Astronomers have found a record explosion (Monthly Notices of the Royal Astronomical Society, Volume 522, Issue 3, July 2023, Pages 3992-4002, https://doi.org/10.1093/mnras/stad1000) . The total energy output was stated to be 1.5 x 10^53 ergs. (People still use ergs??) An erg is 10^-7 of a Joule, so the explosion generated 1.5 x 10^46 Joules. These numbers are sort of mind-boggling. Try thinking of tonnes of TNT. The equivalent would be about 3.4 x 10^36 tonnes, or well over 10^28 of the largest hydrogen bombs ever exploded. That is 10 with 28 zeros after it. Of course we can hardly see it. It is about eight billion light years away, which is probably just as well. It is more than ten time brighter than any supernova ever recorded and so far has been going for three years. The “fireball” is about 100 times the size of the solar system and that mass is two trillion times brighter than the sun. If you want to impress your friends, the explosion is known as AT2021lwx. Astronomers have charming names for things.
So what could have caused it? One option would be a tidal disruption event. This is essentially when a star is tidally disrupted by the black hole and the black hole disrupts the star, leading to star matter pouring into the black hole. Oddly enough, this depends on a tidal radius, which in turn depends on the density of the star, so that for a given stellar mass and radius there is a corresponding upper black hole mass for which this cannot occur. Larger is not better for this. For this to be the cause of this event, the star would have to be almost fifteen times the mass of the sun, which is somewhat unlikely because such a massive star only lives for about 15 million years. It is hard to see how such a massive star could be born there, because the black hole would consume the gas first, and it is hard to see how it could move there, so that is probably not the cause.
A better alternative is thought to be a vast cloud of gas, probably thousands of times more massive than the sun, falling into a black hole. The energy is simply gravitational potential energy being converted to heat as the gas falls towards the black hole and it estimated the temperature reached about 13,000 degrees C. The gas and dust was believed to be in a disk circulating the black hole, and something must have dislodged it and made it start to fall into the black hole. However, so far nothing has been modelled.
So, at the end of the day, we don’t know what it is.
On a much much smaller scale astronomers have noticed a optical outburst named ZTF SLRN-2020 (Nature, 617: 38 – 39) that lasted roughly ten days, and then slowly decayed over six months. The start of the burst coincided with infrared emission that lasted long after the optical emission had decayed. The optical radiation was featureless continuous emission at the red end, as well as lines corresponding to molecular absorption.
The first thought was this was a classical nova. This appears to happen when a white dwarf accretes hydrogen from a close companion star. A white dwarf is effectively a dead star, and is what is left over after nuclear fusion has stopped. They have the mass of the sun and the size of Earth, so they are dense. Hydrogen landing on it will trigger nuclear fusion. However, if this were the cause we would expect to see spectral lined from elements entrained in the gas and we don’t.
Another possibility was a so-called red nova, which is caused by two stars merging. The nature of the light is quite similar, except the power output was far too small. Further, the star’s radius did not change appreciably. After some very detailed observing, they found the source was a sun-like star and the power output was consistent with the other object that did the merging being a giant planet. So it appears the star has swallowed a planet.
How would it do that? If planets get too large for the distance between them, gravity drives them into elliptical orbits with exchanged energy, i.e. one goes closer to the star and one goes further. If the exchange of angular momentum leads to the inner one having a very high eccentricity, its perigee can have a close enough approach to the star that frictional interactions cause the orbit to decay. Once that starts there is no escape for the planet.