21.1 Star formation  (Page 18/11)

Learning objectives

By the end of this section, you will be able to:

• Identify the sometimes-violent processes by which parts of a molecular cloud collapse to produce stars
• Recognize some of the structures seen in images of molecular clouds like the one in Orion
• Explain how the environment of a molecular cloud enables the formation of stars
• Describe how advancing waves of star formation cause a molecular cloud to evolve

As we begin our exploration of how stars are formed, let’s review some basics about stars discussed in earlier chapters:

• Stable (main-sequence) stars such as our Sun maintain equilibrium by producing energy through nuclear fusion in their cores. The ability to generate energy by fusion defines a star.
• Each second in the Sun, approximately 600 million tons of hydrogen undergo fusion into helium, with about 4 million tons turning into energy in the process. This rate of hydrogen use means that eventually the Sun (and all other stars) will run out of central fuel.
• Stars come with many different masses, ranging from 1/12 solar masses ( M _Sun ) to roughly 100–200 M _Sun . There are far more low-mass than high-mass stars.
• The most massive main-sequence stars (spectral type O) are also the most luminous and have the highest surface temperature. The lowest-mass stars on the main sequence (spectral type M or L) are the least luminous and the coolest.
• A galaxy of stars such as the Milky Way contains enormous amounts of gas and dust—enough to make billions of stars like the Sun.

If we want to find stars still in the process of formation, we must look in places that have plenty of the raw material from which stars are assembled. Since stars are made of gas, we focus our attention (and our telescopes) on the dense and cold clouds of gas and dust that dot the Milky Way (see [link] and [link] ).

Pillars of dust and dense globules in m16.

Molecular clouds: stellar nurseries

As we saw in Between the Stars: Gas and Dust in Space , the most massive reservoirs of interstellar matter—and some of the most massive objects in the Milky Way Galaxy—are the giant molecular clouds    . These clouds have cold interiors with characteristic temperatures of only 10–20 K; most of their gas atoms are bound into molecules. These clouds turn out to be the birthplaces of most stars in our Galaxy.