the pressure that tends to blow the clump apart, then the cloud will contract and fall toward its center. In a rotating cloud, a disk of gas and dust the size of a solar system may form in orbit around the nascent star. Matter continues to accrete onto the still-contracting central object, now called a protostar. As the cloud and protostar contract, gravitational energy is released as heat, and the protostar glows brightly at infrared wavelengths. When the temperature at the center of the protostar rises to around 10 million degrees celsius, enough to ignite nuclear reactions, a star is born. Depending on the circumstances of their birth, stars have masses ranging from about 0.1 to 100 times the mass of our sun. Smaller masses never get hot enough at their centers to ignite nuclear reactions; larger masses blow themselves apart at formation by the outward force of their own radiation.

This theory of the formation of stars was given support in the 1980s, when the IRAS detected tens of thousands of stars in the process of formation. More specifically, the satellite detected embryonic stars enshrouded in dense cores of gas clouds, during the early phase of collapse before the nuclear reactions had begun (Plate 2.5). Our understanding of star formation was given another boost in the 1980s when telescopes operating at millimeter wavelengths made an unexpected discovery: streams of gas flowing outward in opposite directions as jets from the vicinity of embryonic stars. Theorists argue that these gaseous streams may be aligned by a planet-forming disk around the young star. These jets are observed at millimeter wavelengths and occasionally break out of the surrounding cloud to become visible at optical wavelengths (Plate 2.6).

Because the earliest stages of star formation occur within very dense clouds of gas, impenetrable to visible light, clues must be sought in the radio waves and infrared radiation that can escape the thick clouds (Plate 2.7). During the 1990s, star formation will be a major focus of study with many of the telescopes proposed in Chapter 1, including SIRTF, SOFIA, the MMA, and the infraredoptimized 8-m telescope on Mauna Kea. Each of these telescopes will make unique contributions to these studies. SIRTF will make sensitive spectroscopic measurements at wavelengths inaccessible from the ground. SOFIA will observe submillimeter wavelength radiation from a large variety of molecules and atoms to characterize the conditions of high density and temperature in protoplanetary disks. The MMA and the infrared-optimized 8-m telescope will make high-spatial-resolution studies of the disks and the outflowing jets that will clarify the dynamics of these regions (Plate 2.8).

The systematic, thorough investigation of star formation throughout the galaxy is important, because star formation and the power of hot, young stars is an important energy-generation mechanism in many galaxies. IRAS found that some galaxies emit copious amounts of infrared energy, perhaps due to bursts of star formation thousands of times more intense than anything seen in our own galaxy. We can study the basic physical processes of star formation up close in our own galaxy and apply that knowledge to more distant systems.

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