars compare with those in luminosity class III?

Accepted Answer

Main-sequence stars are classified as luminosity class V and have a range of luminosities. While most main-sequence stars are less luminous than stars in luminosity class III, a few main-sequence stars can be more luminous. Therefore, the correct answer is C.

Question 14

The star Hadar has a spectral classification of B1 III. This tells us that Hadar is a:

Accepted Answer

Hadar's spectral classification of B1 III indicates it is a hot giant. The "B" denotes it is hot, and the "III" indicates it is a giant star, not a supergiant.

Question 15

How do the characteristics of a K5V star compare to those of a K5II star? The K5II star:

Accepted Answer

The Roman numeral II indicates a bright giant star, which, while having the same temperature as a K5V (main sequence) star due to both being classified as K5, would be more luminous and larger due to its advanced evolutionary stage and expanded outer layers.

Question 16

A particular star has an absolute magnitude of 0 and a spectral class similar to that of the Sun. Using Figure 17-18, how would this star be classified?

Accepted Answer

A star with an absolute magnitude of 0 and a spectral class similar to the Sun (G2) would be classified as a giant star, hence G2 III.

Question 17

A particular star has an absolute magnitude of + 12 and a spectral class of A5. Using Figure 17-18, how would this star be classified?

Accepted Answer

The absolute magnitude of +12 is significantly fainter than what is typical for main sequence stars (including A5 V), giants (A5 III), or supergiants (A5 I). This faintness is characteristic of white dwarfs, which are much less luminous than other types of stars with the same spectral class. Therefore, an A5 star with an absolute magnitude of +12 would be classified as an A-type white dwarf.

Question 18

What proportion of visible stars in the night sky are multiple-star systems?

Accepted Answer

About 50% of the visible stars in the night sky are part of multiple-star systems, meaning they have at least one stellar companion. This is a significant proportion, indicating that binary or multiple star systems are quite common in our galaxy.

Question 19

If two stars of unequal mass orbit each other under mutual attraction in circular orbits, what will be the location of the center of these orbits? (See Figure 17-21.(in the text))

Accepted Answer

The center of mass of the system will be closer to the more massive star because its gravitational pull is stronger, causing both stars to orbit around it. This point is known as the barycenter, and it will be located between the two stars but closer to the more massive one.

Question 20

In a binary star system, each star is in an elliptical orbit with the:

Accepted Answer

According to Kepler's laws of planetary motion, the two stars in a binary star system orbit around their center of mass. This means that the center of mass must be located at one focus of each star's elliptical orbit. Therefore, the best choice is D.

Quiz 17: The Nature of the Stars