Transcript How do stars form?

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!
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.
The H-R Diagram
Another version of the H-R
Diagram
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.
Stellar Evolution
and 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.
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.
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 9
(possibly 8) planets and an asteroid belt.
• Earth is one of those nine planets.
• Earth condensed approximately 4.6 billion
years ago.
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