All stars follow the same basic series of steps in their lives:
Gas Cloud 
Main Sequence
Red Giant (Planetary Nebula or Supernova)
Remnant. How long a star
lasts in each stage, whether a planetary nebula forms or a spectacular
supernova occurs, and what type of remnant will form depends on
the initial mass of the star.
A giant molecular cloud is a large, dense gas cloud (with dust) that is cold enough for molecules to form. Thousands of giant molecular clouds exist in the disk part of our galaxy. Each giant molecular cloud has 100,000's to a few million solar masses of material.
One nearby example is the Orion Nebula which you can see as the fuzzy patch in the sword part of the Orion constellation. It is about 1500 light years away and is 29 light years across. The nebula is lit up by the fluorescence of the hydrogen gas around a O-type star in the Trapezium cluster of four stars at the heart of the nebula. The O-type star is so hot that it produces a large amount of ultraviolet light. The ultraviolet light ionizes the surrounding hydrogen gas. When the electrons recombine with the hydrogen nuclei, they produce visible light. Several still-forming stars are seen close to the Trapezium stars. They appear as oblong blobs in the figure below with their long axis pointed toward the hot Trapezium stars. If you select the image, an expanded view of the Trapezium cluster will appear in another window. Both images are from the Hubble Space Telescope (courtesy of Space Telescope Science Institute).
Hydrogen emission nebulae are called ``H II regions'' and they mark sites of star formation because they are formed by hot, young stars. Recall from the table at the beginning of the chapter that O-type stars live just a few million years, a very short time for a star! They do not live long enough to move out from where they were formed. Behind the visible part of the Orion Nebula is a much denser region of gas and dust that is cool enough for molecules to form. Many stars are now forming inside it.
Fragments of giant molecular clouds with tens to hundreds of solar masses of material a piece will start collapsing for some reason all at about the same time. Possible trigger mechanisms could be a shock wave from the explosion of a nearby massive star at its death or from the passage of the cloud through regions of more intense gravity as found in the spiral arms of spiral galaxies. These shock waves compress the gas clouds enough for them to gravitationally collapse. Gas clouds may start to collapse without any outside force if they are cool enough and massive enough to spontaneously collapse. Whatever the reason, the result is the same: gas clumps compress to become protostars.
A protostar will reach a temperature of 2000 to 3000 K, hot enough to glow red, but the cocoon of gas and dust surrounding them blocks the visible light. The surrounding dust warms up enough to produce copious amounts of infrared and microwave energy. This longer wavelength electromagnetic radiation can pass through the dust. Infrared telescopes and radio telescopes have observed bright clumps in many dust clouds in our galaxy. A star remains in the protostar stage for only a short time, so it is hard to catch many stars in that stage of their life. The power of infrared detectors is illustrated in the images below. The part of the nebula above and to the right of the Trapezium stars is actually forming many stars. They can only be seen in the infrared image on the right side of the figure. If you select the figure, an expanded view will appear in another window. Both images are from the Hubble Space Telescope (courtesy of Space Telescope Science Institute).
Fusion starts in the core and the outward pressure from those reactions stops the core from collapsing any further. But material from the surrounding cloud continues to fall onto the protostar. Most of the energy produced by the protostar is from the gravitational collapse of the cloud material.
Young stars are social---fragmentation of the giant molecular cloud produces protostars that form at about the same time. Stars are observed to be born in clusters. Other corroborating evidence for this is that there are no isolated young stars. This observation is important because a valuable test of the stellar evolution models is the comparison of the models with star clusters. That analysis is based on the assumption that the stars in the clusters used to validate the models all formed at about the same time.
The Hubble Space Telescope has directly observed protostars in the Orion Nebula and the Eagle Nebula. The protostars it has observed have been prematurely exposed. The intense radiation from nearby hot O or B-type stars has evaporated the dust and driven away the gas around the smaller still-forming stars. In more than one case in the Orion Nebula, all of the gas has been blown away to leave just the dark dust disk with the protostar in the center. One example of a totally exposed dust disk seen almost face-on is shown in the figure above. It is the black spot to the right of the prominent cocoon nebula around the protostar at the center. The teardrop-shaped cocoon nebula around the center protostar is oriented toward the Trapezium stars to the right of the figure above. The evaporation of the dark, dense fingers of dust and gas in the Eagle Nebula was captured in the famous ``gas pillars'' picture on the right side of the figure below. Selecting the figure will bring up an expanded view of the Hubble Space Telescope image in another window (courtesy of Space Telescope Science Institute).
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last updated: 28 May 2001