21 (1)

Question 1

Each second in the Sun, approximately ……….. ……… tons of hydrogen undergo fusion into helium, with about ……… ……… tons turning into energy in the process.

Answer 1

600 million / 4 million

This rate of hydrogen use means that eventually the Sun (and all other stars) will run out of central fuel.

Question 2

Stars come with many different masses, ranging from ………. solar masses (M_Sun) to roughly ………….. M_Sun.

There are far more ……….-mass than ……….-mass stars.

Answer 2

1/12 / 100–200

low / high

Question 3

• The most massive main-sequence stars (spectral type ……..) are also the most luminous and have the highest surface temperature. The lowest-mass stars on the main sequence (spectral type ………. or ……..) are the least luminous and the coolest.

Answer 3

O / M or L

Question 4

A galaxy of stars such as the Milky Way contains enormous amounts of ……… and ……. —enough to make billions of stars like the Sun.

Answer 4

gas and dust

Question 5

Astronomers coined the term ……… …….. ……… for these structures,

Answer 5

evaporating gas globules (EGGs)

Question 6

the most massive reservoirs of interstellar matter—and some of the most massive objects in the Milky Way Galaxy—are the ………… ……… ………

Answer 6

giant molecular clouds

Question 7

giant molecular clouds have cold interiors with characteristic temperatures of only …………. …; most of their gas atoms are bound into molecules.

These clouds turn out to be the birthplaces of most stars in our Galaxy.

Answer 7

10–20 K

Question 8

The masses of molecular clouds range from a ………….. times the mass of the Sun to about ……… ………. solar masses.

Answer 8

thousand / 3 million

Question 9

The molecular cloud filaments can be up to ……….. light-years long.

Answer 9

1000

Question 10

Within the clouds are cold, dense regions with typical masses of 50 to 500 times the mass of the Sun; we give these regions the highly technical name …………..

Answer 10

clumps

Question 11

Within these clumps, there are even denser, smaller regions called ………….

The ……….. are the embryos of stars.

Answer 11

cores

Question 12

The conditions in these cores—……….. ……….. and ……….. …………—are just what is required to make stars.

Answer 12

low temperature / high density

Question 13

Remember that the essence of the life story of any star is the ongoing competition between two forces: ………. and …………..

Answer 13

gravity / pressure

Question 14

The force of gravity, pulling inward, tries to make a star collapse.

Internal pressure produced by the motions of the gas atoms, pushing outward, tries to force the star to expand.

Answer 14

R 2

Question 15

When a star is first forming, low ……….. (and hence, low …………..) and high ……….. (hence, greater ………….. attraction) both work to give gravity the advantage.

Answer 15

temperature / pressure

density / gravitational

Question 16

In order to form a star we need a typical core of interstellar atoms and molecules to shrink in ……….. and increase in ………. by a factor of nearly ………. It is the force of gravity that produces this drastic collapse.

Answer 16

radius / density / 10²⁰

Question 17

The Orion Nebula also contains a tight cluster of stars called the …………..

Answer 17

Trapezium

Question 18

The Orion Nebula harbors some of the youngest stars in the solar neighborhood. At the heart of the nebula is the Trapezium cluster, which includes ………… very bright stars that provide much of the energy that causes the nebula to glow so brightly.

Answer 18

four

Question 19

Studies of Orion and other star-forming regions show that star formation is not a very efficient process. In the region of the Orion Nebula, about ……….. of the material in the cloud has been turned into stars.

Answer 19

1%

Question 20

Whether gently or explosively, the material in the neighborhood of the new stars is blown away into ………. …………

Answer 20

interstellar space

Question 21

Although we do not know what initially caused stars to begin forming in Orion, there is good evidence that the first generation of stars triggered the formation of additional stars, which in turn led to the formation of still more stars

Answer 21

R 2

Question 22

The basic idea of triggered star formation is this: when a massive star is formed, it emits a large amount of ultraviolet radiation and ejects high-speed gas in the form of a stellar wind. This injection of energy heats the gas around the stars and causes it to expand. When massive stars exhaust their supply of fuel, they explode, and the energy of the explosion also heats the gas.

Answer 22

The hot gases pile into the surrounding cold molecular cloud, compressing the material in it and increasing its density. If this increase in density is large enough, gravity will overcome pressure, and stars will begin to form in the compressed gas. Such a chain reaction—where the brightest and hottest stars of one area become the cause of star formation “next door”—seems to have occurred not only in Orion but also in many other molecular clouds.

Question 23

There are many molecular clouds that form only (or mainly) low-mass stars. Because low-mass stars do not have strong winds and do not die by exploding,……… ………. ……… cannot occur in these clouds.

Answer 23

triggered star formation

Question 24

There are also stars that form in relative isolation in small cores. Therefore, not all star formation is originally triggered by the death of massive stars.

Answer 24

R 2