Start with a giant molecular cloud
Although we may think of the Sun as a giant ball of flaming gas, its center is much denser than steel. Yet stars are made in nebulae so rarefied that, on average, there are only 100 particles in a cubic centimeter - a cubic centimeter of the air we breathe has about 100 quadrillion times that many.
It seems unbelievable that something so substantial as a star is made from something as flimsy as a nebula. However, the gigantic clouds are spread out over distances of tens of light years. So although they are thin, their total mass can be as much as a million times the mass of the Solar System. There's plenty of material available, but what shapes it?
Gravity, the sculptor
Gravity is the force that collapses a nebula into something dense enough to make a star. A giant molecular cloud is a good place for star formation. Not only does it have abundant material, it's also cold enough that atoms have come together to form molecules, and in some places matter has begun to clump together.
The strength of gravity depends on the mass, so an area of higher density can pull more matter into it, increasing its mass and therefore its gravitational attraction. Over a few million years this is how a nebula can collapse. But it's likely that the collapse will have some help. There are a number of possible triggers for star formation, for example, supernova shock waves pushing matter together to form denser regions.
A nebula doesn't collapse all at once. The denser regions grow and the cloud breaks up. This is why stars form in groups. Each fragment collapses individually and is a potential star whose mass will mark out its life story. The Pleiades star clusteris an example of a group of stars that formed from the same giant cloud. The mass of each individual star determines how luminous it will be, how long it will live, and how it will die. Some fragments won't have enough mass to form stars, but may become brown dwarfs, failed stars.
The fragments heat up, rotate, and continue to collapse.
Matter outside the central region has gravitational potential energy, like water held back by a dam. When it falls into the center, the potential energy becomes kinetic (movement) energy, and heat is released.
Angular momentum is the measure of an object's rotation, taking into account its radius and its velocity. The giant nebulae rotate very slowly. But angular momentum is conserved — that means that a fragment of the cloud, having a smaller radius, will rotate faster. A favorite earthly example is an ice skater doing a spin. She starts with her arms outstretched. If she pulls her arms in to her body, the radius of the spin is less, so she spins faster with no extra effort.
Therefore as the fragment collapses, its rotation speeds up. And instead of the irregular shape of the original fragment, the spinning makes it into a more globular shape.
The fragment contains a dense central region that becomes a protostar and then a star. What's left over is dust and gas. As it spins, the loose dust and gas is pushed into a disk around the protostar's equator. Not only may a star one day form from the protostar, but a planetary system can form from this protoplanetary disk.
The protostar grows by attracting disk material. As its mass increases, it continues to contract. Gravitational contraction releases a lot of heat. The hot gas in the core pushes outward, acting against gravity. Therefore although the initial collapse happened comparatively quickly, it slows as the protostar gets hotter. It takes about a million years to get the temperature up to one million degrees Celsius, and that's not nearly hot enough for it to become a star.
Most of the stars we observe are main sequence stars. Their heat and light come from the nuclear fusion of hydrogen in their cores. In order for nuclear fusion to begin, the core temperature has to be at least 10 million °C (18 million °F).
A star is born
When the hydrogen fusion begins, the protostar is a proper baby star. But it has some growing-up to do before it joins the main sequence.
In a main sequence star there is a balance between the outward pressure of the heat from nuclear fusion in the core and the inward force of gravity. This is called hydrostatic equilibrium. It takes a while for the star to finish contracting and for this balance to occur.
The star's mass doesn't increase once nuclear fusion is sustained, because a strong stellar wind blows the disk material away. In fact, within a few million years it clears the dusty disk completely.
The length of a star's main sequence lifetime depends on its mass. Sun-like stars live about 10 billion years, so our Sun is halfway through its life. A red dwarf with half the Sun's mass may live for 80 billion years or more, which is much longer than the current age of the Universe. But massive stars have short lifetimes. A star ten times the mass of the Sun lasts only 20 million years. Stars stay on the main sequence until their hydrogen fuel is exhausted.
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