Astronomy 162: Professor Barbara Ryden
To convert a dense core into a star, we must both compress it (decreasing R by a factor of 4,400,000) and heat it (increasing T by a factor of 580).
Dense interstellar 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)
The protostar is not in isolation; it is surrounded by a rotating accretion 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 a protostar named 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)
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.
Protostars less massive than 0.08 Msun never become hot enough for fusion to start; they become brown dwarfs.
Protostars more massive than 200 Msun literally blow themselves apart before they can become stars. Such massive protostars are very luminous. The bright light they emit pushes on the outer layers of the protostar (just as sunlight pushes on a comet's tail). The outward force due to starlight is greater than the inward force of gravity, and the star is blown away, starting with the outer layer and working inward.
Updated: 2003 Jan 22
Copyright © 2003, Barbara Ryden