Black Holes and their original star

[Johnnb]

Black holes suck in everything around them. My 5 year old knows that one. It's all over TV, etc. But what about the original star? Wouldn't that have just as much gravity? Wouldn't it do the same thing with the strength? Is it just that black holes are more exciting to talk about?

 

[Jonathan Scott]

Black holes suck in everything around them. My 5 year old knows that one. It's all over TV, etc. But what about the original star? Wouldn't that have just as much gravity? Wouldn't it do the same thing with the strength? Is it just that black holes are more exciting to talk about?

From a distance, the gravitational effect of a black hole is very similar to that of the original star, but you can get a LOT closer to it (remember Newtonian field is -Gm/r² where r is the distance to the center) so the field strength gets very much stronger. In General Relativity, it doesn't just get stronger but it effectively becomes irresistible at the "event horizon", so you can't just swing round and back out again.

They don't really "suck in" things at a given distance any more than a non-black hole of the same mass, but anything that gets too close is not going to get away again.

Representations of black holes in fiction are generally totally non-scientific and should be ignored.

 

[Johnnb]

Your comment answers my question. In essence, it sounds like black hole are just more "sexy" for the science type channels for the most part. It seems like they all have a new show about black holes every month.

 

[Cosmo Novice]

Your comment answers my question. In essence, it sounds like black hole are just more "sexy" for the science type channels for the most part. It seems like they all have a new show about black holes every month.

They do make interesting TV!

Its important though to remeber BH's cannot be properly described by Newtonian physics.

The crucial part to remember is that at a distance black holes gravity are the same as any other body of mass, but once something crosses the Event Horizon - even a photon, its never getting out, essentially the gravity is so extreme that all world lines lead further into the BH.

 

[Chalnoth]

One other important thing to realize about black holes is that until matter gets into a rather close orbit, they don't actually suck anything in. If you are a fast-moving bit of matter coming into the vicinity of a black hole, chances are you'll just end up missing the black hole entirely, simply because the black hole is very, very small. But if you find yourself in a relatively close orbit, well, then you'll progressively lose energy to gravitational radiation and spiral inward into the black hole.