BBC Focus Magazine

[MikeG]

Should there be a correlation between the size and mass of black holes. Apparently, a super massive black hole, the size of millions of solar masses has been observed. Shouldn't something with such huge gravity actually be smaller in size than a less powerful black hole? My reasoning is that the universe was born from something the size of an orange. So apparently, the physical size of such entities must be constrained by the power of its gravitation force. How large exactly should a black hole grow to, before it can crunch up the mass it contains to the size of let's say, our moon?

[The Beige Avenger]

The physical size of a black hole is an unknown... some say it is a "point" i.e. it has no size.

What we perceive as a black hole is entirely due to the gravitational effects of the hole which increase with mass i.e. a heavier black hole is "bigger" because it has more gravity and thus has a much farther radius around which the gravity is such that light cannot escape (Schwarzchild radius).

I think the super-massive black holes are observed by looking at the orbits of the larger stellar bodies close to the centre and thus deduce where the centre of mass is. Additionally, there are gravitational lensing effects (that lead to duplicate galaxies being seen) that are used...

[MikeG]

That's quite interesting. I haven't heard of black holes being referred to as "points" before, but that would make sense, and be consistent with big bang theory (as I understand it anyway).

[The Beige Avenger]

That's quite interesting. I haven't heard of black holes being referred to as "points" before, but that would make sense, and be consistent with big bang theory (as I understand it anyway).

That's generally what they mean by the term "singularity"...

[scott fairbrass]

i think im right in beleiveing that a blackhole is what is left behind after a super nova? if thats the case does the size of a blackhole relate to the size of the sun that was before it? and does a blackhole have a life span?

[The Beige Avenger]

i think im right in beleiveing that a blackhole is what is left behind after a super nova?

Yeah

if thats the case does the size of a blackhole relate to the size of the sun that was before it?

Yeah, the star must be at least a certain amount in order to pull sufficiently when fusion goes mental... it's called the "Oppenheimer-Volkoff limit". Smaller than this and you get a very dense neutron star (provided that the star was heavier than the Chandrasekhar limit)

and does a blackhole have a life span?

I dunno. Time has a very different meaning inside a black hole than "here". I can't see how there could be sufficient energy to ever overcome the gravity when even light cannot escape. But who knows what weirdness goes on and if there's any unknown stuff getting chucked out. "Evaporation" is something that is said to occur in black holes and there can be the appearance of radiation by pair production at the event horizon

[Isee]

How wide is the event horizon?
either an average or a range? jsut to get an idea?
is it the size of our solar system? is it the size of our sun?
is it a certan ratio of the galaxy that orbits it?

[The Beige Avenger]

Radius = (2 x Gravitational constant x mass)/(c.c)

gravitational constant, G, is 6.67300 × 10^-11 (m^3 kg^-1 s^-2)
the speed of light in a vacuum, c, is 3 x 10^8 (m s^-1)

So all you need to do is plug in the value of mass... lets take a conservative estimate of 3 solar masses for the minimum mass needed in order for a dead star to collapse inside its Schwarzchild Radius.

1 solar mass = 2 x10^30 Kg

therefore 3 solar masses = 6x10^30 Kg

therefore an object of 3 solar masses has a Schwarzchild radius (event horizon radius) of ~8900 km... a wee bit bigger than Planet Earth (~6400 km).

The largest black holes have silly numbers associated with them that I tend to take with a pinch of salt... numbers like billions of solar masses make for nice startling headlines but that level is beyond me.

[M Paul Lloyd]

The largest black holes have silly numbers associated with them that I tend to take with a pinch of salt... numbers like billions of solar masses make for nice startling headlines but that level is beyond me.

Might help account for some of the 'missing' dark matter though?

[Isee]

Radius = (2 x Gravitational constant x mass)/(c.c)

gravitational constant, G, is 6.67300 × 10^-11 (m^3 kg^-1 s^-2)
the speed of light in a vacuum, c, is 3 x 10^8 (m s^-1)

So all you need to do is plug in the value of mass... lets take a conservative estimate of 3 solar masses for the minimum mass needed in order for a dead star to collapse inside its Schwarzchild Radius.

1 solar mass = 2 x10^30 Kg

therefore 3 solar masses = 6x10^30 Kg

therefore an object of 3 solar masses has a Schwarzchild radius (event horizon radius) of ~8900 km... a wee bit bigger than Planet Earth (~6400 km).

The largest black holes have silly numbers associated with them that I tend to take with a pinch of salt... numbers like billions of solar masses make for nice startling headlines but that level is beyond me.

Nice! thanks TBA, so even the smallest one's horizon is bgger than earth, cool!
I am not clear one one thing though, why does a black hole of 3 sun masses have an event horizon
yet a star of the same mass doesn't?

I mean if the gravity and the total mass are the same is this effect achieved simply by compounding gazillions of atoms into one dot?

[M Paul Lloyd]

I think its because black holes have so much mass in such a small space that they effectively bend spacetime around them which creates a point beyond which nothing can escape and this is called the 'event' horizon.

Or so I believe..

[Isee]

I think its because black holes have so much mass in such a small space that they effectively bend spacetime around them which creates a point beyond which nothing can escape and this is called the 'event' horizon.

Or so I believe..

Is this why some people think it could act as a portal? Could it be that black holes do not actually feed or consume any matter after forming and just swirls it around itself in space, distorting objects the closer they get to the Event Horison? Like a parabole where you cannot ever cross the event horizon, you can just keep getting closer to it infinitely but never actually cross becaus the space is so wrapped and compressed and what looks like a metre to an unaffected observer is still a million miles to the observer who is close?

[M Paul Lloyd]

I think this has been mentioned in another thread by someone more knowledgable on this subject but I'll have a bash. As I understand it the event horizon is also the point at which physics as we understand it no longer applies so although an object may appear stuck in time from our point of view within the confines of the black holes event horizon it is drawn into the singularity along with everything else because the rules change.
Otherwise black holes would never grow beyond a certain effective mass.
I think?

[The Beige Avenger]

Nice! thanks TBA, so even the smallest one's horizon is bgger than earth, cool!
I am not clear one one thing though, why does a black hole of 3 sun masses have an event horizon
yet a star of the same mass doesn't?

I mean if the gravity and the total mass are the same is this effect achieved simply by compounding gazillions of atoms into one dot?

Welcome.

A star of three solar masses has a Schwarzchild radius equal to that of the event horizon radius of a black hole of the same mass. A star is not squeezed inside its Schwarzchild radius... if it was, it would be called a black hole.

Everything with mass has a Schwarzchild radius. Compress something to smaller than this and you get a black hole with an associated event horizon (the point of no return)

[Isee]

Just read up on black holes on Wikipedia
and I have a question
it says there that a black hole with a mass of a car, would evaporate in some part of a nanosecond and at some point will be 200 times brighter than the sun.
Do I understand correctly that this "evaporation" would look more like an explosion to us? As in a whol ecar woul dbe pulverised and smeared all around it in aninstant?

[M Paul Lloyd]

Oh dear, Wiki is at it again, making confusing and really quite meaninglless statements .

Ok, well something with the mass of a car could not be a black hole because it would not collapse in on itself through force of its own gravity and even if you could scoop up a piece of a black hole with that much mass it would be all but invisible anyway.

However it would indeed then 'evaporate' and quite violently too and probably be many times brighter that our sun, but only for a brief moment.
So to answer your question yes it would be "like an explosion" but don't forget Wiki were refering to a black hole with the mass of a car, not an actual car as such.
As I said 'confusing and really quite meaninglless'.

[KingPhillip]

"I am not clear one one thing though, why does a black hole of 3 sun masses have an event horizon yet a star of the same mass doesn't?"

A star, by definition, is "burning" its fuel of non-renewable atoms. In the process, some energy is projected outward, some of that escaping gravity's hold.

A black hole is a star that ran out of fuel and collapsed on itself. Whatever is happening internally, the energy is insufficient to escape. When it feeds on external material via gravity, its mass grows and anything energetic going on inside still can't escape.