In 2014, a NASA telescope observed as the infrared light emitted by a massive star in the Andromeda galaxy gradually grew brighter. The star glowed more intensely with infrared light for around three years before fading dramatically and disappearing, leaving behind a shell of dust. Although a telescope captured the phenomenon at the time, it took years for scientists to notice it.
Now, a research team led by Kishalay De, a Columbia astronomy professor, has an explanation of what they saw: It was a star collapsing and giving birth to a black hole—an event that astronomers have anticipated for decades, but have had limited convincing observational evidence for. The findings were published today in the journal Science.
The star appears to have undergone direct collapse, turning into a black hole without first exploding and becoming a supernova, long-believed to be a common way for stars to become black holes.
"This has probably been the most surprising discovery of my life," De said. "The evidence of the disappearance of the star was lying in public archival data and nobody noticed for years until we picked it out."
The star, a massive, hydrogen-depleted supergiant, named M31-2014-DS1, was in the Andromeda galaxy, which is the closest major galaxy to the Milky Way, and about 2.5 million light years from earth. When newly formed, the star was around 13 times the weight of the sun. At the time of its death, it was close to five times the mass of the sun., having shed most of its mass via powerful winds during its life.
"The dramatic and sustained fading of this star is very unusual, and suggests a supernova failed to occur, leading to the collapse of the star's core directly into a black hole," De said.
"Stars with this mass have long been assumed to always explode as supernovae," De said. "The fact that it didn't suggests that stars with the same mass may or may not successfully explode, possibly due to how gravity, gas pressure, and powerful shock waves interact in chaotic ways with each other inside the dying star."
The manner in which the star turned into a black hole suggests that at the end of its life, its inner core was not pushed out in a normal supernova explosion and instead underwent a complete inward collapse.
The process of direct collapse may have been seen once before, in around 2010, in the galaxy NGC 6946, which is about 10 times further away than this star. But its exact nature has been unclear and debated, because it was 100 times fainter and there was not as high quality data about it.
"We've known that black holes must come from stars. With these two new events, we're getting to watch it happen, and are learning a huge amount about how that process works along the way," said Morgan MacLeod, a lecturer on astronomy at Harvard, who was De's co-author on the paper.
Black holes were first theorized more than 50 years ago, and today we know of dozens in our own galaxy and hundreds of such sources detected from gravitational wave observations in the distant universe. However, scientists still do not have any clear consensus on what stars turn into black holes and how that process plays out. This discovery provides the clearest insights into this and indicates that this kind of stellar collapse may happen more often than scientists had thought.
The team discovered the star by analyzing archival data from NASA's NEOWISE mission. They used a prediction from the 1970s that theorized that when a star underwent direct collapse, it would leave behind a faint infrared glow caused by the dying gasp of the star shedding its outer layers and becoming enshrouded in dust.
They conducted the largest study of variable infrared sources ever done, tracking every star in the Milky Way and other local galaxies to look for these events, and eventually came across M31-2014-DS1. Further analysis showed that the star fit their predictions perfectly.
"Unlike finding supernovae which is easy because the supernova outshines its entire galaxy for a few weeks, finding individual stars that disappear without producing an explosion is remarkably difficult," De said.
"It comes as a shock to know that a massive star basically disappeared (and died) without an explosion and nobody noticed it for more than five years," De said. "It really impacts our understanding of the inventory of massive stellar deaths in the universe. It says that these things may be quietly happening out there and easily going unnoticed."
