Sky Lights

Look sharp—there's change afoot up above

By Corey S. Powell|Friday, November 01, 2002
RELATED TAGS: STARS


While the moon moves through its phases and the planets dance across the celestial stage, the backdrop remains stubbornly unchanged. The endless sameness of the stars is comforting but, let's face it, a bit dull. Imagine how much more exciting backyard astronomy would be if each night brought a fresh, slightly different view.

The last great flash in our galaxy—a supernova—left nothing but a glowing cloud, called Cassiopeia A.
Photograph courtesy of NASA/CXC/SAO.

Actually, you don't need to imagine. Change is all around if you know where to look. Start with the bright red star Betelgeuse in the right shoulder of Orion, rising in the east a couple of hours after sunset. Compare it to Rigel, the prominent blue star marking Orion's left foot. Sometimes Betelgeuse is nearly as brilliant as its rival; most often it is fainter, as little as one-third as bright. You are witnessing the irregular heartbeat of an aging, bloated star. Over weeks to years, Betelgeuse's distended atmosphere shrinks, heats up, billows outward, cools, and falls inward again. At its peak, the star may swell to 1,500 times the diameter of the sun, large enough to fill the orbit of Jupiter.

Toward the south, in the constellation Cetus, an even more extreme stellar transformation is on display. Mira, another giant red star, varies in brightness by a factor of 600 along a 332-day rhythm. Pulsations alone cannot explain such a change. Astronomers Mark Reid and Joshua Goldston of the Harvard-Smithsonian Center for Astrophysics recently showed that Mira's dramatic dimming is probably regulated by titanium oxide, a white pigment used in sunscreens. When the star puffs outward, titanium and oxygen atoms combine to form this opaque compound. Like an enormous coat of sunblock, the titanium oxide absorbs the light of the star until the next round of stellar collapse heats and separates the atoms. Right now, Mira is so obscured that it is invisible to the naked eye. By next July, it will be among the brighter stars in a relatively barren region of sky.

If Mira is miraculous for its range, Delta Cephei is stunning for its precision. Its brightness rises and falls by about a factor of two in an unerring cycle that lasts 5 days, 8 hours, 47 minutes, and 32 seconds. Deep inside the star is an unstable shell of helium undergoing nuclear fusion. Oscillations in this shell produce the metronomelike flickerings.

The period of change in all similar variable stars, called Cepheids, is directly related to their true luminosity, a remarkable pattern that enabled astronomer Edwin Hubble to reckon the distances to other galaxies in the 1920s. You, too, can observe the ups and downs of a Cepheid, using nothing more exotic than an eyeball. Any decent star chart will guide you to the right spot (after a bit of squinting and swearing, perhaps). Then compare Delta Cephei with the surrounding stars. From night to night, you can monitor the unsteady nuclear metabolism of a star 1,000 light-years from Earth.

Not far from Delta Cephei in the northern sky sits Algol, which displays another style of inconstancy: It remains steady but then dims abruptly every 2.87 days. Again, the target is easy to spot if you have a sky map to guide you. John Goodricke, an 18th-century British amateur astronomer born deaf and mute, first deduced that Algol is regularly eclipsed by a darker companion—now known to be a cool orange star. The period of variation is itself varying, growing longer as streamers of hot gas flow between the two stars and change their orbits.

Supernovas, the greatest displays of stellar instability, are unfortunately also the rarest. The last known one in our galaxy occurred around 1680. Its remnant, a tortured nebula called Cassiopeia A, is now visible only through a large telescope. But some astronomers believe that erratic Betelgeuse is ripe to blow. At a distance of just 425 light-years, a Betelgeuse supernova would be visible all day and cast shadows at night—a dramatic retort to those who think there is nothing new over the sun.





The American Association of Variable Star Observers is an enthusiastic group of amateurs and professionals dedicated to monitoring the changeable aspects of the sky: www.aavso.org.
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