It seems like you can’t go a day without hearing news of a merger. Not corporate mergers—those are boring. Mergers between astronomical bodies are where it’s at these days. And it's not just black holes and neutron stars doing the merging. I honestly had no idea, but it seems that it is not so unusual for stars to be the product of a merger.

Of course, it's also possible that a collision between two stars would lead to a massive explosion. And, so far at least, it's been hard to answer the question of what happens when two stars collide: do they explode or go out with a whimper? The observation of a large white dwarf that seems to have been the product of two titchy white dwarfs may support the whimper side.

Stellar end-times

When stars run out of fuel, their mass determines their fate. Very large stars end spectacularly. The star collapses in on itself, followed by a violent explosion—no going silently into the night around here. As the stellar dust settles, the remains include a neutron star or a black hole. This is the ending for show-offs.

(Okay, I admit there are many other versions of the supernova story, but let’s keep it simple for now.)

Smaller stars end in a kind of sullen sulk. First, they redden and bloat as their fuel runs short. The outer mass sloughs off, leaving behind a dimly glowing corpse, otherwise known as a white dwarf. White dwarfs, being small and dim, are not the easiest of things to observe. But we know enough to know that they usually are not very extreme compared to neutron stars. They don’t have huge magnetic fields. They don’t have large stellar winds driving material away from them. In short, they are the stellar equivalent of accountants.

So, finding a white dwarf that doesn’t look like an accountant is interesting. A group of astronomers noticed an odd white dwarf, charmingly named J005311. J005311 is at the center of a nebula and seems to have a lot of gas moving away from it very fast (some 16,000km/s). This speed seemed to be impossible for the white dwarf because that matter should be pushed outward by the glow of the star, which is very dim.

Not adding up

If every photon of light from the white dwarf gave the gas as big a kick as it possibly could, the matter would still be moving too slowly to match the observations. So, something else must be going on.

To unravel the mystery, astronomers turned to other mechanisms. In neutron stars, for instance, very fast gas outflow is possible when the star is rotating quickly. The speedy rotation generates a huge magnetic field that, in turn, accelerates the charged particles in the gas. In this case, the star can be dim, but it needs a large magnetic field—about 10 to 100 times larger than is normal for a white dwarf.

The only way to produce a quickly rotating magnetic field seems to be via a merger between two white dwarfs. As the two coalesce, the conservation of angular momentum—the tendency of things that spin to keep spinning in the same plane—dictates that the star that emerges from a merger spins very fast indeed. That spinning generates flows of charged particles within the white dwarf. And the motion of the charges generates a magnetic field.

J005311 is above the Chandrasekhar limit, which means it is massive enough that it should explode in a supernova. Yet it has not done so. The alternative is that it collapses to form a neutron star, producing a small fiery display as it does so. This has not occurred either. The fact that the supernova has not occurred tells us that when white dwarfs collide, they don’t have to explode. Instead, they can blow off just enough matter to avoid exploding and gently collapse into a neutron star (gently is a relative term here; you don’t want to be orbiting as it happens).

Models indicate that the creation of a neutron star from a large white dwarf might take a few thousand years. That means, in astronomical terms, the J005311 merger basically happened yesterday, and we are still awaiting the fallout. You can bet that "awaiting" will involve a multi-instrument observation campaign.

Personally, I am still coming to grips with the idea that 10 percent of massive main sequence stars and 10 percent of white dwarfs are the product of mergers. How did I not know this before now?

Nature, 2019, DOI: 10.1038/s41586-019-1216-1 (About DOIs)