Nib asked
Do they suck everything in?
A black hole pulls only as strongly as the mass it contains, so from a safe distance it attracts objects just as any object of the same mass would, and planets can orbit it stably.
4 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the myth
Pop culture pictures a black hole as a drain that hauls in everything around it. But the pull of gravity depends on mass and distance, not on what kind of object the mass is packed into. A black hole has no special sucking power; it is simply a lot of mass squeezed into a tiny space.
Gravity depends on mass and distance, not on the object being a black hole.
a common mix-upBlack holes actively suck in everything around them like a vacuum cleaner.
2swap the sun
Imagine the Sun were swapped, in an instant, for a black hole with exactly the same mass. Earth would feel the same gravitational pull as before and keep orbiting just as it does now. It would get cold and dark, since the light would be gone, but the orbit would not change.
Swap the Sun for a same-mass black hole and Earth's orbit stays the same.
a common mix-upA black hole's gravity is somehow stronger at long range than an ordinary object's of equal mass.
If the Sun became a black hole of equal mass, would Earth's orbit shrink?
Show the answer
No. The pull at Earth's distance is unchanged, so the orbit stays the same.
3how close
What changes is how close you can get. A star's surface keeps you far from its center, but a black hole's mass is crushed so small that you can approach much nearer, and gravity grows steeply as distance shrinks. That is why the extreme effects from earlier pages, like the event horizon, only appear very close in.
The difference is that you can get far closer to the mass, where gravity becomes extreme.
a common mix-upBlack holes are mysteriously stronger than stars of equal mass at every distance.
4safe orbits
At a safe distance, matter can circle a black hole in a stable orbit, just as planets circle the Sun. Only matter that strays too close, inside a certain inner orbit roughly a few times the radius of the event horizon, spirals inward and is lost. Plenty of matter near a black hole is not orbiting at all; the point is that distance decides its fate.
Far out, stable orbits are possible; only matter straying too close falls in.
What decides whether an object near a black hole circles safely or falls in?
Show the answer
Its distance: beyond the innermost stable orbit it can circle; inside it, it spirals in.
So a black hole isn't a hungry drain; it's a heavy object whose danger lives only up close, and from far away it's just gravity as usual.