star formation - cornell university
TRANSCRIPT
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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:
103 to 106 Msun of gas and dust in the cloud.
Collapsing Region
• Contains
‒ H, He, etc.
‒ H2, H20, OH, CO, H2CO, etc.
‒ dust of silicates, iron, ices, etc.
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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
High densities & Gravitational Collapse
Problem: But how do the first stars form!
Causing collapse: Method 3
• Compression by supernova blast waves
Exploding Star
Old Star Nearby
Cloud
Compressed cloud
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 Disk
Rotating Central Core
Infall Material
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
HOT
• Gas and Radiation Pressure balance Gravity
• No collapse or expansion.
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Entrance into the H-R diagram
Temperature
Lum
ino
sity
(Lsu
n)
O B A F G K M
Hayashi Contraction Phase
less massive
more massive
Time to form a star
• The time to reach the main-sequence varies with stellar mass.
Mass (Msun) Time (106 years)
15 0.16
5 0.7
2 8
1 30
0.5 100
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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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NGC 3603 - Star Formation
Bok Globules – dark clouds that could form stars
Newborn stars emerging from their birth clouds
Young star cluster
Dying star showing outflows – may go supernova in a few thousand years
Evaporating disks around stars – planet nurseries?
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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:
5.3ML
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Stellar Lifetimes
• Assuming stars “consume” the same fraction of their mass (M), the lifetime, T, is given by:
T
T
Amount of Fuel
Rate of Consumption
M
M
1
M3.5 2.5
T
T
Amount of Fuel
Rate of Consumption
M
M
1
M3.5 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 Lifetime
1 Msun ~1010 yrs
5 ~108
10 ~107