Chapter 16, Star Birth

The following figures show four stages that occur during the formation of a one-solar-mass star. Rank these stages based on the order in which they occur, from first to last.

1. Molecular Cloud Fragment

2. Contracting Cloud Trapping Infrared Gas

3. Protostar with Jets

4. Main-sequence Star

The following figures show four stages that occur during the formation of a one-solar-mass star. Rank these stages based on the central temperature, from highest to lowest.

1. Main-sequence Star

2. Protostar with Jets

3. Molecular Cloud Trapping Infrared Gas

4. Molecular Cloud Fragment

The following figures show four stages that occur during the formation of a one-solar-mass star. Rank these stages based on their rotation rate, from fastest to slowest. (Assume that the angular momentum of the forming star is conserved throughout the formation process, though in fact it may shed some angular momentum by ejecting material into interstellar space.)

1. Main-sequence Star

2. Protostar with Jets

3. Contracting Cloud Trapping Infrared Jets

4. Molecular Cloud Fragment

Which of these colors of light passes most easily through interstellar clouds?

Yellow Light

What happens to a cloud's thermal pressure if its temperature falls while its density rises?

More information is needed to determine what thermal pressure does.

What happens within a contracting cloud in which gravity is stronger than pressure if its temperature remains the same as it contracts?

It breaks into smaller fragments.

What slows down the contraction of a star-forming cloud when it makes a protostar?

trapping of thermal energy inside the protostar

Which kind of pressure prevents stars of extremely large mass from forming?

radiation pressure

Which kinds of stars are most common in a newly formed star cluster?

M stars

By mass, the interstellar medium in our region of the Milky Way consists of

70% Hydrogen, 28% Helium, 2% heavier elements.

What percentage of a molecular cloud's mass is interstellar dust?

1%

The typical density and temperature of molecular clouds are

300 molecules per cubic centimeter, 10-30 Kelvin.

The most abundant molecule in molecular clouds is

H(little 2)

The typical size of an interstellar dust grain is

1 micrometer.

What is interstellar reddening?

Interstellar dust absorbs more blue light than red light, making stars appear redder than their true color.

If you wanted to observe a molecular cloud, in what wavelength of light would you most likely observe?

infrared

What happens to the visible radiation produced by new stars within a molecular cloud?

It is absorbed by dust grains and heats up the cloud.

The thermal pressure of a gas depends on

density and temperature.

The gravitational force in a molecular cloud depends on

density only.

What prevents the pressure from increasing as a cloud contracts due to its gravity?

Thermal energy is converted to radiative energy via molecular collisions and released as photons.

Calculations show that gravity begins to overcome thermal pressure in clouds that are

more massive than a hundred times the Sun.

How do astronomers infer the presence of magnetic fields in molecular clouds?

by measuring the polarization of starlight passing through the cloud

What is the likely reason that we cannot find any examples of the first generation stars?

The first generation stars were all very massive and exploded as supernova.

Why do we think the first generation of stars would be different from stars born today?

Without heavy elements, the clouds could not reach as low a temperature as today and had to be more massive to collapse.

When is thermal energy trapped in the dense center of a cloud?

when excited molecules collide with other molecules before they can release a photon

What happens to the rotation of a molecular cloud as it collapses to form a star?

The rotation rate increases and results in a disk of material around a protostar.

When does a protostar become a true star?

when nuclear fusion begins in the core

When does a star become a main-sequence star?

when the rate of hydrogen fusion within the star's core is high enough to maintain gravitational equilibrium

When does hydrogen first begin to fuse into helium in the star formation process?

when the protostar undergoes radiative contraction

What is the smallest mass a newborn star can have?

80 times the mass of Jupiter

No stars have been found with masses greater than 100 times our Sun because

they would generate so much power that they would blow themselves apart.

Molecular clouds appear more transparent at longer wavelengths

True

Clouds that appear dark in visible light often glow when observed at long infrared wavelengths

True

Most stars are born in clusters containing thousands of stars

True

Photographs of many young stars show long jets of material apparently being ejected from their poles.

True

Although some photographs show what looks like jets of material near many young stars, we now know that these "jets" actually represent gas from the surrounding nebula that is falling onto the stars.

False

There is no limit to the mass with which a star can be born.

False

In any star cluster, stars with lower masses greatly outnumber those with higher masses.

True