We know our sun will eventually end up as a white dwarf star and, in time, it will become a black dwarf (a long, long, long time…so long none exist yet in our universe).
But what about neutron stars? I have never seen anything that suggests what happens to them in the far distant future. Particularly, we see some neutron stars eject jets of matter from their poles (i.e. pulsars). Surely there is an endgame to that. As massive as they are they can’t spout matter forever.
Yeah, but that isn’t an eventual state. One would expect a spinning neutron star to eventually stop, and for most of the internal processes to cease coupling energy to the outside world (probably mostly as a result of it no longer spinning.) But what happens eventually?
It will still radiate energy as a black body, which means it will reach eventually equilibrium with the CMB, which is likely to be shifted to something silly as the universe ages. But one wonders what the end game is. Does electron capture eventually render it a pure mass of neutrons? Is there an intermediate state where quantum uncertainty/tunnelling leaves it as a sort of psuedo-ground state of different sorts of matter in shells?
Neutron stars can absorb matter or combine with other neutron stars and become black holes, but one assumes that isn’t expected to be the long term fate of every one of them.
A neutron star is a stable entity. The gravitational field is so immense that the electrons of the atoms are squeezed into their nuclei to form an incredibly dense sphere that is, in effect, a giant nucleus in and of itself. Virtually nothing can penetrate it much less break it apart. Being a solid neutron core of immense density, there is no internal instability at all. Unless it is swallowed up by a wandering Black Hole, it isn’t going to change at all. It will continue to spin because the void of space is a vacuum, and there is no friction to stop the spinning. It’s magnetic field will remain steady as it spins at a rate that may be 600 times a second. If it’s magnetic poles are disassociated from its rotational axis, it will be the kind of neutron star known as a pulsar. As the old song goes, “That’s all there is.”
But space isn’t 100% empty. There’s meteors, dust, gasses, and gravitational interactions. Earth’s rotation has slowed down by a half hour in the last 70 million years, and six hours since 1.4 billion years ago. That’s almost entirely due to the moon’s gravitational forces, but even without the moon, on an infinite time scale those little things matter.
I know that protons are thought to eventually decay (with a half life > 10^{30} years). I expect neutrons will too. I would expect that the same Hawking radiation that allows black holes to disintegrate would also affect neutron stars.
Kurzgesagt has a video on neutron stars too but I can’t recall if they talk about their ultimate fate. They’re usually very good about that sort of thing, I just can’t re-watch it right now.
Pulsars (mostly) don’t spew matter. They mostly just spew light. There probably is some incidental matter that gets caught up in that, but as they cool and the radiation decreases, and matter-splash would also decrease. And even at its peak, it’s not much.
A neutron star in a binary or other multiple star system might end up with matter accreting onto it from its partner(s), and that can lead to dramatic results like surface fusion or even collapse into a black hole. But isolated neutron stars wouldn’t have any source of accretion worth mentioning.
I was trying to find whether neutron stars “evaporate” like black holes and encountered some randos on other message boards saying that Hawking radiation require an event horizon. So I’m not sure.
They certainly cool. Primarily with neutrinos until much later, when photons dominate. But I haven’t seen anything on what happens if you cool off a neutron star that was barely big enough to become one in the first place.
My understanding of the Universe is that it eventually comprises only of black holes that then will radiate away by Hawking radiation slowly until the end of entropy.
This implies that all neutron stars will end by being consumed by black holes.
Over a really long timescale, any given neutron star will gain mass (either by impacts of other objects or accretion from the surrounding not-quite-vacuum) until it becomes massive enough to collapse into a black hole.
IANAP, but according to Stephen Hawking’s own conceptual explanation of the nature of what is now called Hawking radiation, yes, it absolutely requires the event horizon of a black hole.
As I recall from what was even originally only a simplified explanatory concept in Hawking’s Cambridge Lectures, it has to do with quantum fluctuations in a vacuum which can be thought of as creating virtual particle anti-particle pairs with net zero energy that almost immediately annihilate each other. Since one of the paradoxical properties of an event horizon is that any particle that passes beyond it is cut off forever from all interactions with the universe, if the black hole absorbs one of the virtual particles it allows the other one to become a real particle with positive energy and potentially escape into space as a real particle, perhaps a photon. This stream of real particles is Hawking radiation, and since the black hole has gained negative energy, by energy-mass equivalence it loses a tiny bit of mass. No doubt there are far more accurate characterizations of this phenomenon.
