Transcript Star Formation - McMurry University

Lecture 20
Star Formation
Announcements
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Comet Lovejoy will be a late
night/early morning object through the
rest of the semester, so currently there
is no plan to have a group observing
session.
This is National Dark Sky Week. Turn
off unnecessary outdoor lighting and
enjoy darker skies. Also, be sure to
check out the web site at
http://www.ndsw.org
Review
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We said last time that stars form from
giant molecular clouds.
But how does a giant, cold cloud of
gas become a hot, dense star?
Pressure and Gravity
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Any gas cloud in space with a
temperature above absolute zero has
internal pressure.
Higher temperature = Higher internal
pressure.
A gas cloud’s internal pressure pushes
out.
Makes the cloud expand.
Pressure and Gravity
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Gravity pulls inward.
More mass = more gravity
– Higher density = more mass
– So higher density = more gravity
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Gravity makes a gas cloud shrink down
(collapse).
Stability
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Pressure > Gravity, cloud expands
(gets bigger)
Pressure < Gravity, cloud contracts
(gets smaller)
Pressure = Gravity, cloud stays the
same size (hydrostatic equilibrium)
How Do You Form Stars?
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Molecular Clouds:
– Pressure about equal to
gravity
– Stable, but “primed” for
collapse
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What could trigger a
molecular cloud to
collapse?
– Lower temperature
(reduce internal
pressure)
– Increase density
(increase gravity)
How Do You Form Stars?
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Just need a little
“push” to trigger
the collapse
What provides the
push?
– Shockwaves:
– Supernovae
– Hot winds from new
stars
Star Formation Triggers
More Star Formation
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In a molecular cloud,
star formation
frequently happens in
stages.
The cloud collapses
and fragments.
Massive stars form
very quickly.
They heat up the cloud
and prevent lower
mass stars from
forming.
Called an OB
association.
Star Formation Triggers
More Star Formation
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O and B stars don’t live
very long.
Explode when they die
(supernovae).
The shock waves
spread out into the
surrounding cloud, and
trigger the formation of
smaller, less massive
stars.
Star Formation Triggers
More Star Formation
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Also, the hot winds
from massive stars
can spread out
through the
surrounding gas
clouds, triggering
new star formation.
Let The Collapse Begin!
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Shockwave increases density of cloud.
Gravity wins! Cloud begins to
collapse!
– Temperature through cloud not even.
Coldest spots collapse fastest.
– Causes cloud to “fragment” into
collapsing knots.
– Each knot is where a new star will form.
Time For Pancakes
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As a gas cloud collapses, it begins to rotate
faster and flatten into a disk (conservation
of angular momentum).
The knot at the center is surrounded by a
flattened disk of rotating gas called an
accretion disk.
The accretion disk:
– Feeds matter onto the protostar
– Is where planets form
A Baby Star Begins To
Form
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Matter begins to pile
up at the center of the
knot.
– Gravitational collapse
releases thermal energy.
– Center of knot begins to
heat up as collapse
continues.
– Contracts into a
spherical “glob” of hot
gas at the center of the
knot. Called a protostar.
Daily Quiz 16 – Question 1
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What force causes the contraction of
a cloud of interstellar matter to form
a star?
A.
B.
C.
D.
The
The
The
The
electrostatic force.
strong nuclear force.
weak nuclear force.
gravitational force.
Protostars
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As cloud heats up,
surface gets hotter.
Protostar begins to
generate light.
Starts out relatively
cool, but very large,
giving it high
Luminosity.
Surface temperature
increases very slowly
as collapse continues,
so protostar gets
hotter, but fainter.
Protostars
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Core of protostar gets hotter for two
reasons:
– Thermal energy from gravitational collapse.
– Thermal energy from nuclear fusion.
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Fusion reactions begin as soon as the center of the
protostar gets hot enough (~1 million K)
Hydrogen to Helium fusion starts out slowly
Heat doesn’t provide enough outward pressure to stop
collapse.
As core temperature goes up, fusion reactions happen
faster.
More energy = slower collapse.
Protostars
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Near end of collapse, increasing energy
output from fusion makes protostar
unstable. Ejects lots of material:
– T Tauri stars
– Herbig-Haro objects
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Increasing rate of fusion heats up
protostar’s surface quickly as it collapses.
Gets hotter, but stays the same luminosity.
Daily Quiz 16 – Question 2
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What happens to the temperature
and density inside a collapsing
protostar?
A. Temperature and density both increase.
B. Temperature and density both decrease.
C. Temperature increases and density
decreases.
D. Temperature decreases and density
increases.
Here’s a Picture of a TTauri Star
It’s ejecting a huge bubble of material from its outer surface into
space.
T-Tauri Stars
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Exclusively low-mass
– All are no more than 3 times as massive as the
sun.
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Very unstable
Some nuclear fusion, but not enough to
support the star (still collapsing)
Likely the sun went through this phase 4.6
billion years ago.
May have a role in clearing out nebula gases
from a forming solar system
Here Are Pictures Of
Herbig-Haro Objects
The protostar is in the center
(concealed by dust)
These are jets of material
coming off of the protostar
Herbig-Haro Objects
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Caused by the accretion disk getting in the way of
mass flowing off of the unstable protostar.
Focuses the hot gasses into the two jets seen in
these objects.
Jet
Accretion disk
Jet
Protostars
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Collapse stops when heat released by
nuclear fusion provides enough
pressure to prevent further
contraction.
Newly formed star is now stable. All
energy to prevent further collapse
provided by H to He fusion.
Star becomes main sequence.
Hydrostatic Equilibrium
Outward pressure force must
exactly balance the weight
of all layers above
everywhere in the star.
This condition uniquely
determines the interior
structure of the star.
This is why we find stable
stars on such a narrow strip
(Main Sequence) in the
Hertzsprung-Russell diagram.
Pressure vs. Gravity
Daily Quiz 16 – Question 3
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What eventually halts the slow
contraction of a newly forming star?
A.
B.
C.
D.
A second shock wave.
Electrostatic repulsion.
Nuclear fusion.
Gravity.
How Long Does It Take?
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More massive protostars have more gravity,
so collapse is faster.
Core gets hotter much faster, so stable
hydrogen to helium fusion sets in faster.
From initial collapse to main sequence:
– Takes only about 30,000 years for a massive star
(30 solar mass star).
– Takes about 30 million years for a star like the
sun.
– Takes very long (1 billion years) for a low mass
star (0.2 solar masses).
Mass Limits
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Stars have limits to
their mass.
If the protostar has
less than 0.08 solar
masses:
– Never gets hot enough
for stable hydrogen to
helium fusion.
– Star is “stillborn”
– Starts with a surface
temperature of about
3,500 K, then cools
down.
– Called a brown dwarf
Mass Limits
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If the protostar has
more than about 120
solar masses:
– The core temperature
rises very fast.
– Star’s internal pressure
rises so fast it
completely overcomes
gravity
– Star explodes, shedding
large amounts of mass
– Enough mass is shed to
bring star below 120
solar mass limit
Daily Quiz 16 – Question 4
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Why can’t you form a star with less
than 0.08 Solar masses?
A.
B.
C.
D.
It will become unstable and explode
Such a small mass would never collapse
It never gets hot enough for fusion
You CAN form a star this size, it is called
a blue dwarf
The PressureTemperature Thermostat
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Hotter temperatures:
–
–
–
–
–
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Atomic nuclei move faster.
They “touch” more frequently.
More fusion reactions each second.
More energy released.
HIGHER PRESSURES PUSH OUTWARD.
Cooler temperatures:
– Atomic nuclei move more slowly.
– Less fusion.
– PRESSURE GOES DOWN.
The PressureTemperature Thermostat
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If the fusion reactions in a star slow down:
– Pressure < Gravity
– Star shrinks
– Heats up, increasing fusion rate until Pressure = Gravity.
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If the fusion reactions in a star speed up:
– Pressure > Gravity
– Star swells up
– Cools down, decreasing fusion rate until Pressure =
Gravity.
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Star tends to maintain the same size and
temperature: called the Pressure-Temperature
Thermostat.
The Main Sequence
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More Mass means:
– More gravity, so the star weighs more.
– The star needs to create more internal pressure
to support its weight.
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What Creates More Internal Pressure?
– More nuclear fusion!
– More fusion = more heat and light!
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So more massive stars are brighter and
hotter.
Next Time
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We will finish discussing stellar
structure and start talking about stellar
evolution
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Read Unit 61 on stellar evolution