Black hole caught committing 'stellar homicide'
Astronomers watch as sun-like star shredded and swallowed over weeks
Astronomers have caught a black hole in the midst of a violent act, watching in real time as it tore a sun-like star apart with gravitational forces so strong that helium gas was flowing in a stream moving at 32 million kilometers an hour.
"We saw the messy carnage involved in this stellar homicide," said Dr. Suvi Gezari, associate research scientist with Johns Hopkins University's department of physics and astronomy. Gezari spoke to CBC Radio host Bob McDonald in an interview that airs on Quirks & Quarks on Saturday, May 5, at noon.
Dr. Suvi Gezari talks to Quirks & Quarks May 5 at noon on CBC Radio One
The destruction of the star — which was likely in the later parts of its life before the black hole cut it even shorter — was seen in June 2010 as a luminous flare of radiation by two telescopes watching at two different wavelengths: an ultraviolet NASA satellite and a ground-based optical telescope in Hawaii.
The event, similar in scope to a supernova but taking much longer, reached peak brightness a month later and slowly faded over the course of a year.
It was the most direct evidence ever gathered of a black hole destroying a star. Black holes, regions of space weighing millions of times more than the sun, are extremely difficult to find because they're only detectable if there is gas spiralling into them — something that might happen once every 100,000 years in any given galaxy.
The star's destruction was so bright it outshone the other stars in that galaxy, which is about 2.7 billion light years away.
"We were watching in real-time, but we still actually didn’t figure out what this event was until almost a year later," Gazari said.
The information the astronomers gathered allowed them to calculate that the black hole had the mass of about three million sun-like stars. By comparison, the black hole in the middle of the Milky Way is about 4 million times the mass of the sun.
The astronomers also believe the star had likely already lost its outer layers during a previous pass near the black hole.
"When the star is ripped apart by the gravitational forces of the black hole, some part of the star's remains falls into the black hole while the rest is ejected at high speeds," Gezari told NASA in a separate interview. "We are seeing the glow from the stellar gas falling into the black hole over time. We're also witnessing the spectral signature of the ejected gas, which we find to be mostly helium. It is like we are gathering evidence from a crime scene. Because there is very little hydrogen and mostly helium in the gas, we detect from the carnage that the slaughtered star had to have been the helium-rich core of a stripped star."
Gezari said the discovery should help scientists discover other, similar events more easily — perhaps by the thousands.
The video below shows a computer simulation of a star being shredded by the gravity of a massive black hole.
Credit: NASA, S. Gezari (The Johns Hopkins University), and J. Guillochon (University of California, Santa Cruz)