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.