Stars start out as dense cores in molecular clouds. (Note that ``dense'' in this context means ``dense relative to the surrounding medium''.)
For a dense core in a molecular cloud:
The luminosity of the dense core, using the equation which relates luminosity to radius and temperature, is:
L = 120 Lsun.
Thus, the dense core is over 100 times more luminous than the star it is destined to become! However, since T = 10K, the wavelength of maximum emission is:
Lambdamax = 300,000 nanometers (far infrared - very hard to detect)
To convert a dense core into a star, we must both compress it (decreasing R by a factor of 4,000,000) and heat it (increasing T by a factor of 600).
The conversion is a 3-step process:
Dense cores of molecular clouds are ordinarily
stable. To start their collapse, we must first compress them with
a shock wave traveling through the interstellar medium. One
source of shock waves is supernovas (exploding stars).
Above is an image (taken with the Hubble Space Telescope) of a supernova shock wave, in the constellation Cygnus, slamming into a molecular cloud. The colors are computer-enhanced; the interstellar medium isn't really as gaudy as this picture. Click on the image to see an enlarged, higher-resolution version.
(Image credit: Jeff Hester [Arizona State University], and NASA)
After the initial trigger provided by the passing shock wave, the gas is in free fall. The velocity of the infalling gas becomes greater and greater, and the dense core becomes denser and denser.
What stops the infall of the gas? When the gas reaches the center, it collides with the gas falling inward from the other direction. The gas is shocked; the ordered velocity of the infalling gas is converted to random velocity. In other words, the kinetic energy of the infalling gas is converted to thermal energy. In other words, the gas goes `Splat' and heats up.
The collapsed, heated object is now called a protostar, the intermediate stage between a dense core in a molecular cloud and a star. The protostar is not in isolation; it is surrounded bya a rotating disk in the equatorial plane, and a bipolar outflow at its north and south poles.
The disk is important because it is capable of
fragmenting into planets (the planets around the Sun are thought
to have formed in this way).
The outflow is important because it sweeps away the excess gas and dust in the vicinity of the protostar.
Above are two images (taken 11 months apart, using the Hubble Space Telescope) of the disk and bipolar outflow associated with the protostar HH30. The outflow is aligned horizontally; the disk (which is divided in two by a dust lane down its middle) is aligned vertically. Click on the image to see an enlarged higher-resolution version.
(Image credit: C. Burrows [Space Telescope Science Institute], and NASA)
Note: if the protostar is rotating too rapidly, it becomes unstable, and breaks apart into two protostars orbiting each other. Thus, binary systems are thought to originate from dense cores which are rotating more rapidly.
The protostar is still collapsing inward, although more slowly now. It is becoming smaller and hotter. What stops it from collapsing until it becomes a black hole?
The center of the protostar, which is becoming hotter and denser with time, eventually reaches the point where it is so hot and so dense, hydrogen nuclei start to fuse together to form helium nuclei. At this point, the protostar has a fusion reactor at its center, and is now called a star. When four H nuclei fuse to form helium, energy is released. The energy raises the temperature and the pressure inside the star.
We have now reached the stable situation where the outward force of pressure is sufficient to balance the inward force of gravity. The star stops collapsing, and starts its sedate life as a main sequence star, which lasts as long as the hydrogen holds out.
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