Young stars that jump the light fantastic.
Stellar evolution follows a balancing act: Gravity pulls a star's matter inward while hot gases inside push their way outward. Standard theory states that young stars continually radiate light. But new research and preliminary observations suggest that some budding stars don't emit much light. Instead, these youngsters may remain dim for long periods, keeping energy locked inside while pressure builds. Finally, over a period equivalent to an astronomical eyeblink, light as bright as 500 suns bursts forth from the star's surface.
MIT astrophysicist Steven W. Stahler made his illuminating finding while computer-modeling protostars, clumps of hot gas that give birth to stars. He calculated that some of these star progenitors are much smaller than researchers had theorized. As a consequence, some young, intermediate-mass stars -- in the range of 2.5 to 6 solar masses -- would have only about one-tenth the previously assumed radius before they begin burning hydrogen, he says. And therein lies the rub.
A star in this mass range with a large surface area efficiently reduces gas pressure before its hydrogen ignites, by transporting hot gas outward from its center and converting it into a steady stream of light at the surface. But smaller stars in the same mass range do not. These stars rid themselves of thermal pressure by emitting photons -- particles of light -- deep within their hot interior, Stahler says. Photons, however, take much longer than hot gas to reach the stellar surface because they must penetrate the star's opaque core. Continually absorbed and reemitted by successive stellar layers, the photons may not escape for 100,000 to 10 million years, Stahler's model suggests. But when they finally emerge, the previously dim star erupts in a brilliant glow, jumping 30 times in luminosity. Stahler says his "crude calculation" indicates the brightness jump occurs over a mere century.
Observations have yet to verify Stahler's theoretical work, but a hint of the long, dim epoch that precedes the predicted luminosity jump may lie in infrared observations of the nearest region known to harbor newly formed stars, says astronomer Mary Barsony of the University of California, Berkeley. She and her collegues used a sensitive infrared detector and the 2.1-meter telescope at Kitt Peak National Observatory near Tucson, Ariz., to record the luminosity of young stars hidden in the dark clouds of the constellation Ophiuchus, some 400 light-years from Earth.
Barsony found a puzzling deficit in the number of stars whose infrared brightness corresponds to that of young, intermediate-mass stars. She thinks
the relative scarcity of such stars in the infrared may be the telltale signature of the dim period described by Stahler. While Barsony emphasizes that her data are preliminary and restricted to a single wavelength, she notes that other astronomers observed a similar dip in Ophiuchus several years before, using a detector that was less sensitive but that recorded luminosity over many wavelenghts. Both Stahler and Barsony reported their findings last week at the annual meeting of the American Astronomical Society in Arlington, Va.
A more tantalizing, though perhaps less supportive, set of observations comes from stellar outbursts detected in the Orion constellation. Astronomers in the 1930s saw an unexplained light burst in a star known as FU Orionis, and since then researchers have observed a sudden and dramatic brightening in five more stars in other constellations. While Stahler hopes the outburst correlate with his predicted luminosity jumps, he says his theory, which currently rules out such jumps in all but 4 percent of newly formed stars, cannot explain that many evetns over such a short time. "I feel strongly that the physics [of bursting] is related, but each star would have to have multiple jumps for the [proposed] theory to match the frequency of observations," Stahler says.
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|Title Annotation:||stellar evolution|
|Date:||Jan 20, 1990|
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