Black hole size vs. neutron star size

hkyriazi

Is there any direct evidence that black holes are smaller in size than (less massive) neutron stars?

phyzguy

Although there is strong observational evidence to believe black holes exist, no black holes have been conclusively observed, so the answer is no.

mathman

In theory (things resulting from burnt out stars) black holes have more mass than neutron stars.

bcrowell

I guess this depends on how direct is "direct." The fact that the matter falling into black holes emits higher-frequency electromagnetic radiation than matter falling onto neutron stars suggests that the gravitational potential, which is proportional to M/r, is bigger for black holes than for neutron stars.

An interesting thing about supermassive black holes is that if you take the mass and divide by r^3, where r is the Schwarzschild radius, you get a density that is many orders of magnitude *lower* than the density of a neutron star. Of course the mass is all really believed to be concentrated in a singularity, but the observational measures of size for an object like Sgr A* are all related to the Schwarzschild radius, not the scale of any structure inside the event horizon, which we will never have access to.

Chronos

The least massive suspected black hole is about 3.8 solar masses. See
http://www.space.com/scienceastronom...blackhole.html

The largest known neutron star appears to be between 1.8 - 2.1 solar masses. (See: http://arxiv.org/abs/astro-ph/0605106), although it could be lower (see: http://arxiv.org/abs/0901.4119).

hkyriazi

Thanks, guys. I gather - especially from bcrowell's answer - that we really don't have any way to directly measure the mass density of black holes, since we can't measure their actual size, only an upper bound for it, based on observation data that relates to (approximates?) the Schwartzchild radius. It's interesting that, based on such radii, they'd have an orders-of-magnitude lower mass density than neutron stars. (If it were greater, I'd have my answer. As it is, we can't say, one way or the other.)

twofish-quant

One point is that the premise is wrong. A massive black hole is predicted to be larger in size than your typical neutron star.

There is a big issue here that the "volume" of a black hole is undefined. Remember that black holes warp space and time, which means that the standard rules for volumes don't work near a black hole. What you can do is to measure the radius of a black hole, figure out what the volume would have been had it been in flat space.

Yes. As the mass of a black hole goes up, the assumed density goes down.

George Jones

There is a good chance that, within a decade or so, we should be able to "image" in this way the astrophysical black hole at the centre of our galaxy. See

http://www.scientificamerican.com/ar...f-a-black-hole

http://arxiv.org/abs/astro-ph/0607279.

kai0

you can say that black hole is what comes into existence when its density is more than neutron star to say there are different fate of a star depending on its mass like our sun will end up being turned into red giant and then supernova leaving a carbon star.when a star density is less than chandrashekhar limit it ends up as neutron star or magnetar sometimes.and about size what black hole actually is entirally theoretical but it is supposed to be smaller than neutron star in size but far more massive and thus gravitationally strong.

hkyriazi

I seem to be getting two contradictory claims here. Both bcrowell and twofish-quant say that, theoretically, black holes have less mass density than neutron stars, but kai0 now says precisely the opposite. Judging by the care (spelling) and thoroughness of their respective responses, and their PF credentials, I have to go with the former.

In any case, I do have my answer, which is that observational evidence cannot, at the moment (but may, within the next decade, according to George Jones' posts - thanks for those), say one way or the other.

Chronos

True, but, based on assumptions. You can use the Schwarzschild radius to calculate the 'density' of a black hole [mass divided by the Schwarzschild radius] as about 2e16 for a solar mass black hole - and effectively the mass density of a black hole. The mass density of a typical neutron star can theoretically exceed this limit.

kai0

How can you say that neutron star are more massive black holes are formed from more massive star than the star that make neutron star the mass of these black hole forming star is so high that even neutron fail to prevent gravitational collapse.so by considering the mass of star involved in formation of black hole and neutron star the former(black hole has to be massive) according to our current understanding anyone falsifying this is lying his hand on a major breakthrough in astrophysics.
here are a few link to support my point read the end of following page
http://archive.ncsa.illinois.edu/Cyb...Formation.html
and also this will be use full in discussion- http://edu-observatory.org/eo/black_holes.html

hkyriazi

No one said they were more massive, only that they may have a greater mass density (greater mass per unit volume).