Question 1

As a solar mass protostar moves on to the main sequence,
A) its surface temperature and luminosity increase.
B) its surface temperature increases and its luminosity decreases.
C) its surface temperature and luminosity decrease.
D) its surface temperature decreases and its luminosity increases.
E) its surface temperature and luminosity remain the same.

Accepted Answer

B

Question 2

What happens after the helium flash?
A) The core quickly heats up and expands.
B) The star breaks apart in a violent explosion.
C) The core suddenly contracts.
D) The core stops fusing helium.
E) The star starts to fuse helium in a shell outside the core.

Accepted Answer

After the helium flash, the core of the star quickly heats up and expands, stabilizing the fusion of helium into carbon in the core.

Question 3

What did Carl Sagan mean when he said that we are all "star stuff"?
A) that life would be impossible without energy from the Sun
B) that the Earth formed at the same time as the Sun
C) that the carbon, oxygen, and other elements essential to life were created by nucleosynthesis in stellar cores
D) that the Sun formed from the interstellar medium: the "stuff" between the stars
E) that the Universe contains billions of stars

Accepted Answer

Carl Sagan meant that the carbon, oxygen, and other elements essential to life were created by nucleosynthesis in stellar cores. This means that the building blocks of life, including the atoms in our bodies, were formed in the cores of stars that went supernova, scattering these elements throughout the universe to eventually form new stars and planets, including our own.

Question 4

No stars are expected with masses greater than 150 times our Sun because
A) molecular clouds do not have enough material to form such massive stars.
B) they would fragment into binary stars because of their rapid rotation.
C) they would generate so much power that they would blow themselves apart.
D) they would shine exclusively at X-ray wavelengths and would be difficult to detect.
E) they would be too massive for hydrogen fusion to occur in their cores.

Accepted Answer

Stars with masses greater than 150 times that of the Sun would generate immense radiation pressure, which would overcome gravitational forces and cause the star to blow itself apart.

Question 5

Helium fusion results in the production of
A) hydrogen.
B) oxygen.
C) carbon.
D) nitrogen.
E) iron.

Accepted Answer

Helium fusion involves combining helium nuclei to form heavier elements, such as carbon. Therefore, the correct answer is option C) carbon.

Question 6

Which of the following masses separates low mass stars from high mass stars?
A) About 150 solar masses
B) About 50 solar masses
C) About 2 solar masses
D) About 1 solar mass
E) About 0.08 solar masses

Accepted Answer

The mass of a star determines its fate. Low mass stars (like our Sun) eventually evolve into red giants and then white dwarfs, while high mass stars evolve into supergiants and then explode as supernovae. The dividing line between low mass and high mass stars is around 2 solar masses. Therefore, the correct choice is C) About 2 solar masses.

Question 7

Which two energy sources can help a star maintain its internal thermal pressure?
A) nuclear fusion and gravitational contraction
B) nuclear fission and gravitational contraction
C) nuclear fusion and nuclear fission
D) chemical reactions and gravitational contraction
E) nuclear fusion and chemical reactions

Accepted Answer

Nuclear fusion and gravitational contraction are the two energy sources that can help a star maintain its internal thermal pressure. Nuclear fusion occurs when atomic nuclei come together and form heavier nuclei, releasing energy in the process. This energy helps maintain the star's internal thermal pressure. Gravitational contraction is the process by which gravity compresses the star's material, causing it to heat up and maintain its internal thermal pressure. Nuclear fission and chemical reactions do not occur in stars, while nuclear fusion and gravitational contraction are the primary sources of energy that power stars.

Question 8

What can we learn about a star from a life track on an H-R diagram?
A) how long ago it was born
B) when it will die
C) where it is located
D) what surface temperature and luminosity it will have at each stage of its life
E) all of the above

Accepted Answer

A life track on an H-R (Hertzsprung-Russell) diagram shows the changes in a star's surface temperature and luminosity throughout its life, but it does not directly provide information about the star's age, death, or location.

Question 9

What is a planetary nebula?
A) a disk of gas surrounding a protostar that may form into planets
B) what is left of its planets after a low-mass star has ended its life
C) the expanding shell of gas that is no longer gravitationally bound to the remnant of a low-mass star
D) the molecular cloud from which protostars form
E) the expanding shell of gas that is left when a white dwarf explodes as a supernova

Accepted Answer

A planetary nebula is the expanding shell of gas that is no longer gravitationally bound to the remnant of a low-mass star. It is formed when such a star exhausts its nuclear fuel and sheds its outer layers, which ionize and glow due to the intense radiation from the hot core. The resulting nebula is often a colorful and intricate structure that lasts for a few tens of thousands of years before dispersing into the interstellar medium. The name "planetary" comes from the mistaken belief of early astronomers that these objects resembled the disks of Uranus or Neptune, but they have no connection to planets or exoplanets.

Question 10

When does a star become a main-sequence star?
A) when the protostar assembles from its parent molecular cloud
B) the instant when hydrogen fusion first begins in the star's core
C) when the rate of hydrogen fusion in the star's core is high enough to sustain gravitational equilibrium
D) when a star becomes luminous enough to emit thermal radiation
E) when hydrogen fusion is occurring throughout the star's interior

Accepted Answer

A star becomes a main-sequence star when the rate of hydrogen fusion in its core is sufficient to maintain gravitational equilibrium, balancing the inward pull of gravity with the outward pressure from fusion.

Question 11

Why does a star grow larger after it exhausts its core hydrogen?
A) The outer layers of the star are no longer gravitationally attracted to the core.
B) Hydrogen fusion in a shell outside the core generates enough thermal pressure to push the upper layers outward.
C) Helium fusion in the core generates enough thermal pressure to push the upper layers outward.
D) Helium fusion in a shell outside the core generates enough thermal pressure to push the upper layers outward.
E) The internal radiation generated by the hydrogen fusion in the core has heated the outer layers enough that they can expand after the star is no longer fusing hydrogen.

Accepted Answer

Once a star exhausts its core hydrogen, it will begin to fuse hydrogen in a shell surrounding the core. This releases energy and generates enough thermal pressure to push the outer layers of the star outward, causing it to expand and grow larger.

Question 12

Compared to the star it evolved from, a red giant is
A) hotter and brighter.
B) hotter and dimmer.
C) cooler and brighter.
D) cooler and dimmer.
E) the same temperature and brightness.

Accepted Answer

A red giant is cooler than the star it evolved from due to its expanded outer layers, but it is brighter because the expansion increases its surface area, allowing it to emit more light.

Question 13

At approximately what temperature can helium fusion occur?
A) 100,000 K
B) 1 million K
C) 10 million K
D) 100 million K
E) 100 billion K

Accepted Answer

Helium fusion, also known as the triple-alpha process, typically occurs at temperatures around 100 million Kelvin.

Question 14

What is the fate of an isolated brown dwarf?
A) It will become a white dwarf.
B) It will become a neutron star.
C) It will become a black hole.
D) It will slowly evaporate to nothing.
E) It will remain a brown dwarf forever.

Accepted Answer

Brown dwarfs are substellar objects that do not have enough mass to sustain hydrogen fusion in their cores. As a result, they do not evolve into other types of stellar remnants like white dwarfs, neutron stars, or black holes. Instead, they will gradually cool and fade over time, remaining as brown dwarfs indefinitely.

Question 15

Which of the following statements about degeneracy pressure is not true?
A) Degeneracy pressure varies with the temperature of the star.
B) Degeneracy pressure can halt gravitational contraction of a star even when no fusion is occurring in the core.
C) Degeneracy pressure keeps any protostar less than 0.08 solar mass from becoming a true, hydrogen-fusing star.
D) Degeneracy pressure is a consequence of the laws of quantum mechanics.

Accepted Answer

Degeneracy pressure does not vary with the temperature of the star. It depends only on the density of the material and the laws of quantum mechanics.

Question 16

What happens when a star like the sun exhausts its core hydrogen supply?
A) Its core contracts, but its outer layers expand and the star becomes bigger and brighter.
B) It contracts, becoming smaller and dimmer.
C) It contracts, becoming hotter and brighter.
D) It expands, becoming bigger but dimmer.
E) Its core contracts, but its outer layers expand and the star becomes bigger but cooler and therefore remains at the same brightness.

Accepted Answer

When a star like the sun exhausts its core hydrogen, the core contracts and heats up, causing the outer layers to expand and cool, making the star bigger and brighter as it becomes a red giant.

Question 17

What happens to the core of a star after it ejects a planetary nebula?
A) It contracts from a protostar to a main-sequence star.
B) It breaks apart in a violent explosion.
C) It becomes a white dwarf.
D) It becomes a neutron star.
E) none of the above

Accepted Answer

After a star ejects a planetary nebula, the core of the star collapses under gravitational pressure and becomes a white dwarf.

Question 18

What type of star is our Sun?
A) A low-mass star
B) An intermediate-mass star
C) A high-mass star

Accepted Answer

Our Sun has a mass that is considered low in comparison to other stars in the universe. It is classified as a main sequence star with a spectral type of G2V. Its mass is about 1.989 x 10^30 kg or 333,000 times the mass of Earth.

Question 19

How many helium nuclei fuse together when making carbon?
A) 2
B) 3
C) 4
D) It varies depending on the reaction
E) Helium cannot fuse into carbon

Accepted Answer

Three helium nuclei, also known as alpha particles, fuse together in a process called the triple-alpha process to form a carbon nucleus.

Question 20

What percentage of a star's total lifetime is spent on the main sequence?
A) 10%
B) 20%
C) 50%
D) 90%
E) 100%

Accepted Answer

Stars spend approximately 90% of their total lifetime on the main sequence, where they fuse hydrogen into helium in their cores.

Quiz 12: Star Stuff

As a solar mass protostar moves on to the main sequence,

What happens after the helium flash?

What did Carl Sagan mean when he said that we are all "star stuff"?

No stars are expected with masses greater than 150 times our Sun because

Helium fusion results in the production of

Which of the following masses separates low mass stars from high mass stars?

Which two energy sources can help a star maintain its internal thermal pressure?

What can we learn about a star from a life track on an H-R diagram?

What is a planetary nebula?

When does a star become a main-sequence star?

Why does a star grow larger after it exhausts its core hydrogen?

Compared to the star it evolved from, a red giant is

At approximately what temperature can helium fusion occur?

What is the fate of an isolated brown dwarf?

Which of the following statements about degeneracy pressure is not true?

What happens when a star like the sun exhausts its core hydrogen supply?

What happens to the core of a star after it ejects a planetary nebula?

What type of star is our Sun?

How many helium nuclei fuse together when making carbon?

What percentage of a star's total lifetime is spent on the main sequence?