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When a star is first forming, low temperature (and hence, low pressure) and high density (hence, greater gravitational attraction) both work to give gravity the advantage. Internal pressure produced by the motions of the gas atoms, pushing outward, tries to force the star to expand. The force of gravity, pulling inward, tries to make a star collapse. Remember that the essence of the life story of any star is the ongoing competition between two forces: gravity and pressure. The conditions in these cores-low temperature and high density-are just what is required to make stars. Within these clumps, there are even denser, smaller regions called cores. Within the clouds are cold, dense regions with typical masses of 50 to 500 times the mass of the Sun we give these regions the highly technical name clumps. The molecular cloud filaments can be up to 1000 light-years long. Molecular clouds have a complex filamentary structure, similar to cirrus clouds in Earth’s atmosphere, but much less dense. The masses of molecular clouds range from a thousand times the mass of the Sun to about 3 million solar masses. These clouds turn out to be the birthplaces of most stars in our Galaxy. These clouds have cold interiors with characteristic temperatures of only 10–20 K most of their gas atoms are bound into molecules.

Scowen (Arizona State University)) Molecular Clouds: Stellar NurseriesĪs 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. (credit a : modification of work by NASA, ESA, and the Hubble Heritage Team (STScI/AURA) credit b: modification of work by NASA, ESA, STScI, J. It is possible that because these EGGs are exposed to the relentless action of the radiation from nearby hot stars, some may not yet have collected enough material to form a star.

Astronomers coined the term evaporating gas globules (EGGs) for these structures, in part so that they could say we found EGGs inside the Eagle Nebula. (b) This close-up view of one of the pillars shows some very dense globules, many of which harbor embryonic stars. The tallest pillar is about 1 light-year long, and the M16 region is about 7000 light-years away from us. These columns are of higher density than the surrounding regions and have resisted evaporation by the ultraviolet radiation from a cluster of hot stars just beyond the upper-right corner of this image. (a) This Hubble Space Telescope image of the central regions of M16 (also known as the Eagle Nebula) shows huge columns of cool gas, (including molecular hydrogen, H2) and dust. 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 Figure 1).įigure 1: Pillars of Dust and Dense Globules in M16. 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.

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.

The lowest-mass stars on the main sequence (spectral type M or L) are the least luminous and the coolest.

The most massive main-sequence stars (spectral type O) are also the most luminous and have the highest surface temperature.

There are far more low-mass than high-mass stars.

Stars come with many different masses, ranging from 1/12 solar masses ( M Sun) to roughly 100–200 M Sun.

This rate of hydrogen use means that eventually the Sun (and all other stars) will run out of central fuel.

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.

The ability to generate energy by fusion defines a star.

Stable (main-sequence) stars such as our Sun maintain equilibrium by producing energy through nuclear fusion in their cores.

Describe how advancing waves of star formation cause a molecular cloud to evolveĪs we begin our exploration of how stars are formed, let’s review some basics about stars discussed in earlier chapters:.

Explain how the environment of a molecular cloud enables the formation of stars.

Recognize some of the structures seen in images of molecular clouds like the one in Orion.

Identify the sometimes-violent processes by which parts of a molecular cloud collapse to produce stars.

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