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View Full Version : What's the threshhold for black hole-ness?

Control Group
2003-Dec-15, 07:25 PM
It&#39;s my first post, so please be gentle...

At how many gravities does a body become a black hole? Alternatively, how does one go about figuring escape velocity (from which information I could presumably determine the answer to the previous)?

Or are these questions nonsensical demonstrations of my fundamental misunderstandings about the universe?

Littlemews
2003-Dec-15, 08:46 PM
At how many gravities does a body become a black hole?
:lol: cant tell you how many gravities, but all I can said is this : A collapsing stellar core that weigh more than 3 Mass of the Sun (3Msun), then it will turn into a black hole, :lol: ost of them are neutron stars.

how does one go about figuring escape velocity
I am not quiet understand what is this question about <_<
a object, such as high massive planets or stars, they usually can capture object that near them, because their gravity is strong enough to do this thing, and thats what we call escape volecity :lol: (Correct me if I am wrong)

Control Group
2003-Dec-15, 10:05 PM
Originally posted by Littlemews@Dec 15 2003, 08:46 PM

At how many gravities does a body become a black hole?
:lol: cant tell you how many gravities, but all I can said is this : A collapsing stellar core that weigh more than 3 Mass of the Sun (3Msun), then it will turn into a black hole, :lol: ost of them are neutron stars.

how does one go about figuring escape velocity
I am not quiet understand what is this question about <_<
a object, such as high massive planets or stars, they usually can capture object that near them, because their gravity is strong enough to do this thing, and thats what we call escape volecity :lol: (Correct me if I am wrong)
That&#39;s actually enough information, I think. Some quick googling gives me 5.96x10^24kg as the mass of the Earth, and 2x10^30kg as the mass of the sun. Some simple division tells me that the sun outmasses the Earth by a factor of about 3x10^5 (it also costs me two sig figs, but I choose not to cry over spilt milk...particularly when it&#39;s my own fault for not getting a more precise number). The Earth&#39;s mass exerts 1G (obviously), so the sun&#39;s mass should exert 3x10^5G. Given three solar masses is, roughly, the critical threshhold for a black hole, that implies we&#39;re talking about approximately 1x10^6G .

Of course, this is treating all bodies as point sources of gravity, which isn&#39;t actually the case, and there are probably any number of other factors I&#39;m not considering, but that should be, I think, a decent guesstimate. If anyone sees something obviously wrong in that reasoning, please let me know.

I think I phrased the escape velocity question poorly. I know there&#39;s a threshhold velocity which an object has to reach in order to break orbit from another body. IIRC, for Earth it&#39;s around Mach 32 (I could be way off on that). Which means, if something is accelerated straight away from the planet and hits Mach 32, it won&#39;t be recaptured by the planet&#39;s gravity; it will just keep on coasting until something else snags it.

That being said, my question was what equation can be used to determine what escape velocity is for a given body. Given that equation, and knowing that the escape velocity of a black hole is greater than c, I could have back-figured how many Gs it had to be exerting.

Of course, you&#39;ve already answered that question for me...thanks&#33; :)

Planetwatcher
2003-Dec-16, 08:50 PM
Control Group, are you a ringer? ;)

You posed a question, asked for gentleness, and responded with more apparent understanding then what it appeared you had. :huh:

Littlemews is pretty much on the money of a steller mass needed to colapse upon itself and become a black hole. I think Steven Hawkings himself placed the value at 3.4 sun masses, but I may be wrong. :unsure:

As for escape velocity of a black hole. It is above C and quite likely by quite a bit, once you&#39;ve crossed the event horizon. Before that it is less then C depending on just how far you are from the event horizon. :o
At any rate I wouldn&#39;t want to use a black hole&#39;s ergoshere to test my new star drive. :D