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Star Birth
• How do stars form?
• What is the maximum mass of a new star?
• What is the minimum mass of a new star?
How do stars form?
Star-Forming Clouds
• Stars form in dark
clouds of dusty gas
in interstellar space.
• The gas between the
stars is called the
Gravity Versus Pressure
• Gravity can create stars only if it can overcome
the force of thermal pressure in a cloud.
• Gravity within a contracting gas cloud becomes
stronger as the gas becomes denser.
• Its called a molecular cloud because the Hydrogen is
cool enough in the form H2 molecules.
Mass of a Star-Forming Cloud
• A typical molecular cloud (T~ 30 K, n ~ 300
particles/cm3) must contain at least a few hundred
solar masses for gravity to overcome pressure.
• The cloud can prevent a pressure buildup as it
collapses by converting thermal energy into infrared
and radio photons that are radiated away to cool the
Fragmentation of a Cloud
This simulation
begins with a
turbulent cloud
containing 50 solar
masses of gas.
Fragmentation of a Cloud
The random motions
of different sections
of the cloud cause it
to become lumpy.
Fragmentation of a Cloud
Each lump of the cloud
in which gravity can
overcome pressure can
go on to become on or
more stars.
A large cloud can make
a whole cluster of stars
containing thousands or
millions of stars.
Glowing Dust Grains
As stars begin to
form, dust grains
that absorb visible
light heat up and
emit infrared light.
So we can find
new stars by using
and infrared
Glowing Dust Grains
infrared light is
brightest from
regions where many
stars are currently
Thought Question
What would happen to a contracting cloud fragment
if it were not able to radiate away its thermal energy?
A. It would continue contracting, but its
temperature would not change.
B. Its mass would increase.
C. Its internal pressure would increase.
Solar system formation is a good example of star birth.
Recall the Nebula Model of the Solar System.
Cloud heats up as gravity causes it to contract due to
conservation of energy. Contraction can continue if
thermal energy is radiated away.
As gravity forces a cloud to become smaller, it begins to
spin faster and faster, due to conservation of angular
As gravity forces a cloud to become smaller, it begins to
spin faster and faster, due to conservation of angular
momentum. Gas settles into a spinning disk because spin
hampers collapse perpendicular to the spin axis.
Rotation of a
cloud speeds
up for the
same reason a
skater speeds
up as she pulls
in her arms.
Collapse of the Solar Nebula
Collisions between particles in the cloud cause
it to flatten into a disk.
between gas
particles also
reduce up
and down
Why Does the Disk Flatten?
Formation of Jets
Rotation also causes jets
of matter to shoot out
along the rotation axis.
These are probably due to
magnetic fields in the new
star. We usually only see
these jets in new stars.
Jets are observed coming from the centers of disks around
A protostar is the name we give a star that is just forming … i.e. it
has not yet reached the main sequence.
The initial disk that forms around the star is called a protostellar
disk (also sometimes called a protoplanetary disk).
Thought Question
What would happen to a protostar that formed
without any rotation at all?
Its jets would go in multiple directions.
It would not have planets.
It would be very bright in infrared light.
It would not be round.
Protostar to Main Sequence
• A protostar contracts and heats until the core temperature
is sufficient for hydrogen fusion.
• Contraction ends when energy released by hydrogen fusion
balances energy radiated from the surface.
• It takes 30 million years for a star like the Sun (less time
for more massive stars).
• Before it reaches the main sequence the cloud\protostar has a larger
radius (it is collapsing) and a lower temperature (fusion has not started
yet) than it will have on the main sequence. So it must approach the
main sequence from the right hand side of the HR diagram.
Summary of Star Birth
Gravity causes gas cloud to shrink and fragment.
Cores of shrinking cloud fragments heat up.
Collapse only continues if the cloud cools by radiating away heat.
If the initial cloud was spinning a protostellar disk is formed.
Protostars approach the main sequence from the right hand side of the HR diagram.
Jets can be formed as the protostar collapses.
When core gets hot enough, fusion H to He begins and stops the collapse.
New star achieves long-lasting state of balance on the Main Sequence (i.e. the
thermostat model that we discussed for the Sun where the rate of nuclear fusion
produced sufficient thermal gas pressure to resist gravitational collapse).
How massive are newborn stars?
A cluster of many stars can form out of a single cloud.
Very massive stars
are rare.
Low-mass stars are
Only about 1 in 200
stars is an O type
star, whereas 90%
of all stars are
either spectral type
K or M.
Upper Limit on a Star’s Mass
• Photons of light 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.
• Hence very large stars are not
stable and quickly fall apart do
to photon pressure.
Upper Limit on a Star’s Mass
• Models of stars
suggest that
radiation pressure
limits how massive
a star can be without
blowing itself apart.
• Observations have
not found stars more
massive than about
Lower Limit on a Star’s Mass
• Fusion will not begin in a contracting cloud if some
sort of force stops contraction before the core
temperature rises above 107 K.
• Thermal pressure cannot stop contraction because the
star is constantly losing thermal energy from its
surface through radiation.
• Is there another form of pressure that can stop
Degeneracy Pressure:
Laws of quantum mechanics prohibit two electrons
from occupying the same state in the same place.
Thermal Pressure:
Depends on heat content
The main form of pressure
in most stars
Degeneracy Pressure:
Particles can’t be in same
state in same place
Doesn’t depend on heat
Brown Dwarfs
• Degeneracy pressure
halts the contraction
of objects with
<0.08MSun before
the core temperature
becomes hot enough
for fusion.
• Starlike objects not
massive enough to
start fusion are
brown dwarfs.
Brown Dwarfs
• A brown dwarf
emits infrared light
because of heat left
over from
• Its luminosity
gradually declines
with time as it loses
thermal energy and
Brown Dwarfs in Orion
• Infrared
observations can
reveal recently
formed brown
dwarfs because
they are still
relatively warm
and luminous.
Stars more
would blow
Stars less
can’t sustain
• Now you make work with your partner(s)
on the homework that has just been given