Who would ever have thought we’d see a massive star become a black hole just like that? And yet that’s the best explanation for the sudden disappearance of what was once a brilliant star 22 million light years away in the galaxy NGC 6946. It took the combined power of three serious telescopes — the Large Binocular Telescope (LBT) and NASA’s Hubble and Spitzer space telescopes to look for remnants of the star’s collapse, only to find that it had disappeared from sight, leaving only a trace of heat (infrared light) from debris falling into the black hole to mark the site.
Most aging, massive stars end their lives in spectacular explosions called supernovas. They run out of burnable fuel in their cores; without the heat and outward pressure produced from burning, gravity assumes complete control and causes a catastrophic collapse of the star. As the star implodes, a rebounding shock wave undulates upward, tearing the sun asunder.
Back here on Earth, the titanic explosion looks like a tiny point of light — a “new star” — inside a distant galaxy. Supernova explosions blast millions of tons of stellar stuff into space, which expands outward at tens of thousands of miles a second to form a glowing supernova remnant. Sometimes the tiny, super-dense stellar core, called a neutron star, lies at the center of the remnant, or even a black hole.
But N6946-BH1 sidestepped all the hoopla, passing from this life to the next with a whimper, not a bang. The red supergiant star, 25 times as massive as the sun, should have exploded in a very bright supernova. Instead, it fizzled out and and left behind a black hole.
“Massive fails” like this one in a nearby galaxy could explain why astronomers rarely see supernovae from the most massive stars, said Christopher Kochanek, professor of astronomy at The Ohio State University and the Ohio Eminent Scholar in Observational Cosmology.
As many as 30 percent of such stars, it seems, may quietly collapse into black holes — no supernova required. This would help explain why astronomers don’t see supernova fireworks from the most massive stars.
Kochanek leads a team of astronomers who published their latest results in the Monthly Notices of the Royal Astronomical Society. Among the galaxies they’ve been watching is NGC 6946, a spiral galaxy 22 million light-years away nicknamed the “Fireworks Galaxy” because supernovae frequently happen there. Ten exploding stars have been discovered there since the early 20th century. The most recent, SN 2017eaw, found in May by Utah amateur Patrick Wiggins, is visible right now in amateur-sized scopes.
Starting in 2009, a star named N6946-BH1, began to brighten weakly as if it might go supernova but by 2015 appeared to have winked out of existence. After an LBT search for failed supernovas turned up the star, astronomers aimed the Hubble and Spitzer scopes to see if it was still there but merely dimmed. They also used Spitzer to search for any infrared radiation (light we feel as heat) from the spot. That would have been a sign that the star was still present, but perhaps just hidden behind a dust cloud.
All the tests came up negative. The star was gone. By a careful process of elimination, the researchers eventually concluded it must have become a black hole. During a 7-year survey, N6946-BH1 was the only likely failed supernova found. “Failed” here means that a massive star falls short of becoming a supernova, instead going directly from collapse to a black hole. During the same period, six normal supernovae occurred in other galaxies surveyed, which suggests that 10-30% of massive stars take the direct route from collapse to black hole.
While I’m a long time watcher of supernovas and love visualizing what’s happening from afar, to think that a 25-solar-mass star could simply disappear into a black hole makes us pause and consider the ultimate power of gravity.