Question 1

_________ 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. They are formed when narrow jets of gas ejected by young stars collide with surrounding material, causing the gas to glow brightly.

Question 2

________ 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 CNO cycle is the process of thermonuclear fusion of hydrogen into helium in the cores of massive stars on the main sequence.

Question 3

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 pink glow of the Great Nebula in Orion is caused by ionized hydrogen (H II), which emits light in the visible spectrum. This makes it an emission nebula, rather than a reflection nebula (B) or Herbig-Haro object (A). While the nebula contains young stars, it also contains more evolved stars, so D is not correct. The age of the nebula is estimated to be around 1 million years, so E is not correct.

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

The lower edge of the main-sequence band is known as ________ and represents the location in the H-R 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) is the stage in the H-R diagram where stars appear at the beginning of their main-sequence life, having just started hydrogen burning in their cores.

Question 6

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)the high temperature and the high density of the gas

Accepted Answer

The high temperature and high density of the gas in a main-sequence star cause the outward gas pressure. This pressure is crucial for balancing the inward pull of gravity, maintaining the star's stability. High temperatures lead to rapid particle movement, exerting pressure outward, while high density increases the number of particle collisions, contributing further to this outward pressure.

Question 7

________ are star-like objects that contain less than 0.08 solar masses and 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 fit the description given in the question, as they are star-like objects that cannot sustain nuclear fusion in their core and fall between the mass range of M dwarfs and massive planets. Herbig-Haro objects, Bok globules, T-Tauri stars, and main-sequence stars do not fit this description.

Question 8

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 the resistance to the flow of radiation (photons) through a gas. It depends on factors such as the density, temperature, and composition of the gas, and determines how easily radiation can transfer heat within the gas or out of it. In astronomy, opacity is an important parameter for understanding the behavior of radiation in stars, planetary atmospheres, and interstellar matter.

Question 9

________ 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 description matches the process of bipolar outflow, where material falls into a disk around the protostar and a strong wind from the star ejects material from its poles. This process is common during the early stages of star formation.

Question 10

There is a main-sequence mass-luminosity relation because
A)helium fusion produces carbon.
B)stars expand when they become giants.
C)more massive stars support their larger weight by making more energy.
D)the helium flash occurs in degenerate matter.
E)all stars on the main sequence have about the same radius.

Accepted Answer

The main-sequence mass-luminosity relation is the correlation between the mass of a star and its luminosity while it is on the main sequence. The more massive a star is, the more energy it makes, which supports its larger weight. This is why more massive stars are brighter and hotter than less massive stars.

Question 11

Due to the dust in the interstellar medium, a distant star will appear to an observer on Earth to be
A)brighter and cooler than it really is.
B)brighter and hotter than it really is.
C)fainter and cooler than it really is.
D)fainter and hotter than it really is.
E)unchanged in brightness or apparent color.

Accepted Answer

Dust in the interstellar medium absorbs and scatters light, making distant stars appear fainter. The scattering is more effective at shorter (bluer) wavelengths, making the star appear redder (cooler) than it actually is.

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

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)the protostar stage is very short, they are surrounded by cocoons of gas and dust, and they radiate mainly in the infrared.
E)they are all so far away that the light hasn't reached us yet.

Accepted Answer

Protostars are difficult to observe for multiple reasons. The protostar stage is short, so it is not easy to catch them in that stage. Additionally, they are often surrounded by cocoons of gas and dust that can obscure them from view. Finally, protostars radiate mainly in the infrared, which makes them difficult to observe with traditional telescopes that can only detect visible light. Therefore, choice D, which includes all of these reasons, is the best answer.

Question 14

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

Accepted Answer

The average star spends approximately 90% of its lifetime on the main sequence, where it fuses hydrogen into helium in its core.

Question 15

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 carbon-nitrogen-oxygen (CNO) cycle is a process in stars where four hydrogen nuclei are fused into one helium nucleus, releasing energy. This cycle is dominant in stars more massive than the Sun.

Question 16

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 other choices are correct.

Accepted Answer

Interstellar gas clouds require a trigger to initiate collapse and eventual star formation. One such trigger is the encounter with shock waves, which can cause the cloud to compress and collapse through gravitational attraction.

Question 17

The H-R diagram main-sequence of stars has a limit at the lower-mass or energy-output end because
A)low-mass stars form from the interstellar medium very rarely.
B)low-mass objects are composed primarily of solids, not gases.
C)pressure does not depend on temperature in degenerate matter.
D)the lower limit represents when the radius of the star would be zero.
E)there is a minimum temperature for hydrogen nuclear fusion (the definition of a star).

Accepted Answer

The minimum temperature required for hydrogen nuclear fusion (about 10 million K) is the defining characteristic of a star. Below this temperature, the object cannot sustain nuclear fusion and is not considered a true star. This sets a lower limit on the mass and energy output of stars, which is reflected in the lower end of the main-sequence on the H-R diagram.

Question 18

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 composed of ionized hydrogen, which is also called HII (pronounced H-two) regions. HI (pronounced H-one) regions are neutral hydrogen clouds that have not been ionized. Bok globules are dark clouds of gas and dust, HeI (pronounced He-one) is singly ionized helium, and reflection nebulae are composed of dust that reflects the light of nearby stars.

Question 19

The diagram below is an H-R diagram. The line indicates the location of the main sequence. Which of the five labeled locations on the H-R 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 H-R diagram....

Question 20

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)mixes the gases of the star and transports the energy outward in the star.

Accepted Answer

Convection in stars plays a crucial role in both mixing the gases within the star and transporting energy from the inner regions towards the outer layers. This process is essential for the star's energy balance and chemical homogeneity.

Quiz 9: The Formation and Structure of Stars

_________ are small luminous nebulae excited by nearby young stars.

________ 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

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

The lower edge of the main-sequence band is known as ________ and represents the location in the H-R diagram at which stars begin their lives as main-sequence stars.

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

Opacity is

________ 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.

There is a main-sequence mass-luminosity relation because

Due to the dust in the interstellar medium, a distant star will appear to an observer on Earth to be

The free-fall contraction of a molecular cloud

Protostars are difficult to observe because

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

The carbon-nitrogen-oxygen cycle

Interstellar gas clouds may collapse to form stars if they

The H-R diagram main-sequence of stars has a limit at the lower-mass or energy-output end because

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

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

Convection is important in stars because it