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Starry weather: partly cloudy, with a chance of flares?

Natural History, Oct, 2002 by Charles Liu

Like all stars, the Sun is a seething ball of heat, light, and magnetic energy. Some of this energy regularly erupts from the Sun's surface, forming beautiful prominences, loops, and solar flares. Particularly powerful events eject millions of tons of protons and electrons into space at millions of miles per hour. When such an outburst points toward Earth, some of this Sun-stuff arrives in our vicinity a day or so later, often causing auroras--the northern and southern lights--as the stream of solar particles crashes into Earth's upper atmosphere. Sometimes the consequences are less innocuous; the particles' electromagnetic effects, for instance, can overload power grids and damage orbiting satellites.

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Public awareness of Sun-induced phenomena has increased as our Earthbound lives have become ever more intertwined with cellular phones, satellite TV, and other space-borne technology. Scientists have even coined a term for this stuff: "space weather." But the name is a little imprecise, since we're really talking about "star weather"--the star being, of course, our Sun. Whatever it's called, it certainly doesn't seem to resemble the wind, clouds, rain, and snow that we're familiar with down here. So, do stars have weather as we do on Earth?

Let's tackle this question by thinking first about the difference between terrestrial weather and space weather. Down here, particles do affect our weather--lightning, for example. But at sea level, Earth's atmosphere has a density of about one kilogram per cubic meter, or roughly 6,000,000,000,000,000,000,000 molecules per cupful of air. Interplanetary space, on the other hand, contains only a few thousand particles per cupful. So the motion of ionized particles dominates weather in space, while the movement of heat--hot and cold air and water, flowing from place to place--drives weather on Earth. Weather patterns readily appear on any planet with a steady energy source--on Earth, that'd be sunlight--and a substantial atmosphere.

Stars have both abundant atmospheres and energy. This suggests they should have planet-like weather. Recently a team of astronomers, led by Adam Burgasser at UCLA, has provided new evidence--what appears to be Earthlike weather in the atmospheres of brown dwarfs--to support this conclusion. (See my column, "When a Star Isn't Born," September 2002, about these so-called failed stars.)

Just as the coils in a toaster glow brighter as they heat up, hotter brown dwarfs should be more luminous than cooler ones. Yet infrared measurements show a puzzling conflict between theory and observation: brighter brown dwarfs appear to be too cool and dimmer ones too hot. Some astronomers have argued that the discrepancy comes from assuming that brown dwarfs have undifferentiated atmospheres, marred only occasionally by flares and prominences. This prompted Burgasser and his colleagues to create a model that assumes the top layers of brown-dwarf atmospheres are patchy, with thick clouds--laden with droplets of liquefied metal-bearing minerals and molten iron--floating atop an otherwise clear "sky."

According to this hypothesis, the clouds obscure part of the brighter dwarfs' light, leading observers to infer that the brown dwarf is cooler than it actually is. As brown dwarfs age, they cool; the clouds on the dwarfs turn into heavy, metallic precipitation. The sky then clears up, allowing more infrared radiation to escape and making the dimmer brown dwarfs appear misleadingly warm. The researchers' new model matches the observations well, suggesting that weather is indeed at work on these almost-stars.

Although brown dwarfs don't fuse hydrogen into helium, they have a lot of other things in common with low-mass stars. So if brown dwarfs have planet-like surface weather, there's a good chance that the faintest stars do too. That's not the whole story, however. The plasma in the guts of stars carries tremendous amounts of heat generated during nuclear fusion, and it can swirl and flow as easily as heat does in Earth's atmosphere. It makes sense to speculate, then, that stars might have weather in their interior as well as near their surface.

Do we have anything to support this remarkable prediction? Yes, indeed. By combining nearly six years of Doppler imaging data from the SOHO (Solar and Heliospheric Observatory) satellite, solar astronomers have now seen tantalizing suggestions of horizontal wind patterns and cyclonic flows deep below the Sun's outer layers. Of course, there's a great deal we don't understand about this result. But with more discoveries like SOHO's, astronomers may one day be able to forecast the imminent clearing of the mysteries of stellar weather.

Charles Liu is an astrophysicist with the Hayden Planetarium. He is also affiliated with Barnard College as a research scientist in the Department of Physics and Astronomy.

Starry weather: partly cloudy, with a chance of flares?

Natural History, Oct, 2002 by Charles Liu

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