Question 1

What is unusual about main-sequence stars about 75% the mass of the Sun?

Accepted Answer

Main-sequence stars are stars that are fusing hydrogen in their cores, and their size and luminosity depend on their mass. Stars with around 75% of the mass of the Sun are still in the main sequence, but they have lower luminosities and surface temperatures than the Sun. There is nothing unusual about their fusion reactions, as they still fuse hydrogen like other main-sequence stars. Therefore, the correct answer is C, as all stars of this mass range are on the main sequence.

Question 2

When the Sun entered the main sequence,its core was approximately 75% hydrogen and 25% helium,by mass.What was the ratio of hydrogen nuclei to helium nuclei?

Accepted Answer

The mass ratio of hydrogen to helium is 3:1, but since each helium nucleus is made of 4 hydrogen nuclei, the ratio of hydrogen nuclei to helium nuclei is 12:1.

Question 3

The star zeta Pegasi (in the constellation Pegasus,the Flying Horse)has a spectral luminosity class of B8 V,which gives it a surface temperature of about 11,000 K.According to Table 11-2 of Comins,Discovering the Essential Universe,6th ed.,the expected main-sequence lifetime of zeta Pegasi is:

Accepted Answer

According to Table 11-2, a star with a spectral luminosity class of B8 has a main-sequence lifetime of about 500 million years. Therefore, the expected main-sequence lifetime of zeta Pegasi is about 500 million years.

Question 4

From the time a star of more than 1 solar mass reaches the ZAMS until it leaves the main sequence,the star's evolution on the main sequence is:

Accepted Answer

As a star of more than 1 solar mass evolves on the main sequence, it gradually increases in luminosity and slightly increases in temperature, moving upward and to the right on the Hertzsprung-Russell diagram.

Question 5

Red dwarfs are fully convective.In this,they are like:

Accepted Answer

Brown dwarfs are also fully convective, meaning that their entire interiors are convecting heat outward. White dwarfs, on the other hand, have no convective zones, while the Sun has only a small convective zone near its surface. Variable stars can be of various types and do not necessarily have fully convective interiors.

Question 6

The final stage in the evolution of a red dwarf star is:

Accepted Answer

Red dwarf stars have low mass and do not have enough pressure or temperature to fuse heavier elements after they exhaust their hydrogen fuel. Therefore, they ultimately become stars of pure helium, and will no longer generate energy through nuclear fusion.

Question 7

What is unique about a low-mass red dwarf star compared with stars at other stages of evolution,such as main-sequence stars?

Accepted Answer

Convection occurs throughout the interior of a low-mass red dwarf star, which is unique compared to other stars. This process allows the star to mix its materials from the core to the surface, preventing the formation of distinct core and outer layer regions and contributing to the star's longevity.

Question 8

How long does a star of three times the Sun's mass live compared with the lifetime of the Sun? (See Table 11-2,Comins,Discovering the Essential Universe,6th ed.)

Accepted Answer

According to Table 11-2 in Comins' "Discovering the Essential Universe," a star of three times the Sun's mass has a lifetime of approximately 10 million years, while the Sun has a lifetime of approximately 10 billion years. Therefore, a star of three times the Sun's mass lives about 1/20 as long as the Sun.

Question 9

Of all the low-mass red dwarfs (0.08 M < M < 0.4 M)that have formed since the Galaxy came into being 15 billion years ago,how many are presently on the main sequence?

Accepted Answer

Red dwarfs have extremely long lifetimes, much longer than the current age of the Universe (about 13.8 billion years), so all low-mass red dwarfs that have formed are still on the main sequence.

Question 10

Main-sequence stars can be divided into those with more than 75% of the mass of the Sun and those with less than this amount of mass.What is different about these two groups?

Accepted Answer

Stars with more than 75% of the mass of the Sun will eventually evolve off the main sequence, while the less massive stars will remain on it. Option A is incorrect because both groups are capable of undergoing nuclear fusion. Option B is incorrect because not all stars in the less massive group are fully convective. Option C is incorrect because both groups can be part of binary systems.

Question 11

The end of the life of a red dwarf star is predicted to be a sphere of almost pure helium.But no such spheres have been detected.What do we believe is the reason?

Accepted Answer

Red dwarfs have extremely long lifespans, often longer than the current age of the universe, so none have yet reached their end stage.

Question 12

The spiral galaxy in which we live is roughly 14 billion years old.Which statement about the stars that formed when our Galaxy was very young (say,during the first billion years)is TRUE? (See Table 11-2,Comins,Discovering the Essential Universe,6th ed.)

Accepted Answer

According to Table 11-2, stars with a mass of 3/4 of the Sun or less have lifetimes longer than 14 billion years and are still on the main sequence, while more massive stars have already evolved off the main sequence. Therefore, option B is the correct statement.

Question 13

The main-sequence lifetime of a star with half the mass of the Sun:

Accepted Answer

The main-sequence lifetime of a star is primarily determined by its mass, with lower-mass stars burning their fuel more slowly and thus having longer lifetimes. A star with half the mass of the Sun would burn its nuclear fuel more slowly than the Sun, resulting in a longer main-sequence lifetime.

Question 14

Some protostars,when they evolve onto the main sequence,are fully convective and use convection to transport material all the way from the core to the surface.Which stars show this characteristic?

Accepted Answer

Only the smallest protostars, of less than about 0.4 solar mass, are fully convective when they evolve onto the main sequence.

Question 15

What does the term hydrostatic equilibrium mean in reference to stars?

Accepted Answer

Hydrostatic equilibrium in reference to stars means that each layer within a star is in balance with respect to pressure and gravity, which prevents the star from collapsing or expanding. The weight of the material above a given layer creates a pressure that is balanced by the pressure generated by the energy released from nuclear fusion. This balance maintains the star's size and shape over time.

Question 16

A low-mass red dwarf (0.08 M < M < 0.4 M)will eventually NOT become a:

Accepted Answer

A low-mass red dwarf will eventually become a brown dwarf, not a red giant or a white dwarf.

Question 17

A low-mass red dwarf (0.08 M < M < 0.4 M)experiences which fusion reaction as the last fusion reaction in its core?

Accepted Answer

Low-mass red dwarfs do not have enough mass or temperature in their cores to undergo helium, carbon, or silicon fusion. The last fusion reaction they undergo is hydrogen fusion, which gradually slows down as the star ages and ascends the red giant branch.

Question 18

If you were able to return to Earth 1 million years into the future,which view of the sky would be MOST likely?

Accepted Answer

1 million years is a relatively short time period in astronomical terms, so the majority of the stars would still be in the same positions. However, some nearby stars may have moved, and some blue stars may have changed color due to evolution. So, the view of the sky would be very much as it is now with minor differences.

Question 19

The present estimate of the age of the universe is about 14 billion years.How long will a half-solar-mass star spend on the main sequence,compared with the present age of the universe? (See Table 11-2,Comins,Discovering the Essential Universe,6th ed.)

Accepted Answer

The answer of The present estimate of the age of...

Question 20

When does a star achieve static equilibrium?

Accepted Answer

A star achieves static equilibrium when the inward gravitational force is balanced by the outward pressure force due to the energy released by nuclear fusion in its core. This occurs for a star when it becomes a main-sequence star.

Quiz 11: The Lives of the Stars From Birth Through Middle Age