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13.6 Young Stars

After the initial collapse of a gas cloud, a proto star forms. A proto star is a cloud of interstellar dust and gas that is dense and cool enough to begin contracting gravitationally into a star. An interstellar cloud may hover on the verge of this state for millions of years. A nearby exploding star or some other disturbance might compress a gas cloud and thus trigger the collapse, which then proceeds very rapidly. For instance, typical proto stars contract to stellar dimensions in a hundred thousand years — a wink of the cosmic eye. This explains why astronomers use the term collapse: the initial contraction is very rapid in terms of astronomical time. This process is also called gravitational free-fall, where gas accelerates towards a central location without impediment, although in practice the process is not that simple.

Since we see stars in groups and clusters rather than isolated in space, we know that the usual situation is a very large cloud of grad and dust forming dozens or hundreds or even thousands of stars. Within the large cloud, particular regions condense on different scales, a process called fragmentation. Even the collapse of a gas cloud to form a single star may not be smooth; the inner core of the cloud may collapse first, later absorbing the surrounding material. Also, under certain conditions a single rotating cloud may break up into two or more stars. Of course, once the cloud begins to reach stellar dimensions (a few astronomical units, or 10-5 pc), the atoms of gas collide frequently enough to produce substantial outward pressure and slow the collapse. At this point the object becomes a pre-main-sequence star.

Astropedia Image
The Omega Nebula. Click here for original source URL.

Astropedia Image
Hertzsprung-Russell diagram. Click here for original source URL.

What do pre-main-sequence stars look like? Where do they lie on the H-R diagram? Are they found among the many stars in space? Can we actually see stars being born? In fact, sites of active star formation are often shrouded in gas and dust. They’re opaque to visible light. The clearest view is at infrared wavelengths. For example, M17, the Omega Nebula, is obscured by a factor of a million in visible light, but only by a factor of four at the infrared wavelength of 2 microns. The pre-main-sequence star stage covers the evolution from the end of the protostar stage to the main sequence.

The Japanese theorist Chushiro Hayashi did pioneering calculations of the evolutionary tracks and appearance of stars contracting toward their main-sequence configurations. Scientists have shown that the energy during most of the pre-main-sequence period does not yet come from nuclear reactions. Instead, gravitational contraction causes a release of energy. After only a few thousand years of collapse, surface temperatures reach a few thousand Kelvin, causing visible radiation. Hayashi's work showed that convection would transport large amounts of energy from the interiors of most newly forming stars, making them very bright for short periods of time. A star like the Sun, for example, contracts in less than 1000 years from a huge cloud to a size about 20 times bigger than the Sun with a luminosity about 100 times greater than the Sun's present luminosity.

The calculations of forming stars have three important implications. First, they show that stars have complicated, if short-lived, evolutionary histories even before nuclear reactions start. Second, they show that newly forming stars have properties that place them above and to the right of the main sequence in an H-R diagram. The position of a star on the H-R diagram helps us identify it as a new star by direct observation. Third, the calculations indicate that stars spend only a small fraction of their lifetimes in the pre-main-sequence stage.

More massive stars evolve faster to the main sequence than less massive stars. For instance, a 15 solar mass star reaches the main sequence in only 100,000 years. A 5 solar mass star takes about 1 million years, while a Sun-like star takes tens of millions of years; the lowest-mass stars, around 0.1 solar mass, take over 100 million years. Stars all begin their lives as very cool objects, but with a wide range in luminosity. As in the early solar nebula, dust forms in the cooling cloud and blocks the outgoing starlight. After a few hundred thousand years the nebula will eventually dissipate, revealing the star. Before the nebula is lost, like the cocoon cast off by a butterfly, the star will be invisible in optical light. It will only be seen as an infrared star.