l Start with cool gas (a "chilly" 10 K - 50 K!) like the nebula shown above, which begins to collapse. l It cannot be too hot or the gas pressure will be to high and nothing will happen. l Thus it keeps away from hot stars, while dust to block the starlight aides in this. l It takes somewhere around several million years for the collapsing stage to take place. l After that, the collapsing cloud fragments into numerous clouds that each continue to collapse. l These, shown below, are called "cloudlets".

l The cool gas is attracted to the center by gravity. l Recall that every atom attracts every other atom. The closer the atoms are, the bigger the attraction.

l The cloud gets smaller and denser. l After the collapsation starts slowing, due to the heat in the core, the cloud becomes very dense (10^6 particles/cc) l Then the core heats more and more as material from the envelope continues to rain onto the core. l Material piles up at the cloud center and density rises in the center l The core begins to collapse faster than the outer envelope.

l Then it starts to rotate because of conservation of angular momentum. l The easiest way to see this is to demonstrate it -- it's like a figure scater doing a spin.

l The outside parts form into a disk. l A disk is the only possible shape for gas in orbit around a central mass. l The disk configuration keeps gas cloudlets from hitting each other. l The disk is called a protoplanetary disk.

l As the central part of the disk continues to contract, it gets warmer. l Whenever you compress a gas, it gets warm. l This is energy conservation at work: gravitational potential energy is turned into heat energy.

l As the temperature goes up to several thousand K, the protostar starts to radiate. l The inside continues to heat up as it contracts. l When the center gets dense enough and hot enough ( a few x 106K), nuclear fusion begins. l The core reaches 2000K at this point, which also causes the H2 molecules break apart. Due to the fact that this takes a lot of energy, the core collapses again l An outer envelope - like a womb - shields the entire event from view at optical wavelengths. The dust in the envelope heats up and becomes a very large region glowing brightly in the infrared part of the spectrum. l Then the outer envelope is blown away and a pre-main sequence star emerges. l This is a violent process - a rapidly spinning and still collapsing outer disk collides with a now expanding shell from the newly formed star. l Motion above and below the rotational plane of the nebula is easier and hence material "squirts" away at right angles to the disk. l The images show clearly the jet like structures (bi-polar flows) ejected from the collapsing stellar cloud and protostar.

l Associated with jets are bright knots of light called Herbig-Haro objects. l We think that these are the result of shock-wave heating produced by supersonic matter ejected along the jets. l As this matter plows into dust and gas left over from the collapsing nebula it heats the gas and produces these bright, fast moving "blobs". l At approximately 10,000K the cloud begin to look like a star, and at this point it is called a Protostar.

l After the Protostar stage, the central heat source stabilizes the pressure, halting further contraction. l Now the energy radiated is supplied by nuclear fusion instead of further contraction.