A reprint from American Scientist the magazine of Sigma Xi, The Scientific Research Society This reprint is provided for personal and noncommercial use. For any other use, please send a request to Permissions, American Scientist, P.O. Box 13975, Research Triangle Park, NC, 27709, U.S.A., or by electronic mail to [email protected]. ©Sigma Xi, The Scientific Research Society and other rightsholders Protostars “Stellar embryology” takes a step forward with the first detailed look at the youngest Sun-like stars Thomas P. Greene natomists have been studying the of stellar birth is more than 400 light- in the past decade, astronomers have Aembryonic development of ani- years away. At such distances, a prena- now been able to observe these seclud- mals for centuries. Their detailed de- tal star the size of our Sun is exceed- ed stellar embryos. My colleagues and I scriptions of tissue growth may have ingly dim. have been among the privileged few to reached a high art, but a theoretical un- Despite such impediments, astro- witness some of the earliest stirrings of derstanding of how the embryo under- physicists have been able to piece to- a star’s life. goes its remarkable transformation gether the broad outlines of how a low- from a single cell to a crying infant lags mass (Sun-like) star forms. It’s a Baby Steps to Stardom far behind. Until recently, the field of surprisingly complex process. Al- The process of making a star begins in- astrophysics had been struggling with though it may involve the simplest of side enormous clouds of interstellar a nearly opposite problem. Theoretical elements and molecules, the making of gas and dust that lie primarily within models of how a star forms—its em- a star is directed by a maelstrom of the plane of the Galaxy (Figure 1). Con- bryonic development from a cold competing forces—including gravita- sisting chiefly of molecular hydrogen cloud of interstellar gas to a blazing tional collapse, magnetic fields, nuclear (H2), the largest of these giant molecular furnace of nuclear fusion—had out- processes, thermal pressures and fierce clouds may be several hundred light- paced the astronomer’s ability to ob- stellar winds—all of which wish to years across and weigh as much as serve the process. It’s as if develop- have their way with the unformed star. 100,000 solar masses. These clouds mental biologists had identified the Because the interaction of these forces have an average density of about 100 major stages in the growth of an em- is not fully understood, there is much H2 molecules per cubic centimeter, but bryo with only the crudest views of that remains mysterious about the they are far from being homogeneous. their subject. birth of a star. How exactly does a Some regions may have a density of This peculiar state of affairs in astro- growing star accrete matter from its physics is largely due to the reclusive surroundings, and how does the nature of embryonic stars. A stellar process stop? Why do stars form in the embryo grows in a cloudy womb of numbers and range of sizes that they molecular gas, which is so choked with do? And why do some stars form plan- dust that visible light has little hope of etary systems? These are fundamental passing through. A view of these mole- questions that cannot be answered cular clouds in even the largest optical without actually observing the process telescope would reveal little more than of star formation. a dark patch of sky. Observations are Fortunately, the field of stellar em- further hindered by the forbidding dis- bryology has recently turned a corner tances to the stars. The nearest region as an observational science. Although visible light is unable to penetrate the Thomas Greene is an astrophysicist at NASA’s dusty cloud that swaddles a prenatal Ames Research Center near Mountain View, star, the longer wavelengths of infrared California. He has also held positions at the radiation can easily slip through the Lockheed Martin Advanced Technology Center dust and so escape the inner confines and at the University of Hawaii Institute for of the cloud. Such radiation has been Astronomy. In addition to his work on young detected for decades, but until recently stars, he has also done research in developing infrared telescopes have lacked the sen- infrared detectors and various astronomical instruments. In his spare time he bicycles and sitivity and resolution to provide de- hikes with his wife Deb and their dog Sputnik. tailed information about the youngest Figure 1. Stellar embryos—protostars—lie Address: NASA Ames Research Center, MS prenatal stars, the protostars. With the hidden within the dust of the Rho Ophiuchi 245–6, Moffett Field, CA 94035–1000. Internet: development of giant telescopes and dark cloud, a star-forming region about 400 [email protected] extremely sensitive infrared detectors light-years away, near the common border of © 2001 Sigma Xi, The Scientific Research Society. Reproduction 316 American Scientist, Volume 89 with permission only. Contact [email protected]. Anglo-Australian Observatory/Royal Observatory, Edinburgh the constellations Ophiuchus and Scorpius. Optical-wavelength images, such as this one, cannot reveal these young stellar objects, but a new generation of large telescopes and sensitive infrared detectors has allowed astronomers to make detailed observations of these objects for the first time. Light from Rho Ophiuchi (near top)—a hot young star associated with the dark and dusty cloud—is reflected as a bluish glow. The aging red-giant star Antares (lower left) is not related to the star-forming region. © 2001 Sigma Xi, The Scientific Research Society. Reproduction 2001 July–August 317 with permission only. Contact [email protected]. a dark cloud b gravitational collapse c protostar envelope bipolar flow disk dense core 10,000 to 200,000 AU 10,000 AU time = 0 500 AU 100,000 years d T Tauri star e pre-main-sequence star f young stellar system bipolar flow planetary debris central disk star protoplanetary disk central planetary star system 100,000 to 3,000,000 to after 100 AU 3,000,000 years 100 AU 50,000,000 years 50 AU 50,000,000 years Figure 2. Early development of a young, Sun-like star can be described in a series of stages that span more than 50 million years. Star for- mation begins inside dark interstellar clouds containing high-density regions (a) that become gravitationally unstable and collapse under their own weight (b). The collapsing core forms a protostar (c), a phase of stellar evolution defined by the rapid accumulation of mass from a circumstellar disk and a surrounding envelope of gas and dust. As the dusty envelope dissipates, the object becomes visible at optical wavelengths for the first time as a T Tauri star (d). These objects can often be recognized in telescopic images by the presence of a protoplanetary disk (see Figure 5). After a few million years the dusty disk dissipates, leaving a bare pre-main-sequence star at its center (e). In some instances, a debris disk with newly formed planets may continue to orbit the star. The star continues its gravitational collapse to the point where its core temperature becomes hot enough for nuclear fusion, and the object becomes a main-sequence star (f). (AU = astronomical unit, the average distance between the Sun and the Earth.) 10,000 molecules or more per cubic about 100,000 years) is defined by the which is the product of rotational ve- centimeter, or more than 1,000 times rapid accumulation of mass from the locity and radius—of the core material the density of the most rarefied parts of surrounding envelope of gas and dust remains constant, so material rotates the cloud. A molecular cloud may have (Figure 3). It does so at the rate of a few faster as it gets closer to the protostar. many such dense cores (each of which millionths of a solar mass—equivalent Slowly moving material falls directly may become a star), but the star-forma- to one Earth-sized planet—every year. onto the protostar, but some of the gas tion process is not very efficient, as a Coinciding with the onset of accretion and dust is moving so quickly that it 100,000-solar-mass cloud never yields is a steady outflow of material in pow- travels in an orbit instead. Since all of 100,000 Sun-sized stars. In fact, the con- erful winds that emanate from the the material in the envelope surround- version efficiency of cloud mass to stel- poles of the young star. These bipolar ing the protostar rotates in the same di- lar mass probably averages less than jets are telltale signs of a protostellar rection, the matter falls into orbits of 10 percent. system, and it is ironic that protostars various sizes depending on its velocity, Giant molecular clouds are support- are often detected by the jets that shed and so forms a circumstellar disk. Most ed against their weight by thermal mass from the system, rather than the matter eventually flows onto the proto- pressure, turbulent gas motions and process of accretion (Figure 4). star through the disk, but some of it re- magnetic fields within, but at some Throughout this period, the proto- mains in orbit. As the surrounding en- point their dense cores become gravita- star progressively increases in density velope of dust disperses, the accretion tionally unstable and begin to collapse as it shrinks in size. The infalling mate- process stops, and the central globe of (Figure 2). The center of the collapsing rial, which had been rotating relatively gas is no longer considered to be a pro- core becomes a protostar, which, as the slowly around the dense core, begins to tostar; it is now a pre-main-sequence (or name suggests, is the first step toward speed up as the radius of the protostar PMS) star.