Supernovas are remarkable events. To help you imagine one, make the sun 10 times more massive and considerably larger than it is in our sky today, let it age until it runs out of nuclear fuel in its core and then sit back in a recliner and watch it self-destruct in a titanic explosion. Searing heat and radiation would fry the Earth 8 minutes later (that’s how long it takes to cross 93 million miles), followed by a powerful shock wave that would obliterate the planet. Poof! No more.
Thank goodness, the sun’s too small for this scenario to ever happen, so you can safely go back to taking a nap in that chair. But in 2015, the All Sky Automated Survey for SuperNovae (ASAS-SN) discovered what was thought at the time to be the brightest supernova ever. The object, named ASASSN-15lh was so over the top, it was dubbed a superluminous supernova, the explosion of an extremely massive star at the end of its life. At peak brightness, the object was 20 times brighter than the total light output of the entire Milky Way!
But new observations from several observatories, including the Hubble Space Telescope, have now cast doubt on this classification. Instead, some astronomers propose that something much more exotic occurred — a rapidly spinning black hole ripping apart a passing star that came too close.
An international team, led by Giorgos Leloudas at the Weizmann Institute of Science, Israel, and the Dark Cosmology Center, Denmark, has now made additional observations of the distant galaxy, about 4 billion light-years from Earth, where the explosion took place and they propose an entirely different take:
This animation shows how the ASASSN-15lh event most likely happened. A sun-like star strayed too close to a rapidly spinning supermassive black hole As it grew closer, it was stretched and torn – spaghettified – by the hole’s powerful gravity. When it finally gets ripped apart close to the event horizon, the “edge” of the black hole, it created a bright flash, that could resemble a brilliant supernova.
“We observed the source for 10 months following the event and have concluded that the explanation is unlikely to lie with an extraordinary bright supernova. Our results indicate that the event was probably caused by a rapidly spinning supermassive black hole as it destroyed a low-mass star,” explains Leloudas.
Once the star strayed into the “area of influence” of the object, the hole’s extreme gravitational force ripped it apart. In the process, the star was “spaghettified”, that is, it was stretched so thin and compressed so strongly that it resembled a strand of spaghetti. For instance, if you crossed the event horizon and fell into a black hole, the gravity you’d feel at your feet would be much stronger than that at your head with the result that your body would be stretched. Soon enough, you’d snap!
Shocks in the colliding debris and other factors led to a burst of light and gave the event the appearance of a very bright supernova explosion, even though the star would not have become a supernova on its own, since like the sun, it didn’t have enough mass.
The mass of the host galaxy implies that the supermassive black hole at its center has a mass of at least 100 million times that of the Sun. A black hole of this mass would normally be unable to disrupt stars outside of its event horizon — the boundary within which nothing is able to escape its gravitational pull. However, if the black hole happens to be rapidly spinning — a so-called Kerr black hole — the situation changes and this limit no longer applies.
Astronomers may never know for sure if spinning black hole was the culprit in this mighty blast, but it’s the best explanation to fit the observations.