Once a star like the sun has consumed most of its hydrogen fuel, its outer layers swell up and cool off; the star becomes a "red giant." Our sun will evolve into a red giant star in another eight billion years or so, and when it does, its radius will expand out to beyond the current orbit of Mercury. The details of how a star becomes a red giant are still being unraveled by astronomers. One complication is that not all red giants are identical, with many variations due to mass, age, chemical composition, and other properties. Understanding the nature of our sun, and thus its past and probable future, is important to our understanding its current behavior. One of the key unsolved problems is how the outer atmosphere of a red giant is heated: perhaps by magnetic processes, for example, as is commonly suspected, or perhaps including some component of heating via collisions of atoms set into motion by acoustic sound waves in the atmosphere.
Our Milky Way galaxy is a flattened, spiral disk of stars, gas and dust, surrounded by a large, spherical, diffuse halo of material. That halo also contains red giant stars -- and these red giants are different from
those in the disk, although no one knows why. SAO astronomers Andrea Dupree and Timothy Li, together with a colleague, have been studying the nature of red giant stars in the halo, and published their new results using the Hubble Space Telescope in last month's Astronomical Journal. By measuring with high precision the light emitted by ionized magnesium atoms in the outer atmosphere of thirteen halo red giants, and then by theoretically analyzing that light, the scientists were able to conclude that these halo red giant stars had fast winds blowing from their surfaces, and furthermore that both magnetic and acoustic effects were contributing at significant levels to the heating. They note that these halo red giant stars appear to have a more noticeable contribution from acoustic heating than does our sun, perhaps because they are among the oldest stars known in the galaxy and a star's magnetic effects decline in relative importance with age. The results are part of ongoing studies to understand how sun-like stars will evolve, and to explain the differences among stars in different locations in the galaxy.
SAO and Sputnik
Fifty years ago tomorrow, on October 4, 1957, Spunik I was launched to the surprise and amazement of the world. No one in the US had been quite prepared for this dramatic unveiling of the Soviet space program. SAO and its Director, Fred Whipple, were ready, however. As part of his ongoing research on meteors, Fred had proposed establishing an optical satellite tracking network, and meanwhile had assembled a network of volunteers armed with small telescopes and stopwatches ("Moonwatch"). When Spunik I was launched, SAO was the only US group able to determine its orbit, and did so after only four days. Soon afterwards, SAO's official Satellite Tracking Network was up and running. It provided information about satellites to NASA into the 1970s, while it simultaneously helped to determine the shape of the earth and the density of its atmosphere.