Black hole smaller than a neutron star?

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Discussion Overview

The discussion revolves around the comparison between black holes and neutron stars, focusing on their sizes, densities, and the nature of their structures. Participants explore theoretical aspects, properties, and implications of both astronomical objects.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants propose that neutron stars are extremely dense due to the removal of empty space between atoms, while questioning if black holes are larger or denser than neutron stars.
  • It is noted that neutron stars have an upper mass limit, estimated at around three solar masses, while black holes do not have such a limit and can reach billions of solar masses.
  • Participants discuss the concept of the event horizon of black holes, which marks the point beyond which nothing can escape, and the singularity within, which current theories cannot fully explain.
  • There is a suggestion that the size of a black hole, defined by the radius of its event horizon, increases with mass, but the nature of the singularity remains uncertain.
  • Some argue that unlike neutron stars, black holes lack a physical surface, and crossing the event horizon does not present any observable boundary for an infalling observer.

Areas of Agreement / Disagreement

Participants express differing views on the nature of black holes and neutron stars, particularly regarding their sizes and the implications of their structures. The discussion remains unresolved with multiple competing perspectives on the properties of these objects.

Contextual Notes

There are limitations in understanding the singularity within black holes and the implications of the event horizon, as well as the definitions of physical surfaces in relation to these astronomical entities.

Slyster
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OK so some stars evolve into neutron stars which I assume are so small (10's of kms) because ALL empty space is removed between the atoms.. absolute highest density possible?

Now.. is a black hole larger (massive and in diameter) than a neutron star and called a black hole only because the gravity is so strong that light cannot escape? ... Or does a black hole actually squash matter ever MORE somehow.
 
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Slyster said:
OK so some stars evolve into neutron stars which I assume are so small (10's of kms) because ALL empty space is removed between the atoms.. absolute highest density possible?

Now.. is a black hole larger (massive and in diameter) than a neutron star and called a black hole only because the gravity is so strong that light cannot escape? ... Or does a black hole actually squash matter ever MORE somehow.
A couple things to know here:
1. There is an upper limit on the mass of a neutron star. It is estimated that a neutron star could have as much mass as 3 times that of the sun, but so far, the most massive neutron star observed has about twice the mass of the sun. This star also would have a radius no less than about 11 km.

https://en.wikipedia.org/wiki/Neutron_star

2. There is no upper limit on the mass of a black hole. Indeed, there are BH in the centers of galaxies which have a mass equal to billions of suns. The radius of the event horizon (the distance at which the escape velocity from the BH = the speed of light) can grow to be larger than the size of the solar system if the BH is sufficiently massive.

https://en.wikipedia.org/wiki/Black_hole

3. BH are called that because no light or other EM radiation can escape from within the event horizon. It is not understood what happens to the matter trapped inside the event horizon, so physical concepts, such as density, as they exist outside the event horizon may no longer have any meaning inside the event horizon.
 
A black has no surface at all in the usual sense.
It does have an event horizon which marks the 'point of no return' both for light and for every kind of material objects.
Once inside the event horizon any object is fated to be pulled deeper into the gravity well until it encounters the dreaded 'singularlty'.
The singularity isn't a 'thing' though with certain dimensions, it just means that present theories can't explain what happens.

The main point here is that a black hole has no physical surface, there is no known kind of matter that a physical surface could be made from.
 
How about a huge black hole.. billions of solar masses.. besides having much more gravity, it it going to be the same 'size' at a much less massive black hole. Thanks.
 
Slyster said:
How about a huge black hole.. billions of solar masses.. besides having much more gravity, it it going to be the same 'size' at a much less massive black hole. Thanks.
The radius of the event horizon is directly related to the mass of the BH.

If the sun were turned into a BH, the radius of its event horizon would be approx. 2.95 km.
If a star with 10 solar masses were turned into a BH, the radius of the event horizon would be 29.5 km, and so on.

BHs also can become more massive as they gobble up additional matter after their initial formation.
 
Slyster said:
How about a huge black hole.. billions of solar masses.. besides having much more gravity, it it going to be the same 'size' at a much less massive black hole. Thanks.
I think the answer to that has to be 'unknown' at the present time.
We just don't know what that singularity really is.
Event horizon can be of differing radius depending on mass, but that isn't a physical surface, it's just the radius from within which there can be no escape.
 
rootone said:
I think the answer to that has to be 'unknown' at the present time.
We just don't know what that singularity really is.
Event horizon can be of differing radius depending on mass, but that isn't a physical surface, it's just the radius from within which there can be no escape.

A physical surface doesn't matter. A black hole is defined by its mass and the radius of the event horizon, because these are properties apart of the black hole. It is perfectly fine to state the radius of the black hole by the radius of the event horizon.
 
Yes but unlike a neutron star which does have a physical surface, as the OP points out this may be as small as 10km,
there is no shrinkage from 10km to something smaller for a BH.
An observer crossing the event horizon (which could be bigger a lot bigger than 10km) wouldn't see anything special about it, they'd just carry on falling inward.
 
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