Otterbein College Department of Physics and Astronomy

FORMATION OF STARS

FORMATION OF STARS LIKE THE SUN

Common theme: balance between gravity and pressure.

Stage 1	Lasts: 2 million y Central T: 10 K Size: tens of parsecs	cloud starts to contract under the influence of its own gravity. Typically breaks up into fragments. Collapse is triggered by some external event, required because ordinarily gravity is opposed by pressure within the gas itself
Stage 2	Lasts: 30,000 y Central T: 100 K Surface T: 10 K Size: ~ 100 x solar system	As cloud contracts, it warms slightly and gives off energy in the form of black body radiation
Stage 3	Lasts: 100,000 y Central T: 10,000 K Surface T: 100 K Size: ~ Solar system	cloud becomes so dense that it becomes opaque; the dust prevents black body radiation from escaping the core. The core starts to heat up.
Stage 4	Lasts: million years Central T: 10⁶ K Surface T: 3000 K Size: ~ orbit of Mercury	Star continues to contract and radiate energy from its contraction. Heating at core produces pressure which opposes contraction.
Stage 5	Lasts: 10 million y Central T: 5x10⁶ K Size: 10 x sun	T Tauri stage. Violent surface activity; strong solar wind (blows out remaining solar nebula)
Stage 6	Lasts: 30 million y Central T: 10x10⁶ K Surface T: 4500 K Size: slightly larger than Sun	Young star. Temperature and density at core become high enough to sustain nuclear fusion
Stage 7	Lasts: 10 billion years Central T: 15x10⁶ K Surface T: 6000 K Size: ~ Sun	Main sequence star. Star shrinks a little more and surface heats a little more. Pressure and gravity are balanced; rate of energy production from nuclear fusion and rate of radiation from surface are balanced.

DIFFERENCES FOR STARS OF OTHER MASSES

Larger stars surface temperatures, luminosities, radius larger

take less time to form
have a shorter main sequence lifetime

Smaller stars surface temperatures, luminosities, radius lower

take longer to form
have a longer main sequence lifetime

All have in common that the main sequence lifetime is much longer than the time of formation.

BROWN DWARFS Jupiter-like bodies that are too small to start fusion but give off heat from contraction. There may be much mass in the universe in the form of brown dwarfs.

OBSERVATIONAL CONFIRMATION

M20 (Trifid Nebula) shows regions of warm condensing molecular clouds (stage 2), and hot young stars (stage 6 or 7)
Orion nebula (M42) contains molecular clouds that may be stage 3 and objects that emit in the IR, which may be stage 4
IRAS revealed smaller objects emitting in the IR, such as Barnard 5, which may be solar-sized stage 5 objects

WHAT HAPPENS AFTER STAR FORMATION

Stars form in clusters from the clouds. (No evidence of formation of isolated stars is seen.)
The remaining gas is blown out by stellar winds
The clusters break up (stars are thrown out of the cluster by gravitational boosts or galactic tidal effects)

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