Lecture 17: Star Formation
Astronomy 101/103
Terry Herter, Cornell University
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Lecture
Topics
  • Molecular clouds
    • The birth place of stars
  • The fragmentation and collapse of molecular clouds
  • The formation of protostars
  • The birth of a star

Star
Lifetimes
  • The mass of a star determines how long it will live.
  • More massive stars evolve faster.


Mass

(Msun)

Lifetime

(years)

1
~1010
5
~108
10
~107


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?
  • When stars are born, so are planets!

Molecular
Clouds
  • Stars form in giant clouds of gas and dust called molecular clouds.
  • The term "molecular cloud" is used since molecules form there.
  • The large amount of gas and dust in the cloud shields the molecules from UV radiation from stars in our galaxy.

 

Stellar
Factory

Anatomy of a Stellar Factory


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.

 

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 reaching this critical mass is not so easy.

 

Collapses
Methods

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

Collapse: Method 2

  • Gravity and Radiation Pressure

Collapse: Method 3

  • Compression by supernova blast waves

 


The Path
to Collapse
  • Gravity makes the cloud collapse.
  • Two hindrances to collapse, both which favor expansion of the cloud:
    1. Internal heating
      • Causes pressure build-up
    2. Angular momentum
      • Causes high speeds
      • (exemplified by a figure 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.

Angular
Momentum
  • Angular momentum
    • A = mass x vel. of rotation x radius
    • A = mvr
  • Conservation of angular momentum.
    • A = constant for a closed system.
  • As the cloud fragment shrinks due to gravity, it spins faster.
  • Collapse occurs preferentially along path of least rotation.
  • The cloud fragment collapses into a central core surrounded by a disk of material.

Disk
Formation


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!

 


Protostars
  • The central core is called a protostar.
  • Surface ~ 300 K
  • Undergoing continuous gravitational contraction.
  • Self-compression heats the central core.

Protostar Diagram

  • As core shrinks and temperature rises, energy emitted in the infrared.
  • L = 4*pi*R2 *sigma*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.

 


The
New-Born
Star
  • The protostar continues to collapse while the central core heats up to millions of degrees.
  • Fusion reactions start => A star is born

 


Stopping
the
Collapse?
  • Collapse is halted by the pressure of the heated gas and radiation which counteract gravity.
    • Pressure is force in outward direction, gravity is force pulling star in.
  • An equilibrium is reached between opposing forces.

Stable Star

  • Forces in balance => no collapse or expansion

 


Entrance
into the H-R
Diagram

 


Time to
Form a Star
The time for a star to reach the main-sequence varies with its mass.

 

Mass

(Msun)

Time

(106 years)
15
0.16
5
0.7
2
8
1
30
0.5
100

 


Making the
Stars
Visible

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 is then becomes "visible."
  • Prior to this it could be seen only in the radio and the infrared.


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