Astronomers have observed for the first time the thunderclap of x-rays that announces a star has exploded into a supernova. Researchers monitoring spiral galaxy NGC 2770, approximately 88 million light-years away, observed a brief but intense flash of x-rays in early January, followed by a prolonged afterglow of visible and ultraviolet light—the hallmark of a supernova.

Although the x-ray outburst lasted only seven minutes, it flashed 100 billion times brighter than the sun in that time. Based on that brightness and the duration of the flash, researchers conclude that the star (SN 2008D) was approximately 20 times the size of the sun and was blown apart by a shock wave expanding outward at 70 percent the speed of light.

Writing in Nature, the group says the discovery offers the first direct evidence for astrophysical models of supernova shock waves that date to the 1970s.

Decades of research indicate that when a star heftier than eight suns burns up its nuclear fuel, it will begin to collapse under its own gravity, starting at the core. The implosion sends a shock wave outward through the star and blows it apart, releasing a flash of x-rays into space [see animation below]. Or that was the idea, anyway. Researchers had never confirmed the basic model, because they always spotted supernovae by their visible afterglow, which lingers after the explosion for several weeks.

Astronomer Alicia Soderberg, a postdoctoral fellow at Princeton University and the first author of the report, was using the Burst Alert Telescope, an instrument on NASA's orbiting Swift observatory, on January 9 to study a supernova in NGC 2770 that was then two weeks in progress (but still 88 million years old, given the transit time of light). In a stroke of luck, the same galaxy suddenly flared with x-rays. "The probability of that happening is about one in 10,000," she says. "It was really exciting. We caught the whole thing on tape, basically."

To confirm that it was really a supernova, she and her co-workers followed up two days later with the eight-meter (26-foot) Gemini North Telescope in Mauna Kea, Hawaii. The supernova glowed visibly for about 20 days, and the pattern of light indicated that the supernova was a type called IIbc, born from a star rich in helium.

Prior to the discovery, the closest astronomers had come to witnessing a supernova's x-ray outburst was in 1987, when researchers saw strongly ionized gas around a supernova in the Large Magellanic Cloud, which implied that a powerful blast of x-rays had occurred.

The result confirms that researchers understand the basics of exploding stars, but "we're still unsure about the details," says astrophysicist Roger Chevalier of the University of Virginia in Charlottesville, who wrote an editorial accompanying the Nature report.

"You need to have something to drive the explosion. It remains controversial exactly what that is," he says. One possibility is that a burst of neutrinos, ghostly particles produced constantly in nuclear reactions, heats the material around the core and causes it to expand.

Soderberg notes that future experiments may provide clues by monitoring supernovae for neutrinos as well as ripples in spacetime known as gravitational waves. The new findings show that by pinpointing supernovae as they happen, x-ray telescopes could help confirm that a burst of neutrinos and a supernovae match up.

"We can detect supernovae from now on using an entirely different method," she says.

Animation of Supernova:

Credit: Dana Berry/NASA