Transcript How do stars form?

How did the Earth form?
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What processes were involved?
When did it happen?
How long did it take?
How do we know?
How do stars form?
The Nebular Hypothesis
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.
Our Sun: an example
of 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.
Example of a nebula
Nebula begins to contract
• As the molecules of gas and dust move
closer together, they experience stronger
gravitational attraction.
• Newton’s Law of Universal Gravitation
• Fg = g(m1m2)/d2
• Most mass concentrates at the center as the
nebula contracts.
Nebula begins to take a more
definite shape.
Shape becomes spherical with
equatorial disk
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!
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?
Heavy Elements
• Since all elements heavier than H are
produced by fusion in stars,
• We are made of Stardust !!!
Summary: Nebular Hypothesis
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.
Our Solar System
• Sun began to radiate energy about 5 billion
years ago.
• Surrounding disk condensed into 8 planets,
an asteroid belt and other objects.
• Earth is one of those eight planets.
• Earth condensed approximately 4.6 billion
years ago.
Evidence of Age of the System
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Oldest Earth rock: 3.98 Ga Acasta Gneiss
Oldest Earth minerals: 4.4 Ga
Chemistry of the Sun and rate of fusion
Age of oldest Moon Rocks: 3.3 - 4.2 Ga
Age of Meteorites: 4.5 Ga