Caught on camera: the moment a star dies
Astronomers witness the birth of an “ultra-stripped” supernova for the first time. Andrew Masterson reports.
The violent death of a distant star, captured by observatories around the world, marks what is likely the first ever recorded birth of a compact neutron star binary system, researchers say.
Writing in the journal Science, a team led by astronomer Kishalay De from the California Institute of Technology, US, report the advent and rapid decline of an exploding star, known as a supernova, dubbed iPTF 14gqr, located on the outskirts of a spiral galaxy some 920 million light years from Earth.
The iPTF 14gqr event was unusual for several reasons, leading De and colleagues to conclude that it represents the final act in the creation of an “ultra-stripped supernova” – the first time such a phenomenon, first theorised a few years ago, has ever been observationally confirmed.
The sudden flash that marked the onset of the star’s titanic explosion was first detected by the Palomar Observatory in California. As the rotation of Earth took iPTF 14gqr out of the facility’s line of sight requests were sent to other observatories around the world to keep up the vision.
It was a timely call-out, because even in terms of supernovae, which are by nature short-lived, it turned out to be a blink-and-you’ll-miss-it spectacle.
A supernova explosion occurs when the core of a massive star, having run out of fuel, collapses in on itself. It then rebounds in a colossal explosion that strips off and blasts away all its outer layers, leaving only a small but incredibly dense neutron star.
In normal circumstances, the amount of material ejected in the explosion is equivalent to several times the mass of the sun. When iPTF 14gqr detonated, however, it blasted off matter comprising only one-fifth of a single solar mass – a comparatively tiny amount that intrigued the researchers.
“We saw this massive star's core collapse, but we saw remarkably little mass ejected,” says co-author Mansi Kasliwal.
“We call this an ultra-stripped envelope supernova and it has long been predicted that they exist. This is the first time we have convincingly seen core collapse of a massive star that is so devoid of matter.”
The observation raised important questions. It’s accepted that collapsing stars can’t explode unless they have a mass at least eight times that of the sun, so this one must have done so. The amount ejected, however, was much, much less – so where did the rest go?
The researchers concluded that it must have been propelled elsewhere. In short, they realised that iPTF 14gqr must have been subject to the gravitational pull of another, invisible object – a white dwarf, perhaps, a neutron star or a black hole.
De and colleagues at length concluded that the hidden actor in the drama was indeed a neutron star – and that it and the newly formed one are locked in a close cosmological tango.
It is a dance that is destined – one day – to come to an end. Eventually, the two stars will be drawn by their gravitational fields closer and closer together until they collide – producing gravitational waves in the process.