How do stars form?

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How do stars form?

• The Nebular Hypothesis

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Observations

• Stars can be seen in various stages of formation.
• Stars seem to have been forming continuously
  since the formation of the Universe.
• Star formation continues today.
• Observations synthesized into the Nebular
  Hypothesis.

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Our Sun an exampleof stellar evolution

• Our Sun began as a nebula, approximately 5
  billion years ago.
• A nebula is an enormous cloud of gasses (mainly
  Hydrogen) and dust
• Nebula may become disturbed by shock waves, for
  example from a nearby supernova.

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Example of a nebula
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Nebula begins to contract

• As the molecules of gas and dust move closer
  together, they experience stronger gravitational
  attraction.
• Newtons Law of Universal Gravitation
• Fg g(m1m2)/d2
• Most mass concentrates at the center as the
  nebula contracts.

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Nebula begins to take a more definite shape.
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Shape becomes spherical with equatorial disk
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Will a star form?

• If mass is sufficient, gravity at the center of
  the sphere may be great enough to squash atoms
  together.
• 4 H atoms are fused to form 1 He atom in a
  nuclear reaction (not chemical).
• Nuclear reaction is nuclear fusion, which
  releases tremendous energy.
• A star is born!

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Stable stars

• Radiant energy produced by fusion causes the star
  to expand.
• Gravity holds the star together.
• Gas pressure/radiant energy is balanced by
  gravity, so the star is stable.
• The Hertzsprung-Russell (H-R) Diagram shows these
  stars as the main sequence.

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The H-R Diagram
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Another version of the H-R Diagram
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H-R Diagram

• Shows relationships among size, temperature and
  brightness (luminosity or magnitude).
• Larger, stable stars are hotter and brighter.
• Large, hot stars burn out faster than smaller,
  cooler stars.

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Stellar Evolutionand the H-R Diagram

• What happens when a star exhausts its nuclear
  fuel?
• Depends on size
• Star core collapses on itself, but heats the
  outer envelope.
• Result may be White dwarf, white dwarf with
  planetary nebula, red giant, neutron star or
  black hole.

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Stellar collapse

• May result in gravitational heating and eventual
  burnout.
• May result in renewed fusion (He is fused this
  time).
• Collapse may produce explosion (supernova).
• Remnant of supernova may be a neutron star or a
  black hole.

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Products of fusion

• H is fused to form He
• He is fused to form C and other, heavier chemical
  elements.
• Heavier elements are recycled into new nebulae,
  and/or new stars and planets.
• Implication?

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Heavy Elements

• Since all elements heavier than H are produced by
  fusion in stars,
• We are made of Stardust !!!

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Summary Nebular Hypothesis
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So what happened to the disk?

• The disk that surrounds the central star may
• 1) be swallowed as the star initially expands.
• 2) remain as a disk or a series of rings
• 3) may form planets that orbit the central star.

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Our Solar System

• Sun began to radiate energy about 5 billion years
  ago.
• Surrounding disk condensed into 9 (possibly 8)
  planets and an asteroid belt.
• Earth is one of those nine planets.
• Earth condensed approximately 4.6 billion years
  ago.

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How do we know the timing?

• Age of Sun via chemical composition and known
  rate of fusion about 5 Ga
• Oldest Earth rock 3.98 Ga
• Age of oldest Moon Rocks 4.2 Ga
• Age of Meteorites 4.5 Ga
• Ga Giga-annum billion years