Silver_Back

Illyr

So, once again, the Doctor was right. And, of course he was: his people practically invented them.

Dunny0

I'm kinda disappointed.

Lizzie24601

I reject your reality and substitute my own, one with Ernest Borgnine and V.I.N.C.E.N.T.

[en.wikipedia.org]

Maddrjeffe

workingonyourinvoice

Interplanetary spacecraft make use of both of these: if you want to make use of a gravitational slingshot around Jupiter (say), you come zipping in aimed to just miss it, it bends your path around to come out at a different angle, and then you shoot off again. If instead you want to slow down and land on Mars, you aim so close to it that as you're whipping past, you dip into the atmosphere and it slows you down. (If this sounds tricky to get right, it is, and it's why we've lost some Mars spacecraft.)

Black holes are incredibly tiny - it's hard to fall into the Sun, but if you made it into a black hole it would be only a few kilometers across, which is much much harder to fall into.

But black holes do manage to gobble up some matter in spite of their tininess. This usually happens in an accretion disk, when you've got a disc-shaped cloud of gas orbiting the stars. The gas is normally mostly in orbit, but this means the inner parts of the disc have to move faster than the outer parts. So some viscosity in the disc (which we don't fully understand) causes the outer parts to slow the inner parts, which lets the inner parts drift further inward. Eventually the gas moves so far inward it falls in. In the process all that viscosity heats the gas to millions of degrees Kelvin, so hot it produces X-rays we can see (which is how we know there are black holes).

aarchiba

We don't believe there is a geometrical center of our universe, although there do seem to be massive black holes at the center of most galaxies. This is an interesting and puzzling result to many astronomers.

@AdamL:

In most of the cluster, the force from the black hole is negligible compare to the force from the bulk of the stars. Very, very close to the black hole stars can spiral in and be eaten by the hole, but this relatively uncommon. Remember, the stars (as a group) have a total mass hundreds of times that of the hole.

@Illyr:

This guy is only 20,000 light-years from here, so the stars are only 20,000 years older than we see them. We expect no appreciable difference in that time in the number of stars, though it is possible that few more may have wandered into the hole by then. This cluster is many billions of years old.

gStar

Illyr

AdamL

Illyr