Star Birth

interstellar medium

The gas between the stars

Stars form in

dark clouds of dusty gas in interstellar space

We can determine the composition of interstellar gas from

its absorption lines in the spectra of stars

Most of the matter in star-forming clouds is

in the form of molecules (H₂, CO, etc)

These molecular clouds

have a temperature of 10–30 K and a density of about 300 molecules per cubic centimeter

Most of what we know about molecular clouds comes from

observing the emission lines of carbon monoxide (CO)

Tiny solid particles of _____ block our view of stars on _____ of a cloud

interstellar dust; the other side

Particles are

< 1 micrometer in size and made of elements like C, O, Si, and Fe.

Stars viewed through the edges of the cloud look redder because

dust blocks (shorter-wavelength) blue light more effectively than (longer-wavelength) red light

Long-wavelength infrared light passes

through a cloud more easily than visible light

Observations of infrared light reveal

stars on the other side of the cloud

Visible light from a newborn star is often _____ where the _____

trapped within the dark, dusty gas clouds; star formed

Observing the infrared light from a cloud can

reveal the newborn star embedded inside it

Dust grains that absorb visible light

heat up and emit infrared light of even longer wavelength

Long-wavelength infrared light is

brightest from regions where many stars are currently forming

Emission lines from molecules in a cloud can prevent _____ by _____ into _____

a pressure buildup; converting thermal energy; infrared and radio photons

Gravity can create stars only

if it can overcome the force of thermal pressure in a cloud

Emission lines from molecules in a cloud can prevent _____ by _____ that escape the cloud

a pressure buildup; converting thermal energy into infrared and radio photons

A typical molecular cloud must contain

at least a few hundred solar masses for gravity to overcome pressure

A cloud must have even more _____ to begin _____ if there are additional forces opposing gravity

mass; contracting; dditional forces opposing gravity

Both magnetic fields and turbulent gas motions

increase resistance to gravity

Gravity within a contracting gas cloud becomes

stronger as the gas becomes denser

Gravity can overcome _____, causing it _____, each of which may go on to form a star

pressure in smaller pieces of the cloud; to break apart into multiple fragments

The random motions of different sections of the cloud

cause it to become lumpy

Each lump of the cloud _____ can _____

in which gravity can overcome pressure; go on to become a star

A large cloud can make

a whole cluster of stars

Gravity can overcome pressure _____ if _____

in a relatively small cloud; the cloud is unusually dense

unusually dense clouds may make

only a single star

What would happen to a contracting cloud fragment if it were not able to radiate away its thermal energy?

Its internal pressure would increase

Elements like _____ had not yet been made when _____

carbon and oxygen; the first stars formed

Without CO molecules to provide cooling, the clouds that formed the first stars

had to be considerably warmer than today's molecular clouds

The first stars must have been more massive

than most of today's stars for gravity to overcome pressure

the proportion of heavy elements rose

quickly in the early universe

Simulations of early star formation suggest

the first molecular clouds never cooled below 100 K, making stars of ~ 100M_Sun

Stars form in

dark, dusty clouds of molecular gas with temperatures of 10 - 30 K

As contraction packs _____closer together, it becomes harder for _____

the molecules and dust particles of a cloud fragment; infrared and radio photons to escape

Contraction slows down, and the center of the cloud fragment becomes

a protostar

Matter from the cloud continues to fall onto the protostar until

either the protostar or a neighboring star blows the surrounding gas away

The nebular theory of solar system formation illustrates

the importance of rotation

The _____ increases as _____

rotation speed of the cloud from which a star forms; the cloud contracts

Rotation of a contracting cloud speeds up

for the same reason skaters speed up as they pull in their arms

Collisions between particles in the cloud

cause it to flatten into a disk

Collisions between gas particles in cloud gradually

reduce random motions

Collisions between gas particles reduce

up and down motions

The spinning cloud _____ as it _____

flattens; shrinks

Rotation causes jets of matter to

shoot out along the rotation axis

Jets are observed coming from

the centers of disks around protostars

It is thought that the spinning disk twists _____ of the star, channeling _____

the magnetic field; the jets

The jets ram into _____, heating it and _____

interstellar gas; causing it to glow

What would happen to a protostar that formed without any rotation at all?

It would not easily form planets

A protostar looks starlike after _____, but its thermal energy comes from _____

the surrounding gas is blown away; gravitational contraction, not fusion

Contraction must continue until

the core becomes hot enough for nuclear fusion

Contraction stops when

the energy released by core fusion balances energy radiated from the surface. the star is now a main-sequence star

With very low temperatures and luminosities, where would protostars be located on the H-R diagram?

Lower right

A life track illustrates

a star's surface temperature and luminosity at different moments in time

Luminosity and temperature grow as

matter collects into a protostar

Surface temperature remains near

3000 K while convection is main energy transport mechanism

Luminosity remains _____ during _____, while _____

nearly constant; late stages of contraction; radiation transports energy through star

Core temperature continues to

rise until star begins fusion and arrives on the main sequence

Models show that the Sun required about _____ to go from _____

30 million years; protostar to main sequence

Higher-mass stars form

faster

Lower-mass stars form

more slowly

The contraction of a cloud fragment slows when _____ because _____ can no longer escape.

thermal pressure builds up; infrared and radio photons

Conservation of angular momentum leads to

the formation of disks around protostars

Fusion will not begin in a contracting cloud if

some sort of force stops contraction before the core temperature rises above 10⁷K

Thermal pressure cannot stop contraction because

the star is constantly losing thermal energy from its surface through radiation

Thermal Pressure

Depends on heat content and is the main form of pressure in most stars

Degeneracy Pressure

Doesn't depend on heat content and particles can't be in same state in same place

Degeneracy pressure halts the contraction of objects with < 0.08M_Sun before

core temperature becomes hot enough for fusion

brown dwarfs

Starlike objects not massive enough to start fusion

A brown dwarf emits

infrared light because of heat left over from contraction

A brown dwarf’s luminosity

gradually declines with time as it loses thermal
energy

Infrared observations can reveal _____ because they are still _____

recently formed brown dwarfs; relatively warm and luminous

Photons exert

a slight amount of pressure when they strike matter

Very massive stars are so luminous that

the collective pressure of photons drives their matter into space

Models of stars suggest that radiation pressure limits

how massive a star can be without blowing itself apart.

Maximum star mass thought to be around

150M_Sun

Observations of star clusters show that

star formation makes many more low-mass stars than high-mass stars