Question 1

Convection is important in stars because it
A) increases the temperature of the star.
B) mixes the gases of the star.
C) transports energy outward in the star.
D) carries the neutrinos to the surface of the star where they can escape.
E) b and c

Accepted Answer

Convection in stars not only helps to transport energy outward but also helps to mix the gases within the star, allowing it to maintain stability and avoid the build-up of heavier elements in the core.

Question 2

Protostars are difficult to observe because
A) the protostar stage is very short.
B) they are surrounded by cocoons of gas and dust.
C) they radiate mainly in the infrared.
D) all of the above
E) they are all so far away that the light hasn t reached us yet.

Accepted Answer

All three options listed are valid reasons why protostars are difficult to observe. The protostar stage is relatively short-lived, lasting only a few hundred thousand years, making it difficult to observe. Additionally, protostars are surrounded by gas and dust, which obscures their visible light and makes observations challenging. Finally, protostars radiate primarily in the infrared, which is less easily observable by telescopes on Earth. All three options are correct. The protostar stage is relatively short (compared to the star's lifetime) and usually hidden by cocoons of gas and dust. Additionally, protostars radiate mainly in the infrared, which is difficult to observe from Earth without specialized equipment.

Question 3

What causes the outward gas pressure that balances the inward pull of gravity in a main sequence star?
A) The rapid outward flow of gas.
B) The rapid inward flow of the gas.
C) The high temperature of the gas.
D) The high density of the gas.
E) c and d

Accepted Answer

The high temperature and high density of the gas in a main sequence star cause the outward gas pressure that balances the inward pull of gravity.

Question 4

Which of the following is NOT evidence of the existence of an interstellar medium?
A) extinction of light from distant stars
B) very narrow calcium absorption lines in the spectra of O and B stars
C) reddening of more distant stars
D) dark cloud silhouetted against background stars
E) molecular absorption lines in the spectra of cool stars

Accepted Answer

Molecular absorption lines in the spectra of cool stars are not evidence of the interstellar medium. These lines are typically associated with the atmospheres of the stars themselves, rather than with material between the stars.

Question 5

____ are small luminous nebulae excited by nearby young stars.
A) T Tauri stars
B) Herbig-Haro objects
C) O associations
D) Bok Globule
E) Giant molecular clouds

Accepted Answer

Herbig-Haro objects are small luminous nebulae that are excited by nearby young stars. T Tauri stars are also associated with the formation of Herbig-Haro objects, but they are not the luminous nebulae themselves. O associations and giant molecular clouds are not directly related to the formation of Herbig-Haro objects, and Bok Globules are dark clouds of gas and dust.

Question 6

Opacity is
A) the balance between the pressure and force of gravity inside a star.
B) the force that binds protons and neutrons together to form a nucleus.
C) the force that binds an electron to the nucleus in an atom.
D) a measure of the resistance to the flow of radiation (photons) through a gas.
E) the temperature and density at which a gas will undergo thermonuclear fusion.

Accepted Answer

Opacity is a measure of how easily radiation (photons) can pass through a material. In a gas, opacity depends on factors such as temperature, density, and the type of atoms or molecules present. High opacity means that radiation is slowed down or absorbed by the material, while low opacity means that radiation can easily pass through.

Question 7

The average star spends ____ of its lifetime on the main sequence.
A) 1%
B) 2%
C) 10%
D) 20%
E) 90%

Accepted Answer

The main sequence is the stage in a star's life where it is primarily converting hydrogen into helium through nuclear fusion. This stage lasts for the majority of a star's lifetime, and for stars like the Sun, it can last for about 90% of their total lifespan. Therefore, choice E is the best answer.

Question 8

The carbon-nitrogen-oxygen cycle
A) operates at a slightly lower temperature than the proton-proton chain.
B) is most efficient in star less massive than the sun.
C) occurs when carbon and oxygen combine to form nitrogen, which produces energy.
D) produces the energy responsible for bipolar flows.
E) combines four hydrogen nuclei to form one helium nucleus, which produces energy.

Accepted Answer

The answer of The carbon-nitrogen-oxygen cycle
A) operates at a slightly...

Question 9

In the proton-proton chain
A) no neutrinos are produced.
B) energy is released because a hydrogen nucleus has a greater mass than a helium nucleus.
C) no photons are produced.
D) carbon serves as a catalyst for the nuclear reaction.
E) energy is produced in the form of gamma rays and the velocity of the created nuclei.

Accepted Answer

The proton-proton chain is a series of nuclear reactions that occur in the core of the Sun. In this process, hydrogen nuclei fuse together to form helium nuclei, releasing energy in the form of gamma rays and the velocity of the created nuclei. No carbon is involved in the process, and neutrinos are also produced. Therefore, choices A, B, C, and D are incorrect. In the proton-proton chain, energy is produced in the form of gamma rays as well as the velocity of the created nuclei. No neutrinos are not produced in this process. The energy release occurs due to the fusion of four protons into one helium nucleus, which has a slightly lower mass than the combined mass of the four protons. Carbon does serve as a catalyst for a later stage of the reaction, but not for the initial proton-proton fusion.

Question 10

The lower edge of the Main Sequence band is known as ____ and represents the location in the HR-diagram at which stars begin their lives as main sequence stars.
A) the Coulomb barrier
B) hydrostatic equilibrium
C) the birth line
D) the zero-age main sequence
E) the evolutionary track

Accepted Answer

The zero-age main sequence (ZAMS) represents the location on the HR-diagram where stars begin their lives as main sequence stars. It is the lower edge of the Main Sequence band. The zero-age main sequence (ZAMS) marks the location on the HR-diagram where stars begin their lives as main sequence stars, and is located at the lower edge of the Main Sequence band.

Question 11

Emission nebulae are also called ____ because they are composed of ionized hydrogen.
A) HI regions
B) HII regions
C) Bok globules
D) HeI regions
E) reflection nebulae

Accepted Answer

Emission nebulae are also known as HII regions because they are composed of ionized hydrogen (HII).

Question 12

The free-fall contraction of a molecular cloud
A) can be initiated by shock waves from supernovae.
B) can be initiated by nearby spectral type G stars.
C) can be initiated by the rotation of the cloud.
D) causes the cloud to become transparent to ultraviolet radiation.
E) causes the particles in the cloud to decrease the speed with which they move.

Accepted Answer

Shock waves from supernovae can compress the gas in a molecular cloud, initiating its collapse under gravity, leading to star formation.

Question 13

____ are star-like objects that contain less than 0.08 solar masses an will never raise their core temperatures high enough that the proton-proton chain can begin. Other minor fusion reactions do occur in these objects. They fall in a gap between the low-mass M dwarf stars and the massive planets in which nuclear fusion never occurs.
A) Brown dwarfs
B) Herbig-Haro objects
C) Bok globules
D) T-Tauri star
E) Main sequence stars

Accepted Answer

Brown dwarfs are objects that fall in between the characteristics of low-mass M dwarf stars and massive planets. They are star-like objects that contain less than 0.08 solar masses and will never reach the core temperatures required for sustained nuclear fusion via the proton-proton chain. However, they still undergo minor fusion reactions, such as deuterium fusion. Brown dwarfs fit the description given, as they are star-like objects that do not undergo nuclear fusion and fall in the gap between low-mass stars and massive planets. Herbig-Haro objects are not star-like objects and are instead jets of gas ejected from young stars. Bok globules are dense regions of dust and gas where stars can form, but are not themselves stars. T-Tauri stars are young stars that have not yet reached the main sequence, and main sequence stars are stars that are able to undergo nuclear fusion.

Question 14

The diagram below is an HR diagram. The line indicates the location of the main sequence. Which of the five labeled locations on the HR diagram indicates a luminosity and temperature similar to that of a T Tauri star? 
A) 1
B) 2
C) 3
D) 4
E) 5

Accepted Answer

The answer of The diagram below is an HR diagram....

Question 15

Interstellar gas clouds may collapse to form stars if they
A) have very high temperatures.
B) encounter a shock wave.
C) rotate rapidly.
D) are located near main sequence spectral type K and M stars.
E) all of the above.

Accepted Answer

Interstellar gas clouds must encounter a shock wave in order to collapse and form stars. High temperatures and rapid rotation do not necessarily lead to star formation. The presence of main sequence K and M stars in the vicinity may influence star formation but is not the primary factor.

Question 16

____ occurs when most of the material collapsing to form a protostar has fallen into a disk around the star and a strong wind from the warm protostar ejects material from its poles.
A) An emission nebula
B) Hydrostatic equilibrium
C) The proton-proton chain
D) The thermonuclear fusion of hydrogen
E) A bipolar outflow

Accepted Answer

The scenario described in the question is characteristic of a protostar in the bipolar outflow phase. As material falls from the surrounding cloud onto the protostar, it forms an accretion disk around the equator of the protostar, with material spiraling inwards due to angular momentum conservation. The protostar itself is still growing and contracting, and its inner regions become hot and dense enough for nuclear fusion to eventually start. In the bipolar outflow phase, strong magnetic fields are generated near the protostar, and these fields can channel some of the infalling material along the magnetic poles of the protostar, leading to powerful jets of gas (outflows) that emerge from the two poles. These jets carry away angular momentum and can also help clear out some of the surrounding material, allowing the protostar to continue growing. The outflows are often visible as bipolar lobes or jets in radio, infrared, and optical images of protostars.

Question 17

____ is the thermonuclear fusion of hydrogen to form helium operating in the cores of massive stars on the main sequence.
A) The CNO cycle
B) The proton-proton chain
C) Hydrostatic equilibrium
D) The neutrino process
E) none of the above

Accepted Answer

The answer of ____ is the thermonuclear fusion of hydrogen...

Question 18

The visual pink glow of the Great Nebula in Orion
A) is a Herbig-Haro object.
B) is a reflection nebula of dust.
C) is an emission nebula of thin gas.
D) contains only young low mass stars.
E) is believed to be about 5 billion years old.

Accepted Answer

The visual pink glow of the Great Nebula in Orion is an emission nebula of thin gas. This gas is ionized by radiation from nearby stars, causing it to emit light in various colors, including the pink hue seen in photographs.

Question 19

The ____ is the nuclear fusion of three helium nuclei to form a carbon nucleus.
A) proton-proton chain
B) Carbon-Nitrogen-Oxygen cycle (CNO cycle)
C) triple-alpha process
D) Coulomb barrier
E) infrared cirrus

Accepted Answer

The triple-alpha process is the nuclear fusion of three helium nuclei to form a carbon nucleus. The other options do not describe this process. The triple-alpha process is the nuclear fusion process where three helium nuclei combine to form a carbon nucleus. This process occurs in the cores of red giant stars. The proton-proton chain and CNO cycle are other nuclear fusion processes that occur in stars, but they do not involve the fusion of helium nuclei. The Coulomb barrier and infrared cirrus are not related to nuclear fusion.

Question 20

The nuclear reactions in a star s core remain under control so long as
A) luminosity depends on mass.
B) pressure depends on temperature.
C) density depends on mass.
D) weight depends on temperature.
E) temperature depends on mass.

Accepted Answer

Nuclear reactions in a star's core occur as a result of high temperature and pressure. The pressure inside a star's core depends on its temperature, such that a decrease in temperature would result in a decrease in pressure. This decrease in pressure would cause the nuclear reactions to slow down, as the atomic nuclei would be less likely to collide and undergo fusion. Therefore, the nuclear reactions in a star's core remain under control so long as pressure depends on temperature.

Quiz 10: Structure and Formation of Stars

Convection is important in stars because it

Protostars are difficult to observe because

What causes the outward gas pressure that balances the inward pull of gravity in a main sequence star?

Which of the following is NOT evidence of the existence of an interstellar medium?

____ are small luminous nebulae excited by nearby young stars.

Opacity is

The average star spends ____ of its lifetime on the main sequence.

The carbon-nitrogen-oxygen cycle

In the proton-proton chain

The lower edge of the Main Sequence band is known as ____ and represents the location in the HR-diagram at which stars begin their lives as main sequence stars.

Emission nebulae are also called ____ because they are composed of ionized hydrogen.

The free-fall contraction of a molecular cloud

The diagram below is an HR diagram. The line indicates the location of the main sequence. Which of the five labeled locations on the HR diagram indicates a luminosity and temperature similar to that of a T Tauri star?

Interstellar gas clouds may collapse to form stars if they

____ occurs when most of the material collapsing to form a protostar has fallen into a disk around the star and a strong wind from the warm protostar ejects material from its poles.

____ is the thermonuclear fusion of hydrogen to form helium operating in the cores of massive stars on the main sequence.

The visual pink glow of the Great Nebula in Orion

The ____ is the nuclear fusion of three helium nuclei to form a carbon nucleus.

The nuclear reactions in a star s core remain under control so long as