In the classic Sherlock Holmes story “Silver Blaze,” the famed detective solves a murder mystery by noticing something that does not happen: a watchdog's failure to bark in the middle of the night. Astronomers are now using a similar inference to solve the cosmic mystery of a black hole's birth—looking for stars that fail to explode.
Stars many times more massive than our sun often end their lives with a super-nova, a cataclysmic explosion caused by the collapse of the star's heavy core. Because they are so bright, supernovae across the universe can be observed and studied here on Earth. Modern astronomers have yet to see one in our Milky Way but have managed to witness a few dozen in nearby galaxies with known progenitor stars. Strangely, though, none of those stars was bigger than about 17 solar masses, even though much more massive stars abound and should also be dying as supernovae.
Theorists suspect black holes could explain this curious observation. When the cores of certain “red supergiant” stars collapse, instead of making supernovae, they may form black holes that simply swallow up the disintegrating star. Seen from afar, the disappearance of the star could then announce the birth of a new black hole. “We call these ‘failed supernovae,’” says Stan Woosley, an astrophysicist at the University of California, Santa Cruz, who has modeled the process. “Now you see a red supergiant. Now you don't.”
In 2008 Chris Kochanek and his colleagues at Ohio State University proposed a way to search for these elusive deaths. Unlike most supernovae surveys, which look for bright bursts of light, Kochanek would monitor about 30 nearby galaxies for curious patches of darkness where a star had suddenly disappeared. Last year, based on observations with the Large Binocular Telescope Observatory in Arizona, Kochanek and his colleagues Jill Gerke and Kris Stanek announced their discovery of one convincing failed supernova candidate, a red supergiant in the galaxy NGC 6946 that briefly flared and then seemed to wink out of existence.
Now there are two possible sightings of black hole births. In July, Thomas Reynolds, Morgan Fraser and Gerard Gilmore, all at the University of Cambridge, reported they had seen another supergiant fade to black amid a star cluster in archival Hubble Space Telescope observations of the galaxy NGC 3021. The results from both teams were published in separate papers in the Monthly Notices of the Royal Astronomical Society.
Certainly there are more prosaic explanations for both candidates: the stars could be variable and wildly fluctuating in brightness, or they might have drifted behind clouds of dust. The research teams plan more in-depth observations with space telescopes to strengthen the case for the dark births of black holes.
Among the best sights they could see would be nothing at all. “Death has a special property that other sources of stellar variability do not,” Kochanek says. “Death is forever.” If the stars reappear, Fraser says, “then they clearly haven't exploded,” and the search for failed supernovae will go on or fade away.