[[Tycho?]]

I was wondering, how large are the event horizons of black holes? Not any sort of miniature black holes, but ones likely to form from the collapse of very massive stars. Also, is there any easy way to calculate it?

[swansont]

'[Tycho?] said:

I was wondering, how large are the event horizons of black holes? Not any sort of miniature black holes, but ones likely to form from the collapse of very massive stars. Also, is there any easy way to calculate it?

The Schwarzschild radius for a black hole of mass M is given by r_s=2GM/c², where G is Newton's gravitation constant.

[[Tycho?]]

swansont]The Schwarzschild radius for a black hole of mass M is given by r[sub]s[/sub]=2GM/c[sup]2[/sup said:

, where G is Newton's gravitation constant.

Heh, yeah I just discovered that, good old wikipedia. At first I thought, well, it just says radius, it doesn't say radius of the event horizon. Of course then I realized that no other part of a black hole could really have a radius...

Thanks.

[Martin]

'[Tycho?] said:

I was wondering, how large are the event horizons of black holes? Not any sort of miniature black holes, but ones likely to form from the collapse of very massive stars. Also, is there any easy way to calculate it?

for M equal to the mass of the sun, that formula
namely 2GM/c²
gives you about 3 kilometers

then if you want to calculate other BH sizes its is really simple

If someone tells you that in center of some galaxy like Milky Way there is a 10 million solar mass black hole
you can reckon its radius is 30 million kilometers.

this works for the simplest situation where you dont assume anything fancy about the BH like that it is electrically charged or rotatating fast enough to effect the shape of its event horizon. as long as there are no extra things to worry about you just multiply the solar masses by 3

so a 10 solar mass hole has radius of 30 kilometers.

planet jupiter is very roughly 1/1000 of mass of sun so a black hole
with jupiter mass would have radius roughly 3 meters
so you could keep it in your swimming pool
or just leave it out in the back yard