Death spiral of a star mapped into the heart of a black hole

21 Mar 2017

An artist’s rendering of the tidal disruption event named ASASSN-14li. Image: NASA's Goddard Space Flight Center

NASA scientists looking into the depths of space have observed the death spiral of a star as it was dragged into a black hole, producing some amazing results.

Like a bee to nectar, a star is instantly attracted to a neighbouring black hole, albeit not willingly – the latter’s sheer gravitational pull renders the star helpless.

Some stars being sucked into black holes have been found to have some fight in them, continuing to orbit their gravitational masters for some time. Many more are simply torn apart.

One such example is ASASSN-14li. NASA’s Swift Mission has mapped the star’s gradual death spiral into a nearby black hole. It left behind a trail of stellar gas over 290m years ago.

A team of astrophysicists from MIT has now analysed the data from the distant cosmic event and has determined that the long, gassy streak was produced when a sun-like star wandered too close to a 3m-solar mass black hole, similar to the one at the centre of our own galaxy.

For a star to be torn apart like this, all that is needed is a black hole with 10,000 or more times the star’s mass, as tidal forces outstrip the star’s own gravity in a tidal disruption event.

However, there were a number of puzzling discoveries with ASASSN-14li that did not line up with similar finds in the cosmos.

Collision of the clumps

For instance, this emission appeared to be located significantly closer to the black hole than where tides would usually shatter the star.

Also, the gas emitting the light seemed to remain at steady temperatures for much longer than expected.

Now, in a paper published to The Astrophysical Journal Letters, Dheeraj Pasham and his MIT team believe the answer may lie in infalling debris that looped around the black hole and into the original stream.

“Returning clumps of debris strike the incoming stream, which results in shock waves that emit visible and ultraviolet light,” said Bradley Cenko, a member of the research team.

“As these clumps fall down to the black hole, they also modulate the X-ray emission there.”

The next stage of research will need future observations of other tidal disruption events to further clarify the origin of optical and ultraviolet light.

Colm Gorey was a senior journalist with Silicon Republic