Astronomers have caught a galactic version of Jack the Ripper tearing its victim to shreds in a horrific case of heavenly homicide. The cosmic criminal is a supermassive black hole that captured an unsuspecting star which wandered too close to its powerful pull.
As it pulled the star to pieces, the black hole’s violent attack was witnessed by space scientists using a variety of telescopes on the ground and in space.
It allowed them, for the first time, to identify the innocent victim that was destroyed by its encounter with the invisible monster. They observed that it was a star rich in helium gas that had become a red giant and so was already near the end of its natural life.
The international team of astronomers, led by The Johns Hopkins University in Baltimore and including researchers from the Universities of Edinburgh, Durham and Belfast, were keeping watch on the centre of a galaxy 2.7 billion light years away. They suspected that, just like other giant galaxies, it hid a supermassive black hole.
The team was looking for a bright flare in ultraviolet light from the galaxy’s nucleus where the black hole might be sleeping. They spotted one in June 2010. Astronomers continued to monitor the flare as it reached peak brightness a month later and slowly faded during the next 12 months.
The brightening event was similar to the explosive energy unleashed by a supernova, but the rise to the peak was much slower, taking nearly a month and a half.
Professor Andy Lawrence, of the University of Edinburgh, said: “As the object faded, it stayed hot, so we knew it wasn’t a supernova. They cool down. The ultra-fast gas velocity is something we also see in active galactic nuclei, but seeing only helium was like nothing i’ve ever seen in an active nucleus.”
To completely rule out the possibility of an active nucleus flaring up in the galaxy, the team used NASA’s Chandra X-ray Observatory to study the hot gas. Chandra showed that the characteristics of the gas didn’t match those from an active galactic nucleus.
The astronomers were led by Suvi Gezari of the Johns Hopkins University in Baltimore, Maryland. She said: “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.
“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.”
The observations suggested that it was not the first time the unlucky star had had a brush with the powerful black hole. The team believes the star’s hydrogen-filled envelope surrounding its core was stripped away by the same black hole in a previous encounter.
Astronomers predict that similarly stripped stars circle the supermassive black hole that lies at the centre of our own Milky Way galaxy. They believe such close encounters in a galaxy are rare, occurring roughly every 100,000 years.