# Black hole smaller than a neutron star?

1. Dec 20, 2015

### Slyster

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.

2. Dec 20, 2015

### SteamKing

Staff Emeritus
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.

3. Dec 20, 2015

### rootone

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 in to 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.

4. Dec 20, 2015

### Slyster

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.

5. Dec 20, 2015

### SteamKing

Staff Emeritus
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.

6. Dec 20, 2015

### rootone

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.

7. Dec 21, 2015

### OmegaOm

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.

8. Dec 21, 2015

### rootone

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.

Last edited: Dec 21, 2015