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Which two processes can generate energy to help a star or gas cloud maintain its internal thermal pressure?

nuclear fusion and gravitational contraction

About what percentage of the mass of a molecular cloud is in the form of dust?


How do we learn the chemical composition of the interstellar medium?

By studying spectra of interstellar gas clouds.

What happens to the visible light radiated by stars located within a dusty gas cloud?

It is absorbed by dust, which heats the dust grains so that they emit the absorbed energy as infrared light.

Under which circumstances can you be sure that the thermal pressure within a gas cloud is increasing?

the cloud's temperature and density are both increasing

Which process is required to allow a gravitationally-collapsing gas cloud to continue to collapse?

The cloud must radiate away much of its thermal energy.

According to current understanding, how did the first generation of stars differ from stars born today?

They were much more massive than most stars born today.

Close binary star systems are thought to form when

gravity pulls two neighboring protostars quite close together, but angular momentum causes them to orbit each other rather than colliding.

Generally speaking, how does the surface temperature and luminosity of a protostar compare to the surface temperature and luminosity of the main-sequence star it becomes?

A main-sequence star is hotter and dimmer than it was as a protostar.

Where does a 1-solar-mass protostar appear on an H-R diagram?

to the right of the main sequence, and higher up than the Sun

Why does the rotation of a protostar slow down over time?

Magnetic fields can transfer angular momentum to the protostellar disk and protostellar winds can carry angular momentum away.

The surface of a protostar radiates energy while its core

shrinks and heats

The core of a protostar that will eventually become a brown dwarf shrinks until

the type of pressure called degeneracy pressure becomes important

If a star is extremely massive (well over 100 solar masses), why isn't it likely to survive for long?

It may blow itself apart because of radiation pressure.

Where would a brown dwarf be located on an H-R diagram?

below and to the right of the lowest part of the main sequence

What do we mean by the interstellar medium?

the gas and dust that lies in between the stars in the Milky Way galaxy

The interstellar clouds called molecular clouds are

the cool clouds in which stars form

Interstellar dust consists mostly of

microscopic particles of carbon and silicon

Which part of the electromagnetic spectrum generally gives us our best views of stars forming in dusty clouds?


Suppose you look by eye at a star near the edge of a dusty interstellar cloud. The star will look _______ than it would if it were outside the cloud.

dimmer and redder

Most interstellar clouds remain stable in size because the force of gravity is opposed by _______ within the cloud.

thermal pressure

What kind of gas cloud is most likely to give birth to stars?

a cold, dense gas cloud

What effect are magnetic fields thought to have on star formation in molecular clouds?

They can help resist gravity, so that more total mass is needed before the cloud can collapse to form stars.

What is a protostar?

a star that is still in the process of forming

What law explains why a collapsing cloud usually forms a protostellar disk around a protostar?

conservation of angular momentum

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

the surface temperature and luminosity the star will have at each stage of its life

When does a protostar become a main-sequence star?

when the rate of hydrogen fusion becomes high enough to balance the rate at which the star radiates energy into space

Approximately what core temperature is required before hydrogen fusion can begin in a star?

10 million K

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