December 2, 2015
Astrophysicists have observed an extremely rare event of a supermassive black hole sucking in a Sun-sized star, taking it apart, keeping most of its material and ejecting the rest via jet of particles.
Black holes are objects so dense, nothing that comes near escapes their gravitational pull, including light. That’s why they are called black - we can’t see them because not even light can escape from them. Supermassive black holes, presumed to be at the centre of most galaxies, are even bigger and the area around them is densely populated with stars and filled with gas which creates an accretion disc as it spirals toward the black hole.
Take the centre of the Milky Way Galaxy for example - we cannot directly observe the supermassive black hole residing there, because it is obscured by all the radiation coming from the heated gas and the sheer number of stars around it.
It is thought that the only time material can escape the gravitational pull of a black hole is when too much of it crosses the event horizon - the line after which there is no going back. Excess material, be it a star that’s been sucked in or gas from the above mentioned accretion disc, that still hasn’t crossed the event horizon is ejected into the space in form of jets made of magnetised elementary particles, reaching speeds near the speed of light.
The team of international scientists led by Sjoert van Velzen, at John Hopkins University, Baltimore, USA, took an opportunity and observed as a Sun-sized star gets pulled into the black hole, taken apart and excess material jettisoned back out.
“These events are extremely rare,” van Velzen said. “It’s the first time we see everything from the stellar destruction followed by the launch of a conical outflow, also called a jet, and we watched it unfold over several months. Previous efforts to find evidence for these jets, including my own, were late to the game.”
The event was first observed by another team, at the Ohio State University, Columbus, USA and posted it on social media. Soon after hearing the news, van Velzen contacted his colleague Rob Fender at the University of Oxford, Oxford, UK, and soon they were observing the event unfold in front of their eyes, using both ground and space telescopes at different wavelength to get as complete picture as possible.
Compared to other galaxies that were observed in hopes of catching this kind of event and the jet of particles that is its best giveaway, this galaxy was relatively near - about 300 million light-years away. This enabled the team to confirm the increase in the galaxy’s luminosity was really true to a star being sucked into the black hole, and not by an accretion disc.
“The destruction of a star by a black hole is beautifully complicated, and far from understood,” van Velzen said. “From our observations, we learn the streams of stellar debris can organize and make a jet rather quickly, which is valuable input for constructing a complete theory of these events.”