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.

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.

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.

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.

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

____ 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

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

A. 1%

B. 2%

C. 10%

D. 20%

E. 90%

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

____ 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

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

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.

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.

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.

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.

____ 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

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

Molecular clouds can be observed

A. using infrared telescopes to detect ionized gas in the clouds.

B. using x-ray telescope to observe the x-ray radiation from the molecules in the cloud.

C. using radio telescopes to observe the CO emission from the clouds.

D. by looking for blue wispy regions near star clusters.

E. by looking for the 21-cm radiation from hydrogen.

The HR 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).

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.

The lowest mass object that can initiate thermonuclear fusion of hydrogen has a mass of about

A. 1 solar masses

B. 80 solar masses

C. 8 solar masses

D. 0.08 solar masses

E. 0.001 solar masses

Stars with masses less than ____ produce most of their energy via the proton-proton chain.

A. 0.01 solar masses

B. 0.1 solar masses

C. 1.1 solar masses

D. 11 solar masses

E. 110 solar masses

The creation of ____ requires that a young hot star (T=25,000 K) be relatively nearby.

A. Emission nebulae

B. HI regions

C. Molecular clouds

D. The cool gas of the interstellar medium

E. 21 cm radiation

The extinction of starlight due to the interstellar medium

I. is the greatest in the ultraviolet.II. is the greatest in the infrared.III. is caused by ionized hydrogen.IV. is caused by dust particles.

A. I & III

B. II & III

C. I & IV

D. II & IV

E. only IV

____ are small dark nebulae about 1 light-year in diameter that contain 10 to 1,000 solar masses.

A. HI regions

B. HII regions

C. Emission nebulae

D. Bok globules

E. Reflection nebulae

Absorption lines due to the interstellar medium indicate that some components of the interstellar medium are cold and of a very low density because

A. the lines are blue shifted.

B. the lines are red shifted.

C. the lines are extremely broad.

D. the lines are extremely narrow.

E. the lines are much darker than the stellar lines.

Stars are born in

A. reflection nebulae.

B. dense molecular clouds.

C. HII regions.

D. the intercloud medium.

E. the local bubble.

Absorption lines due to interstellar gas

A. are wider than the lines from stars because the gas is hotter than most stars.

B. are narrower than the lines from stars because the gas has a lower pressure than stars.

C. indicate that the interstellar medium contains dust.

D. indicate that the interstellar medium is expanding away from the sun.

E. none of the above.

What is the lifetime of a 10 solar mass star on the main sequence?

A. 3.2107 years

B. 320 years

C. 3.21012 years

D. 1109 years

E. 11011 years

What is the lifetime on the main sequence of a 2 solar mass star compared to the approximately 10 billion year life of our Sun? Pick the best.

A. 1.81011 years

B. 1.81010 years

C. 1.8109 years

D. 1.8106 years

E. 1.8103 years

The H-R diagram of a young star cluster shows

A. that high mass stars have not yet reached the main sequence yet.

B. that low mass stars have not yet reached the main sequence yet.

C. mainly giant stars.

D. no stars since none have reached the main sequence yet.

As a star begin to form the initial energy source is from

A. nuclear fusion.

B. nuclear fission.

C. gravitational potential energy.

D. magnetic fields.

While on the main sequence a star's primary energy source comes from

A. nuclear fusion.

B. nuclear fission.

C. gravitational potential energy.

D. magnetic fields.

What must occur for an object to be considered a main sequence star?

A. Hydrostatic equilibrium

B. Nuclear fusion reaction in the core

C. Protostar life begins

D. Both A and B

What force(s) are responsible for the collapse of an interstellar cloud?

A. nuclear

B. gravity

C. electric

D. magnetic

E. both C and D

Star clusters are important to the study of stellar evolution because stars in a given cluster have the same

A. temperature.

B. mass.

C. age.

D. luminosity.

Why do higher mass stars live shorter lives on the main sequence than lower mass stars?

A. Higher mass stars burn through their nuclear fuel faster.

B. Lower mass stars don't get their energy from that same nuclear fusion source as higher mass stars.

C. Higher mass stars have less hydrogen fuel to burn.

D. Lower mass stars spend a longer time evolving to the main-sequence.

E. All of the above are false.

Why do nuclear fusion reactions only take place in the interior of a star?

A. The magnetic fields are strongest there.

B. The temperature is the hottest in the center.

C. The density of material is very high in the center.

D. Both B and C.

E. FALSE; Nuclear fusion reactions can take place in the core and corona where temperatures are high enough.

What characteristic of a star primarily determines its location on the main sequence?

A. age

B. distance from the galactic center

C. mass

D. space motion

E. radius

Some young star clusters contain large numbers of protostars.

1. True

2. False

Ninety percent of all stars fuse helium to form carbon and lie on the main sequence.

1. True

2. False

The sun has a core in which energy travels outward primarily by radiation.

1. True

2. False

The sun makes most of its energy by the CNO cycle.

1. True

2. False

The sun makes most of its energy by the proton-proton cycle.

1. True

2. False

Energy flows by radiation or convection inside stars but almost never by conduction.

1. True

2. False

Hydrostatic equilibrium refers to the balance between weight and pressure.

1. True

2. False

The Orion region contains young main sequence stars and an emission nebula, but the original molecular cloud they formed out of has been dispersed.

1. True

2. False

T Tauri stars are believed to be young, high mass main sequence stars.

1. True

2. False

The dust in the interstellar medium can make distant stars look redder than they really are.

1. True

2. False

Molecular clouds are mapped using CO instead of hydrogen because CO is much more abundant than hydrogen in molecular clouds.

1. True

2. False

HII regions are found near stars cooler than 25,000 K because large amounts of ultraviolet photons would totally destroy the hydrogen atoms in the gas.

1. True

2. False

The pressure of a gas depends on the temperature and density of the gas.

1. True

2. False

Some massive stars live only a few million years.

1. True

2. False

Pre-main sequence stars are above and to the right of the main sequence.

1. True

2. False

High mass stars evolve more slowly to the main sequence than lower mass stars.

1. True

2. False

Main sequence stars many times the mass of the sun produce their energy by the set of nuclear reactions called the CNO cycle.

1. True

2. False