Stellar Birth

Overview

Nebulae, discussed in the Nebulae page of this section, are what form stars. However, it takes a long time before a nebulae becomes a full-fledged star. The steps and processes that form stars from these vast clouds of dust and gas is the topic of this page.

The basic process of star formation is that they emerge due to accretion of enough matter to reach a critical mass* of approximately 80 times Jupiter's, at which point internal pressures raise the core temperatures high enough to ignite nuclear fusion.

*Brown dwarfs are a type of failed star. They do not fuse hydrogen and so are technically not considered to be stars. They do not fuse because their masses are below approximately 80 times Jupiter's, so there is not enough pressure to raise internal temperatures to those necessary for fusion. If two brown dwarfs were to merge, and the resulting body had more than that critical mass, then it would theoretically start fusion and become a bona fide star.

Gravitational Collapse

The first step in the birth of a star is to wait. Dust, gas, and other materials sit around in nebulae, and wait for eons until a passing star, shockwave, or other gravitational disturbance passes through or by the nebula.

Once this happens, its gravity causes swirls and ripples. It would be like spreading marbles out on a trampoline, and then rolling a large lead ball around the edge, or through the middle. The other marbles would roll around, and clump together near the path the lead one took. It is no different in a nebula when a star passes by. To add to the marble analogy: When the marbles gather in places, the dip in the trampoline causes other marbles to accumulate in the same spot until there are just a few piles of marbles, with very few marbles in between. This process is called accretion, and causes the stars - or marble clusters - to grow larger.

However, the molecules in the nebula have energy of their own, which resist this collapse. The cloud will only collapse if its mass is large enough to allow this - a mass called the "Jean's Mass." This is derived from the Virial Theorem (left). Rearranging this, and substituting in the equations for kinetic an potential energy, the equation becomes:

Now, the number of particles in the cloud is equal to the mass of the cloud divided by the mass of the particles, , and assuming that the cloud has a constant density, you can relate its size to its mass by the equation to the right. Rearranging, combining, and simplifying, we get the equation for Jean's Mass, M_J:

If the cloud's mass is larger than this critical mass, then it will collapse. Otherwise, it will continue to swirl and clump, but the clumps will not be permanent, and they will dissolve in the cloud.

Continuing on the road of accretion, assuming that the cloud's mass is above the Jean's Mass, the clumps of matter continue to group together in the nebula until they are gigantic clumps of dust and gas. By this time, the clumps have reached sun-like sizes, and by that stage, the gas is dense enough that it no longer loses heat to the surrounding nebula. It has become "adiabatically opaque," and the heat that it generates is retained, and it starts to heat up. At this stage, the clump is called a protostar. From the start of the collapse to this stage, typical time scales are on the order of a few hundred thousand years.

Protostars - Pre-Main Sequence

As the protostar becomes larger, gravity squeezes it tighter, causing pressure to build and for the heat to increase. If you have ever pumped a bicycle tire, you know that when the air becomes compressed, it becomes hotter.

On the Hertzsprung-Russell Diagram, the large swath of stars through the center is called the "Main Sequence," and it is where most stars live most of their lives. There is a period of time between when protostars are formed and they reach the main sequence, and this is called the "pre-main sequence."

At this point, the stars technically are still proto, not having ignited fusion. They are still contracting.

The pre-main sequence to the right shows theoretical temperature vs. luminosity of the protostars for several different masses. Low mass stars contract and drop in luminosity until the interior opacity drops and the energy comes flooding out, resulting in an increase in surface temperature and luminosity. High mass stars have low opacity to begin with due to high temperatures, and simply heat up as the contract.

Then, when the pressure in the center causes the core to reach a temperature of 10,000,000 K (18,000,000 °F), hydrogen fusion is initiated. Now, the protostar has become a star. It shines with its own light. Its solar wind quickly pushes away the rest of the dust and gas in its vicinity.

NOTE: A protostar that does not become hot enough to begin fusion, yet is no longer surrounded by its parent nebula is called a brown dwarf. A brown dwarf usually has between 1/12 and 1/100 of the Sun's mass. It can still produce heat by contracting very slowly (i.e. decreasing its equatorial diameter by a few millimeters a year), yet does not shine as a star does. Jupiter produces heat in this way, although it is too small to be considered a brown dwarf. Most brown dwarfs have an average surface temperature of 1,800 K (2,700° F). There are an estimated one trillion brown dwarfs in our galaxy alone, and some think they may be a source of the universe's missing mass.