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Star Formation
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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
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.
Reflection nebulae do
not emit their own light.
Dust scatters and
reflects light from
nearby stars.
Reflection nebulae
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.
2.
3.
4.
Ultraviolet radiation from O and B stars
Nuclear fusion
Dust scattering light from stars
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
Protostars form in cold, dark nebulae
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.
Gravitational collapse
Which configuration has more potential energy?
A
B
Potential energy due to gravity
m1m2
F G 2
r
m1m2
U    F ( x)dx  G
r
Gravitational collapse
Which configuration has more potential energy?
A
B
Potential energy due to gravity
Sphere of mass M and radius R
3 M
U  G
5
R
2
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 21041 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:
ΔE
2 10 J
26
L

 3  7 10 W  1-2 L
14
T
3-6 10 s
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 diagram
Hotter
Why does temperature increase as star contracts?
• Note that luminosity remains constant.
• To produce constant luminosity as radius
decreases, need increase in temperature
LR T
2
1/ 4
4
L

 T  1/ 2
R
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
lower T
higher T
• Temperature is proportional to the average kinetic energy
per molecule
1 2 3
K  mv  kT
2
2
k = Boltzmann constant =
1.3810-23 J/K = 8.6210-5 eV/K
Energy of gas cloud
Gravitational potential energy:
3 M
U  G
5
R
Sphere of mass M and radius R
Kinetic energy of N atoms
3
M 3
K  N kT 
kT
2
mH 2
2
Energy of gas cloud
M 3
3 M
E
kT  G
mH 2
5
R
2
If E < 0 then gas cloud collapses
If E > 0 then gas cloud can support itself
Density of gas cloud is n
M  R n  mH
4
3
3
Critical size of gas cloud
3kT
3G  4nmH 
5/ 3
E
M

 M
2mH
5  3 
1/ 3
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
15kT
RJ 
8GmH2 n
M J  18M 
T in Kelvin, n in atoms/cm3
3
T
n
Critical size of gas cloud
If we have a cloud at T = 100 K and n = 1 cm-3, how large
pieces does it fragment into?
M J  18M 
T3
1003
 18M 
 18000M 
n
1
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.
Critical size of gas cloud
If we have a cloud at T = 30 K and n = 300 cm-3, how large
pieces does it fragment into?
M J  18M 
T3
303
 18M 
 170M 
n
300
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.
Critical size of gas cloud
The dense cores can reach n = 300,000 cm-3, how large
pieces do they fragment into?
M J  18M 
T3
303
 18M 
 5.4M 
n
300,000
Therefore, the dense cores fragment into individual stars.
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?