Passage IV

NATURAL SCIENCE: This passage is adapted from the article “Not Dead Yet: A Dying Star Is Caught Flaring Briefly Back to Life” by Charles Liu (©2005 by Natural History Magazine, Inc.).

About a billion years before a sunlike star “dies,” or stops generating energy via nuclear fusion, it becomes a red giant, growing dramatically to a hundred times its original diameter. Then, as the red-giant phase ends, the star blows off its outer layers, giving rise to an expanding gas cloud called a planetary nebula. The planetary nebula, in turn, swells in size and drops in density for at most another 100,000 years, exposing the remaining stellar core at its center. That core becomes a white dwarf—the most common celestial cadaver visible in the sky. The white dwarf usually radiates its leftover heat into space for billions of years, and it slowly fades to black.

Some soon-to-be white dwarfs, however, seem to heed the counsel of poet Dylan Thomas: “Do not go gentle into that good night.” According to the theory of stellar evolution, the temperature in the stellar core can fluctuate wildly, and sometimes spikes as high as tens of millions of degrees. For a little while at least, the core may even flicker back into stellar life as a giant star, generating new energy with new. flares of nuclear fusion.

Alas,such a giant can’t last long, because the core is, in essence, running on fumes. Without a substantial fuel source to sustain fusion, a nuclear re-ignition of this kind runs out of gas within a few centuries, and the star heads back toward white dwarfhood. But during its brief return to fusion-powered life, its interaction with the surrounding cloud of gas creates a fascinating astronomical laboratory for the study of stellar and interstellar processes.

The star FG Sagittae, a highly variable star in the constellation Sagitta, seems to be a case in point. FG Sagittae lies at the heart of a planetary nebula called He 1-5. In the past thirty years the star’s temperature has dropped from more than 30,000 degrees Fahrenheit to less than 10,000 degrees, though its brightness has changed erratically from year to year. As with an old, grease-choked diesel engine struggling to start back up, the star’s efforts to restart nuclear fusion create puffs of thick smoke—carbon atoms coughed up from the fading stellar core. The smoke absorbs the star’s radiating heat and periodically obscures the visible light it emits. To see through the haze and examine the goings-on near the star’s surface, astronomers must look at its radiation in less obscured wavelengths, such as infrared light.

A research team led by Robert A. Gehrz of the University of Minnesota in Minneapolis has now done just that. Recently the team published the results of twenty years of monitoring the infrared properties of FG Sagittae with three telescopes equipped with infrared photometers—in effect, photon counters. One instrument is in Minnesota, one in Arizona, and one in Wyoming. Gehrz and his colleagues discovered that, though the star’s overall brightness and temperature have changed dramatically through the years, carbon dust from the surface of FG Sagittae has been shining more or less steadily at a temperature of about 1,200 degrees F (650 degrees Celsius). That’s roughly hot enough to melt aluminum, but substantially cooler than the core of any star undergoing active nuclear fusion. Gehrz and his colleagues conclude that, besides giving rise to clouds of obscuring gas, FG Sagittae is powering a strong stellar wind peppered with this carbon dust. They think this dust has been glowing continuously for the past decade. On the basis of the measured amount of emitted infrared radiation, Gehrz’s team estimates that the wind is carrying between 1.5 and 7.5 quadrillion tons of stellar material away from FG Sagittae each second—or about eight to forty Earth masses each year.

Sooner rather than later the current burst of new nuclear fusion will cease, and the dusty stellar wind will cease. The stellar core, no longer obscured by a thick, dusty blanket, will turn once more into a hot white dwarf. If, as theoretical models predict, the stellar renaissance of FG Sagittae lasts a few hundred years, the wind will deposit thousands of Earth-masses’ worth of carbon-rich matter into the star’s surroundings. The carbon atoms, as they cool down, could become seeds for the buildup of interstellar dust grains一which, in turn, could seed the formation of asteroids, moons, planets, and perhaps eventually even life as we know it. Maybe the astronomers of the twenty-fourth or twenty-fifth century will look toward FG Sagittae and see, in its surroundings, the potential makings of a new and distant earth.