For the first time ever, astronomers might have witnessed
a star actually become a black hole right before our eyes... or
telescopes. In this visible light image from the Hubble
Space Telescope, we can see a large star about 25 times the
mass of our Sun around 22 million light years away in the
galaxy NGC 6946. This was in 2007. But in a Hubble image from
2015, looking with the same filters at the same wavelengths,
the star appears to be gone. One possible explanation - the star
died and became a black hole. But it gets weirder. The most
prevalent theory for how a black hole forms is through a
supernova - if a star is big enough, at the end of its life
it will eject its outer layers at high velocity in a massive
explosion while the inner core collapses into a very tiny
space, creating a gravity well so great that light can’t
escape. Literally, a black hole. So did we see this star go
supernova? No, not really. A team of astronomers was
monitoring this star with the Large Binocular Telescope in
Arizona and saw the star get brighter in 2009, but not nearly
as bright as a supernova. They call it a failed supernova. The
star does expel its outer-most layer, but relatively gently and
not in a big explosion. Ok, so this star got brighter in
visible light in 2009, and then disappeared in visible light.
How do we know it’s not just hidden behind a cloud of dust or
something? The team checked for that; they looked at infrared
observations from the Spitzer Space Telescope, which would be
able to see the heat of dust warmed by the star. What we see
with Spitzer is there is some emission in the mid-infrared,
but it’s fading and fainter than what you’d expect to see with a
hidden star. The team thinks instead that this infrared light
is from the heat of gas falling back onto the newly formed black
hole. To help confirm that this star is now a black hole, the
team plans to analyze observations taken with the
Chandra X-Ray Observatory, which would be able to reveal X-rays
being emitted by the gas falling into the black hole. The team
also wants to continue monitoring the star’s location
in visible light with Hubble, in case the star is still there and re-appears, and
they’ll want to look at the location with the upcoming James
Webb Space Telescope to check if there’s a surviving star hidden
by cooler dust than can be observed with Spitzer. So if
this really is a black hole birth, what does that mean for
astronomy? First of all, this would show that a star doesn’t
need to go supernova to form a black hole. Astronomers actually
haven’t seen as many supernovas occur with the largest stars as
they would expect to see, and they’ve been wondering why this
is. Perhaps 10 to 30 percent of massive stars don’t go supernova
and are still able to simply form a black hole. If future
observations confirm this team’s findings, this would be the
first birth of a black hole ever witnessed and the first failed
supernova ever discovered, both of which would usher in an
exciting era of astronomy research.
www.nasa.gov/hubble @NASAHubble