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 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.

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 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.

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