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I wonder, why stars take a really long time to become stars? Is it because it needs to gain mass? Or heat up? Something like that? And could it be sped up at all?

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2 Answers 2

The time to "produce" a star actually depends on the mass of the star. Let's star with a solar type star.

For a typical solar type star, the formation time is about 10 million years (you can see this image on Wikimedia that gives an overview of the star formation process, adapted from Philippe André's figure in Observations of protostars and protostellar stages in The cold Universe, 1994). You can distinguish different epochs in the star formation process, that are the signature of different dominent physical processes. The very first stage of star formation is a gravitational collapse that leads to the formation of the protostar itself. The timescale for this collapse is the so-called free-fall time which depends only on the density of the object. When you get a central object in a hydrostatic equilibrium, things become more subtle: the core will contract adiabatically (without heat transfer) and when a temperature of about 2000 K is reached, dihydrogene dissociated (which is a highly endothermal reaction) which lead to a second phase of collapse, leading to the formation of the protostar itself. It takes about 1000 years to get to this stage, from the hydrostatic core.

The next stage, the protostellar phase, is mostly an accretion phase. It means that the timescale for this epoch is given by an acretion time, that varies with the accreted mass (which is quite low for a solar type star). It takes about 200 000 years to accrete 90% of the final mass of the star.

Then, since the star is still contracting, the temperature at its center is increasing; when a temperature of 1 million Kelvin is reach, the protostar starts to burn its deuterium. At this stage, the Kelvin-Helmholtz mechanism allows the protostar to contract and to radiate away its gravitational energy. The meaningful timescale is then the Kelvin-Helmholtz time (which varies as the square of the mass and the inverse of the radius and the luminosity), that is much longer than the previous timescales. Temperature continues to rise, up to 10 million Kelvin, when hydrogen eventually starts to burn, which is the birth certificate of a star. It takes around 10 million years to get to this point.

But, as I said, this scenario depends on the mass of the star. It is valid for Sun-like stars, but quite not for massive stars. It is much faster for massive stars, and the star formation process is quite different. In particular, the accretion rate is much higher, the radiation pressure of the protostar is insanely higher, and their interplay is not completely understood. However, there are some theroretical works that gives an estimate of about 100 000 years to form a massive stars (see for example works from McKee and Tan).

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When the gas sphere that will become a star first condenses out of the interstellar gas cloud, the sphere is quite large in radius (compared to the Sun). It must shrink in size, squashing itself under its own gravity until, because of the pressure of the squashing, the central temperature reaches 10 million degrees Kelvin and fusion starts (it becomes a star). The shrinking is only possible since the sphere is losing energy from its surface (light is escaping), and so the material is settling down under gravity. Once the central temperature rises to 10 million Kelvin, fusion starts, energy is released by the fusion, replacing the lost energy from the surface, and the sphere stops contracting - it is now a star like the Sun. The process of losing energy, and shrinking, while the central temperature rises as the pressure builds, all takes 10 million years or more.

Why so long? Well, it's a combination of two things. First, such a large amount of gas (as is contained in the star) must lose a large amount of energy (by light lost from the surface) before it can settle down to the radius of the Sun. Second, it is losing that energy at a specific rate dictated by the way energy is transported up the surface from the interior. Both of these facts determine how long the entire shrinking process will take place.

Source: Frequently Asked Questions About Stars, Compiled by Dr. John Simonetti of the Department of Physics at Virginia Tech.

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nice answer, a suggestion - how about including some linked references. –  user8 Sep 26 '13 at 2:25

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