
How big is a neutron star? - raattgift
https://www.symmetrymagazine.org/article/how-big-is-a-neutron-star
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JackFr
Great hard SF novel about creatures living on the surface of a neutron star:
[https://en.wikipedia.org/wiki/Dragon%27s_Egg](https://en.wikipedia.org/wiki/Dragon%27s_Egg)

Author was a physicist, engineer and gravity expert.

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jessriedel
I just recently read this and enjoyed it. One criticism is that although the
_physics_ is "hard" (i.e., relatively accurate), the sociology and psychology
of the creatures is silly. Despite dramatically different biology, the
creatures act extremely similar to humans with only a few bizarre tweaks
(e.g., widespread polyamory and orgies, which play essentially zero role in
plot development).

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watersb
Yes, Forward does not have characters, in terms of agents with motivations and
opinions. They are engineering constraints.

If you can treat these books as something like Socratic dialogue, or Medieval
theatre, then you can get a feel for Forward's facility with gravitational
dynamics. Quadrupole radiation. Relativistic mechanics.

~~~
jessriedel
This is a different criticism than the one I was making, although it's also
basically valid.

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a_imho
The study found the radius of a 1.4-solar-mass neutron star is between 10.4
and 11.9 kilometers, a substantial improvement over other estimates.

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leeoniya
i got curious why neutron stars don't collapse to black holes:

[https://www.forbes.com/sites/startswithabang/2018/06/13/the-...](https://www.forbes.com/sites/startswithabang/2018/06/13/the-
surprising-reason-why-neutron-stars-dont-all-collapse-to-form-black-holes/)

although it did not explain how black holes form then. do they need a
sufficient density of bosons? do those only form in supernovae?

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pdonis
_> it did not explain how black holes form then. do they need a sufficient
density of bosons?_

No. Even degeneracy pressure from the Pauli exclusion principle has a limit;
it can't hold an object up against gravity for an arbitrarily large mass. So
neutron stars, like white dwarfs, have a maximum mass limit; any object over
that mass cannot be a neutron star because neutron degeneracy pressure can't
hold it up against its own gravity. (The article briefly mentions this; the
limit is the Tolman-Oppenheimer-Volkoff limit.)

We don't know exactly what that maximum mass limit is for neutron stars,
because we don't know their equation of state with sufficient accuracy
(whereas we know the white dwarf equation of state accurately enough to know
that the maximum mass limit for them is 1.4 times the mass of the Sun), but we
know it's somewhere betweena about 2 and 3 times the mass of the Sun. Objects
more massive than that can only be black holes.

As far as how black holes form, they form when a sufficiently massive object
collapses under its own gravity. Usually this happens when a star runs out of
nuclear fuel. (More precisely, this is how black holes of roughly stellar mass
form. For supermassive holes, like the one at the center of our galaxy, things
are more complicated, because they probably don't form in one single process,
they form over a long period of time as a hole that started out a lot smaller
swallows other massive objects.) If the collapsing object is over the maximum
mass limit for a neutron star, degeneracy pressure from the Pauli exclusion
principle cannot stop the collapse.

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danenania
Did all supermassive black holes begin as stellar mass black holes? Is there
some other way for them to form?

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cipherzero
My understanding is that we're not totally certain if all black holes form
from stellar mass black holes.

There is a theory that black holes can form from direct collapse. In this
case, as I understand it, it can skip the star phase and just collapse
straight into a black hole.... but I really am not an expert.

You can find more here:
[https://en.wikipedia.org/wiki/Direct_collapse_black_hole](https://en.wikipedia.org/wiki/Direct_collapse_black_hole)

And I HIGHLY recommend the PBS NOVA episode "Black Hole Apocalypse" (Jan 10,
2018, Season 45 Episode 1). It was on Netflix for a while (and may still be.)

Edit: here is the full episode on PBS (thank you NOVA!!!)
[https://www.pbs.org/video/black-hole-apocalypse-
yj34qi/](https://www.pbs.org/video/black-hole-apocalypse-yj34qi/)

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raxxorrax
The sun has a radius of about 700,000 km, but the question is interesting and
there should be a fairly small upper border, since too much mass would end in
a black hole.

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jacquesm
Question: if the sun has a 'fixed' radius but it is radiating constantly
doesn't that mean that at some point that radius will change due to the
changing radiation pressure from within the sun itself? If so how fast does
that change, and how large was the sun say a billion years ago?

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spaetzleesser
The sun will grow to a red giant in a billion years or so. The earth will
possibly be swallowed by it but at least in a few hundred million years earth
will be so hot that oceans boil off and life will be impossible on earth. I
hope I got the numbers right...

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bluedevil2k
Boil off where? What would happen to the water vapor if it floats off the
Earth into the vacuum of space? Pulled in by the Sun's gravity and forever
lost?

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ardit33
Think Venus.... Scorched surface and lots of hot gases above it

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bluedevil2k
There's no water vapor on Venus though. At a certain point in the atmosphere,
wouldn't it be cool enough and pressure low enough to make clouds still? Maybe
they move around the Earth to the cool side away from the Sun?

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ben_w
Water disassociates into hydrogen and oxygen in UV light, and hydrogen gas
move faster than escape velocity, even at current atmospheric temperatures.

Ok, so it is more complicated than that, the speed of any atom or molecule is
a Maxwellian distribution, but enough move fast enough that it’s an acceptable
approximation, especially over these time scales.

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_Microft
Does anyone know the meaning of radius here? Does it measure from center to
surface along curved spacetime or is it rather the radius that a sphere with
the given surface would have if spacetime were flat (i.e. like the
Schwarzschild radius of a blackhole)?

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baggy_trough
I love how gravity is so strong on a neutron star that time runs 1.4-1.7x
slower there.

