Most Massive Neutron Star Ever Detected

I worked on pulsars in grad school, and I always groused that I had to deal with ultrarelativistic plasma physics and pair-production in curved spacetime. There's a reason it's hard to come up with solid answers for these things.

You seem to be familiar with these things, so I hope you don't mind if I ask a tangential question: if dark matter is WIMPs, then there's a change that under sufficiently extreme conditions the WIMPs produce interactions with detectable outcomes. Maybe neutron stars offer just such conditions. Is anyone looking at neutron stars in search for evidence for dark matter?

>I struggle to visualize that...

Scales are simultaneously intuitive and yet mind bending. Never ceases to amaze me either.

How does this work? Is it the centrifugal force?

If neutron stars can spin that fast, it's good that they don't fling fragments of themselves. Although, if they did, those fragments would likely turn to dust outside the star's gravity.

I'm curious as well. My guess is it would expand because there's not enough mass left to overcome the strong nuclear force.

With the confining pressure of gravity gone, it would instantly and violently expand. The intense 10^32 pascals of degeneracy pressure drop quickly with expansion, but even just assuming it can expand 5 ml at that pressure, the energy released would be 5e26 joules. That's about 1000 times the asteroid that made the dinosaurs extinct. It would certainly imply the end of all multicellular life.

If somehow you could keep it confined, it would drop through whatever it was put on, through the entire Earth, and pop up at the other side. I suppose it would keep oscillating and eventually settle in the core.

Randall Munroe (author of xkcd) addresses this question in his excellent book "What If?", covering what would actually happen (basically a nuclear explosion), what would happen if it was somehow kept magically at the same density without exploding (it will end up oscillating around then settling at the Earth's core), and finally and most interestingly what it would be like if you could keep it near the surface and attempted to approach it.

we can use it as a gravity gun

As I understand it, there is some speculation by observers at LIGO/VIRGO that two neutron stars colliding with each other may eject neutron-degenerate matter into space before they coalesce into a single new compact object.

Not sure what that ejecta would look like...but if I had to guess, it would be release a shitload of energy as it came out from under compression, and would eventually condense into normal matter, much of it comprised of the sort of heavy elements not easily created via stellar nucleosynthesis.

Imagine a cloud of incandescent radioactive gold dust weighing as much as multiple earths, and you've got the idea.

> This is close to the limit of how much mass a compact object can contain before it crushes itself into a black hole.

Obviously it's time for bed because reading this makes me want to toss a quarter in it to see what happens.

Imagine a far future game where bored teenagers grab the family spaceship, find a neutron star just below the blackhole limit, and throw things into it until it makes the switch. How fun would that be?!

They actually measured a mass of 2.14

No information leaks out a black hole. It is opaque to the normal Universe.

Nothing comes out of a black hole, so we can't get observations from the inside.

(There are nit-picky theoretical ways in which a black hole can evaporate, keyword "Hawking Radiation", but as far as I know that hasn't been observed, because it's too weak and too far away. But even then we learn about its surface/event horizon, not its inside).

We know that current theory predicts singularity, which is most likely incorrect.

Also, because we understand neutron degeneracy pressure, but don't yet know what (if any) quantum effect would prevent an actual singularity from being reached once the upper limit of a neutron star was breached. There is speculation that quark matter and quark stars may exist but no evidence of it yet.

That's not quite right, we do know what black holes are made up of: the remains of a star. Well, if they are reasonably small they are made out of the remains of a star, if they are too big they can contain the remains of lots of stars and also gas, dust and maybe bits of planets, but exactly how we get the biggest black holes isn't clear.

The issue is rather that we don't know what all that star stuff becomes once it goes into the black hole, though there are theories. The problem is that the defining feature of a black hole, its even horizon, by necessity means there is no way of knowing what theory is correct since nothing happening inside is supposed to matter to us.

So before we can make any real progress on the interior of black holes we thus need to figure out exactly in what sense the event horizon leaks information, if it does.

In contrast, while we don't really know what happens to matter inside a neutron star, its surface remains visible to us and so we have been able to discard many different theories about what goes inside it. There is still debate about the precise equation of state, which correspond to different internal structure the various models represent and so different ideas of how a neutron star is made up.

Honestly, I don't know how much difference there are between the various models, but pretty crazy sounding ideas are still popping up, so there must be at least some space left.

Its mass would not change, so it should not affect other objects more than it does at the moment (I think).

> Black holes have the same gravitational effects as any other equal mass in their place. They will draw objects nearby towards them, just as any other planetary body does, except at very close distances to the black hole. If, for example, the Sun were replaced by a black hole of equal mass, the orbits of the planets would be essentially unaffected. A black hole can act like a "cosmic vacuum cleaner" and pull a substantial inflow of matter, but only if the star from which it formed was already having a similar effect on surrounding matter.

https://en.wikipedia.org/wiki/List_of_common_misconceptions#...

The mass of the neutron star is _not_ the same as the entire star before it. The neutron star is (mostly) core material from its parent star, the rest of the matter was likely flung out upon that stars death. Gravity is not the only thing at play, strong nuclear forces between atoms prevent the star from collapsing under its own gravity, as well as the angular momentum of the stars spin. The neutron star is one giant atomic nuclei in the sense that all of its atoms are incredibly densely packed together. Imagine a styrofoam ball that you put under immense pressure from all angles (say by putting it deep under water). The air/free space in the styrofoam is squeezed out, and this in a sense is the same thing that happens to the plasma inside the parent star's core as it collapses. All of the atoms are squeezed together, and a balancing act is created whereby the incredible density of the core is trying to pull the matter inwards (into a black hole), while the strong nuclear force/momentum are pushing back to prevent the star from collapsing further.

It's about nuclear density. Neutron stars are held up by neutron degeneracy pressure.

Another fun fact is that the magnetic field energy density alone near the surface of a pulsar (1e12 gauss and up) is about that of lead. (~40 g/cm^3).

The main reason is that while black holes are more "extreme" objects, they are actually physically very simple. The matter collapses to a singularity (as far as we know), and so the physics is just described by the Einstein field equations with a correction for quantum fluctuations near the event horizon. Remarkably, the entire state of the black hole can be described by only the mass, charge, and angular momentum of the black hole.

By contrast, although neutron stars are less "extreme", they end up in a regime where the physics becomes much more difficult. The gravity is strong enough that you still need general relativity, but because the object is extended rather than a singularity, there's also a bunch of "stuff" which needs to be described as well. This means that you need to add in quantum electrodynamics and quantum chromodynamics. Furthermore, it's thought that at these densities you end up with some sort of a superfluid which means you need fluid dynamics and statistical physics to model its behavior, all of which is extremely difficult.

That said, there is another sense in which we could say that black holes are actually more mysterious than neutron stars. Although the physics of neutron stars is very complicated, we at least know what all the fundamental equations are. In the case of black holes if we want to understand physics near the singularity we would need a theory of quantum gravity which we currently don't have.