There’s something strange on the outskirts of the Milky Way.
In a cluster of stars skimming the outer reaches of the galaxy, astronomers have identified something very small, and very dense, locked in an orbital dance with a millisecond pulsar. There are, they say, only two things the mystery object could be – a neutron star or a black hole – but either one of those things would be an exciting find.
If it’s a neutron star, it could be the heaviest of its kind we’ve ever seen. If it’s a black hole, it’s the lightest of its kind.
With a mass equivalent between 2.09 and 2.71 Suns, it sits at the lower end of a compact object desert known as the lower mass gap – a desert between 2.2 and 5 solar masses in which very few neutron stars or black holes have ever been detected.
“Either possibility for the nature of the companion is exciting,” says astrophysicist Ben Stappers of the University of Manchester. “A pulsar-black hole system will be an important target for testing theories of gravity and a heavy neutron star will provide new insights in nuclear physics at very high densities.”
Neutron stars and black holes are very closely related. They’re both super-dense objects that form from the gravitational collapse of the core of a massive star when it dies.
The difference is the mass. A neutron star can be up to around 2.3 times the mass of the Sun. It’s prevented from collapsing completely by something called degeneracy pressure, whereby multiple particles of a similar quantum nature can’t occupy the same state, including a physical space.
But the particles inside a neutron star are packed as tight as they can get; the star is analogous to one big atomic nucleus.
Add more mass, however, and even degeneracy pressure cannot prevent further collapse. So more massive cores should, theoretically at least, collapse completely into black holes.
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