Star Formation

• Why is the sunset red?

• The stuff between the stars

• Nebulae

• Giant molecular clouds

• Gravitational collapse of molecular cloud

• Gravitational contraction of protostars

• Which clouds collapse?

Interstellar medium

• Space between the stars within a galaxy is not empty. • The interstellar medium (ISM) consists of gas and dust.• Gas is mainly hydrogen, but also contains other elements

and molecules.• Density is typically around 1 atom per cubic centimeter.

Clouds and nebula

• The interstellar medium is not uniform, but varies by large factors in density and temperature.

• The clumps in the interstellar medium are clouds or nebulae (one nebula, two nebulae).

• Three types of nebulae:– Emission nebulae– Reflection nebulae– Dark nebulae

Emission nebulae emit their own light because luminous ultraviolet stars (spectral type O,B) ionize gas in the nebula. The gas then emits light as the electrons return to lower energy levels. In this image Red = Hydrogen, Green = Oxygen, Blue = Sulfur.

Emission nebulae

Reflection nebulae

Reflection nebulae do not emit their own light. Dust scatters and reflects light from nearby stars.

Dark nebula

Dark nebula are so opaque that the dust grains block any starlight from the far side from getting through.

Reflection nebulae emit light as a result of

1. Ultraviolet radiation from O and B stars

2. Nuclear fusion

3. Dust scattering light from stars

4. Ionized gas

Molecular clouds

• Dark nebula are usually molecular clouds

• Molecular clouds are relatively dense and are very cold, often only 10 K.

• Giant molecular clouds can contain as much as 104 solar masses (M) of gas and be 10 light years across.

• Molecular clouds are the primary sites for star formation.

Eagle nebula

Eagle nebula in infrared

Star birth can begin in giant molecular clouds

Carbon monoxide

map

Visible (left) and infrared (right) views of the Orion nebula show new stars. These new stars can only been seen in infrared because

the protostar’s cocoon nebula absorbs most of the visible light.

Protostars form in cold, dark nebulae

Gravitational collapse

A

Which configuration has more potential energy?

B

Potential energy due to gravity

221

r

mmGF

r

mmGdxxFU 21)(

Gravitational collapse

A

Which configuration has more potential energy?

B

Potential energy due to gravity

R

MGU

2

5

3

Sphere of mass M and radius R

Gravitational potential energy is released as sphere shrinks

Gravitational collapse

• How much energy is released when 1 M of material collapses from a radius of 10 R to 1 R ?

Evolution of stars

• Stars change over their lifetimes (from formation to death).

• We can track these changes via motion of the star in the HR diagram.

Protostars on HR diagram

Where would a collapsing gas cloud appear on an HR diagram?

Gravitational collapse• About 21041 J of energy is released when 1 M

of material collapses from a radius of 10 R to 1 R . This collapse takes about 10-20 million years. The luminosity is:

L--T

ΔEL 21W1073

s1063

J102 2614

41

• Size of cloud?

• Temperature of cloud?

Cloud collapse to star: on HR diagram

Cloud is transparent. Protostar is when cloud becomes opaque.

Protostars on the HR diagramHotter

• Note that luminosity remains constant.

• To produce constant luminosity as radius decreases, need increase in temperature

Why does temperature increase as star contracts?

2/1

4/142

R

LTTRL

More massive stars form faster

Which clouds will collapse?

• Gravitational force causes objects to collapse.

• What keeps objects from collapsing?

• In the solar system, the motion of the planets keeps them from falling in to the Sun.

• In a gas, the random motions of the gas atoms can support the gas against gravity.

Temperature

• Temperature is proportional to the average kinetic energy per molecule

lower T higher T

kTmvK2

3

2

1 2 k = Boltzmann constant = 1.3810-23 J/K = 8.6210-5 eV/K

Energy of gas cloud

R

MGU

2

5

3

Gravitational potential energy:

Sphere of mass M and radius R

Kinetic energy of N atoms

kTm

MkTNK

H 2

3

2

3

Energy of gas cloud

R

MGkT

m

ME

H

2

5

3

2

3

If E < 0 then gas cloud collapses

If E > 0 then gas cloud can support itself

Density of gas cloud is n

HmnRM 334

Critical size of gas cloud

3/53/1

3

4

5

3

2

3M

nmGM

m

kTE H

H

By increasing the mass, we can always cause the gravity to dominate so that the gas cloud collapses.

Critical size and mass are called the Jean’s length and mass

n

TMM

nGm

kTR J

HJ

3

218

8

15

T in Kelvin, n in atoms/cm3

Critical size of gas cloud

MMn

TMM J 18000

1

1001818

33

Therefore, such clouds will typically form a group of stars rather than a single star.Stars are generally found in groups, called star clusters or OB associations, depending on the type of stars.

If we have a cloud at T = 100 K and n = 1 cm-3, how large pieces does it fragment into?

Critical size of gas cloud

MMn

TMM J 170

300

301818

33

Therefore, such clouds will typically form a group of stars rather than a single star.Stars are generally found in groups, called star clusters or OB associations, depending on the type of stars.

If we have a cloud at T = 30 K and n = 300 cm-3, how large pieces does it fragment into?

Critical size of gas cloud

MMn

TMM J 4.5

000,300

301818

33

Therefore, the dense cores fragment into individual stars.

The dense cores can reach n = 300,000 cm-3, how large pieces do they fragment into?

Protostars form by collapse of molecular clouds

• Clouds must form dense and cold clumps or cores to collapse

• Typically, multiple stars will form from one gas cloud

Star cluster

An OB association is a group of O and B class stars which are producing ionizing radiation, causing an HII nebula glow

(example: Trapezium in Orion Nebula)

Star formation

Watch for:

• Collapse of cloud

• Rotation of cloud

• Formation of disk near protostar

• Show animation

• As the gas/dust falls in, it picks up speed and energy. It is slowed by friction and the energy is converted to heat.

• As long as the protostar is transparent, the heat can be radiated away.

• When the protostar becomes so dense it is opaque, then the heat stars to build up, the pressure increases, and the rapid collapse slows.

• Gas in the cloud keeps falling onto

the protostar.• The collapsing gas tends to start

rotating around the protostar as it falls in forming a disk and a jet.

• Eventually, the protostar develops a wind, like the solar wind but much stronger. This out flowing wind stops the in falling matter.

• The protostar keeps contracting under it own gravity. The protostar is powered by gravity via contraction - not by fusion.

• The protostar becomes a star when it has contracted so much that it is dense and hot enough to begin nuclear fusion.

During the birth process, stars both gain and lose mass

Magnetic field lines are pulled toward the protostar as material is attracted to the protostar. The swirling motions of the disk material

distort the field into helical shapes and some of in-falling disk material is channeled outward along these lines.

Jets, disks form in protostars

Disk and jet of a protostar

Protostar jet

As gas is pulled in towards a protostar which does not occur

1. the gas starts to rotate more rapidly

2. some of the gas is ejected in jets

3. some of the gas forms a disk around the protostar

4. some of the gas undergoes nuclear fusion

Review Questions

• What is a protostar’s source of energy?

• How does a protostar’s radius and luminosity change as it contracts?

• What is the relation between luminosity, radius, and temperature.

• How does a protostar’s mass influence its speed of formation?

• What is the Jean’s mass?