Download formation2

Spiral Density waves initiate star
formation
A molecular cloud passing through the
Sagittarius spiral arm
Spiral arm
Gas outflows
from super
supernova or
O/B star winds
Initiation of
star formation
Supernova bubble
Molecular
cloud
Bubble moving
outward from
supernova
Star formation in a compressed cloud
• A region of the molecular cloud becomes dense.
• This pocket of over density is much bigger than
a single star.
• This over dense region is not uniform, but has
within it other, smaller regions of high density.
• As the over density begins to be drawn together
by gravity, it fragments into smaller pockets of
gas which go on to form individual stars.
• The result is a star cluster. The more massive
pockets from massive stars, the less massive
form smaller stars, like the Sun
Spinning stars and disks
• As material falls into a newly forming star it
begins to spin rapidly.
• This is due to another conservation law. It
is the conservation of angular momentum.
• Angular momentum is similar to regular
momentum in a straight line. Angular
momentum is just the momentum that
keeps things spinning.
Angular momentum is constant
• L = mass X velocity X radius
• Where L is angular momentum and it is
constant in a system.
• L = mvr
• Let’s examine this by first holding the
mass, m, constant.
L = mvr So, what happens if the radius
decreases?
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1. The velocity will
increase
2. The velocity will
decrease
3. The velocity will stay
the same
4. L will decrease
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• If the radius were to decrease, then the
velocity has to increase. Causing the
object to speed up its rotation.
• L = mvr and L is constant. So if r gets
smaller and m is constant, there is no
choice but for v to increase in such a way
as to keep mvr constant.
• As a star begins to form and contracts
(shrinking R), the spinning material falling
in, speeds up (increasing v).
• This causes the proto-star to spin faster
and faster as it shrinks
• The material outside the proto-star spins
fast enough to orbit, and flattens into a
spinning disk.
Pre-main sequence stars with a disk of in
falling material and bi-polar out flows
These are stars that are still forming. As
gas falls into the forming star, some is
redirected out along the poles of the star.
What is happening?
• Material in the spinning disk is falling into the
newly forming star.
• Two things can happen…
• 1) Some of the gas is caught in the magnetic
field of the star and shot out along the poles,
where the magnetic field is the strongest.
• 2) The star has a stellar wind that is attempting
to blow the gas away. The gas is restrained
from moving in the disk, but perpendicular to the
disk it can flow outward quite easily.
Now we will look in detail at the star forming
process.
• A) A large over dense region fragments
into smaller pockets of high density
• B) A individual pocket begins to shrink due
to the influence of gravity.
• Any small amount of spinning in the
extended cloud will cause fast spinning as
the cloud shrinks, due to the conservation
of angular momentum. L = mrv
But why does the cloud shrink at all?
• In terms of energy, the material that is going to
form the star loses potential energy which is
changed into kinetic energy.
• This speeds up the material. So when the
material approaches the center of the cloud it
should be moving very fast.
• This would suggest that in falling material will
simply fly back out, turning its kinetic energy into
potential energy once again.
• If this happened the star would never form.
Mass on a spring
• The mass on a spring starts with lots of
potential energy.
• The potential energy is changed into
kinetic energy making the mass move very
fast.
• Then the kinetic energy is changed back
into potential energy
• The spring oscillates.
Why does the proto-star shrink?
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1. Atoms slow down due to
head on collisions near
the center
2. Atoms collide and
radiate giving up their
kinetic energy
3. Atoms can kinetic energy
but the pull of gravity
slows them down at the
center.
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Proto-star begins to shrink, but in the
process it is radiating. Gravitational
potential energy is being converted into
luminosity.
Let’s remember what luminosity
depends on.
• L = σT4(4πR2)
• There is a battle going on, between the
effects of a shrinking radius and increasing
temperature.
• Let’s look at the H-R diagram to see once
again how these two parameters change.
Temperature is
increasing this
way
Radius is
increasing in
this direction
Temperature is
increasing this
way
So in the first phase (1), the luminosity is
increasing because the temperature is going up.
The radius is actually shrinking, but is losing the
battle to the increase in temperature.
The star move up in luminosity and increases in
temperature
Right here, just before #2, something strange
happens. We see at #2 the luminosity is dropping.
What is causing this drop in luminosity?
What is causing the down turn
at #2
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1. Temperature is
decreasing while the
radius is increasing
2. Radius is constant but
temperature is
increasing
3. Radius is rapidly
decreasing while
temperature is constant
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At this point, convection begins in the proto-star.
Convection is a very efficient way to transport
energy. The atoms give up their kinetic energy
and cool, causing the star to rapidly shrink.
The luminosity is dominated by the shrinking
radius. Radius wins this battle.
At this point, (3) the core temperature is hot
enough for nuclear reactions to begin. As the
reactions increase, the star begins to heat up, and
expand.
Temperature is winning this battle.
Finally at (4) the reaction rates come into
equilibrium with the inward force of gravity. The
star becomes stable, and is now on the main
sequence.
Here are the evolutionary tracks for various mass
stars. Stars that never have convection do not
have the down turn. Also the very massive stars
form fast, due to their large gravity.
Interesting, but does it really happen. Here is
the cluster at the center of the Orion Nebula
This is the HR diagram for the Orion cluster
Main
sequence
line
Massive
stars on
MS, but
lower
mass stars
not.
Close up of low mass stars
Same thing in Lagoon Nebula.
Pleiades, what about them?
Still some gas around
And dust shows up in the infrared image
taken by Spitzer telescope
All stars are on the main-sequence except
the O-stars which are already running out of
fuel and moving off the main-sequence.
It would take 100,000 sun-like stars to
produce the luminosity of 1, O-type star
Quiz #6
• Most stars form in the spiral arms of galaxies
• Stars form in clusters, with all types of stars
forming. O,B,A,F,G,K,M
• Spiral arms barely move, but gas clouds and
stars orbit around the galaxy moving in and out
of spiral arms
• From the HR diagram, by far the most luminous
stars are the O-type stars. Their luminosity can
be 100,000 times the Sun’s.
• Why is the spiral structure in galaxies so
noticeable, even at great distances?