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"Stellar embryology" takes a step forward with the first detailed look at the youngest Sun-like stars

Thomas Greene

Anatomists have been studying the embryonic development of animals for centuries. Their detailed descriptions of tissue growth may have reached a high art, but a theoretical understanding of how the embryo undergoes its remarkable transformation from a single cell to a crying infant lags far behind. Until recently, the field of astrophysics had been struggling with a nearly opposite problem. Theoretical models of how a star forms—its embryonic development from a cold cloud of interstellar gas to a blazing furnace of nuclear fusion—had outpaced the astronomer's ability to observe the process. It's as if developmental biologists had identified the major stages in the growth of an embryo with only the crudest views of their subject.

This peculiar state of affairs in astrophysics is largely due to the reclusive nature of embryonic stars. A stellar embryo grows in a cloudy womb of molecular gas, which is so choked with dust that visible light has little hope of passing through. A view of these molecular clouds in even the largest optical telescope would reveal little more than a dark patch of sky. Observations are further hindered by the forbidding distances to the stars. The nearest region of stellar birth is more than 400 light-years away. At such distances, a prenatal star the size of our Sun is exceedingly dim.

Despite such impediments, astrophysicists have been able to piece together the broad outlines of how a low-mass (Sun-like) star forms. It's a surprisingly complex process. Although it may involve the simplest of elements and molecules, the making of a star is directed by a maelstrom of competing forces—including gravitational collapse, magnetic fields, nuclear processes, thermal pressures and fierce stellar winds—all of which wish to have their way with the unformed star. Because the interaction of these forces is not fully understood, there is much that remains mysterious about the birth of a star. How exactly does a growing star accrete matter from its surroundings, and how does the process stop? Why do stars form in the numbers and range of sizes that they do? And why do some stars form planetary systems? These are fundamental questions that cannot be answered without actually observing the process of star formation.

Fortunately, the field of stellar embryology has recently turned a corner as an observational science. Although visible light is unable to penetrate the dusty cloud that swaddles a prenatal star, the longer wavelengths of infrared radiation can easily slip through the dust and so escape the inner confines of the cloud. Such radiation has been detected for decades, but until recently infrared telescopes have lacked the sensitivity and resolution to provide detailed information about the youngest prenatal stars, the protostars. With the development of giant telescopes and extremely sensitive infrared detectors in the past decade, astronomers have now been able to observe these secluded stellar embryos. My colleagues and I have been among the privileged few to witness some of the earliest stirrings of a star's life.

Figure 1. Stellar embryos . . .Click to Enlarge Image

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