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10/17/2012
Star Formation
Lecture 12
Stellar Birth
• Since stars don’t live forever, then they must
be “born” somewhere and at some time in the
past.
• How does this happen?
• And when stars are born, so are planets!
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Molecular clouds
• Stars form in giant clouds of gas and dust
called molecular clouds.
• The term molecular cloud is used since
molecules are present.
• The large amount of gas and dust in the cloud
shields the molecules from UV radiation from
stars in our galaxy.
Anatomy of a Stellar Factory
• Molecular cloud:
•
Contains
‒
‒
‒
H, He, etc.
H2, H20, OH, CO,
H2CO, etc.
dust of silicates,
iron, ices, etc.
Collapsing
Region
103 to 106 Msun of gas
and dust in the cloud.
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M8 - Lagoon Neb.
Cloud fragmentation
• The molecular cloud does not collapse into a
single star.
• It fragments into many clumps.
• These clumps can further collapse to form
stars.
• 10 - 1000 stars can be formed from the cloud.
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Gravitational Collapse
• When a fragment of a molecular cloud
reaches a critical mass - it collapses to form a
star.
– Gas and dust pulled together by gravity until a star
is formed.
• But to get this critical mass is not so easy.
Causing collapse: Method 1
• Accretion:
– Build up of small clouds of gas and dust into giant
ones.
• Clouds “stick” together and grow.
• Very slow - due to low interstellar densities
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Causing collapse: Method 2
• Gravity and Radiation Pressure
Pressure of
Starlight
Problem:
But how do the
first stars form!
High densities &
Gravitational Collapse
Causing collapse: Method 3
• Compression by supernova blast waves
Old
Star
Nearby
Cloud
Compressed
cloud
Exploding
Star
Shock waves
from Supernova
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M16 Eagle
Nebula
NOAO Image
Pillars in M16
HST Image
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M16: Close-up
M16
10 ly
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The path to collapse
• Gravity makes the cloud collapse.
• Two hindrances to collapse
1. Internal heating
- Causes pressure build-up
2. Angular momentum
- Causes high speeds
(like a skater)
Internal Heating
• Cloud fragments collapse
• Potential energy => Kinetic Energy
– Gas particles speed up and collide.
• The temperature increases.
• This causes a pressure build-up which slows
(or stops) the collapse.
• Energy is radiated away.
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Angular Momentum
• Angular momentum
A = mass  vel. of rotation  radius
= mvr
• Conservation of angular momentum.
– A = constant for a closed system.
• As the cloud fragment shrinks due to gravity, it
spins faster.
Angular momentum
• Collapse occurs preferentially along path of
least rotation.
• The cloud fragment collapses into a central
core surrounded by a disk of material.
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Disk Formation
Rotating
Central
Core
Infall Material
Rotating
Disk
OMC Proplyds 2
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Dust Disks around Stars
Planet formation
• The disk around the central core will fragment
further, producing rings of material.
• The particles in these rings can accrete
together to form planets!
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Protostars
• The central core is called a
protostar.
• It is undergoing continuous
gravitational contraction.
• Self-compression heats the
central core.
• Surface ~ 300 K
• Energy emitted in the infrared.
L = 4 R2  T4 , R is very large.
Overview of the build-up
• Collapse starts out in free fall controlled by
gravity
• Central parts collapse more rapidly => central
core becomes a protostar.
• Core accretes material from the surrounding
envelope
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A Star is Born
• The protostar continues to collapse while the
central core heats up to millions of degrees.
• Fusion reactions start => A star is born
What stops the collapse?
• Collapse is halted by the pressure of the
heated gas which balances gravity.
An equilibrium
• Gas and Radiation
Pressure balance
Gravity
HOT
• No collapse or
expansion.
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Entrance into the H-R diagram
more massive
Luminosity (Lsun)
Hayashi
Contraction
Phase
less massive
O
B
A
F
G
K M
Temperature
Time to form a star
• The time to reach the main-sequence varies
with stellar mass.
Mass (Msun)
15
5
2
1
0.5
Time (106 years)
0.16
0.7
8
30
100
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Making the stars visible
• After a star is born it heats the gas and dust
around it.
• Eventually the gas and dust are pushed away.
• The star then becomes “visible.”
• Prior to this it could be seen only in the radio
and the infrared.
30 Doradus (Opt/IR)
Massive newborn stars are indicated by the arrows. Note that some (2, 3, & 4) are
hidden to visible light. Arrows 1 and 5 indicate a compact cluster of bright young stars.
Sources 6 & 7 may be due to outflow jets from the cluster 5.
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Dying star showing outflows –
may go supernova in a few
thousand years
Evaporating disks around
stars – planet nurseries?
Bok Globules – dark clouds
that could form stars
Young star cluster
Newborn stars emerging
from their birth clouds
NGC 3603 - Star Formation
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Stars Die!
• The fuel in stars is proportional to the
mass, M.
• It is found that the luminosity of stars on
the main-sequence varies with mass as:
L  M 3.5
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Stellar Lifetimes
• Assuming stars “consume” the same
fraction of their mass (M), the lifetime, T, is
given by:
T

T
T

T
Amount of Fuel
Amount of Fuel
Rate of Consumption
Rate of Consumption
M
1
M3.5
12.5
M
M
M 3.5
M 2.5


where M is in solar masses and T is in solar
lifetimes.
The Lifetime of Stars
• The mass of a star determines how long it
will live.
• More massive stars evolve faster.
Mass
1 Msun
5
10
Lifetime
~1010 yrs
~108
~107
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