Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top


If black holes can pull even light, why cant they pull the stars in the galaxy?

share|cite|improve this question
They pull everything. But those things often never reach the black hole. Just like that satellites don't fall down. – Calmarius May 10 '13 at 10:56
up vote 3 down vote accepted

Why would you assume they do not ? Of course they do. But as you probably know, the gravitational pull decreases with distance (inverse square law). From a safe enough distance any other object (star, galaxy) would feel the normal gravitational pull of an object of the black-hole's mass at that distance; it makes no difference to the stars if the source is a black hole or anything else.

share|cite|improve this answer

This is an extremely common misconception so I will try to make my answer as simple as possible. A black hole is black and light can't escape because of mass for a given volume (density), not because of mass alone.

Take our solar system for example. Our sun is very massive and it pulls on all of the planets hard enough that they orbit the sun. As long as the planet's orbits are outside the sun, they do not depend on the size of the sun, only its mass. Mercury feels a greater pull from the sun because it's closer to the Sun's center of mass. The closest we can get to the Sun's center of mass is the surface of the sun itself. If the sun were to shrink in size then it would be possible to get even close to the Sun's center of mass and the pull from gravity would be greater.

Note that making the sun smaller or larger doesn't affect the pull on the planets at all because we orbit the center of mass of the Sun, not the Sun's surface. If the Sun were to shrink the Earth wouldn't notice a change in gravity at all. If the Sun were to continue to shrink smaller and smaller, there would be a point where the surface of the Sun is so close to the center of mass of the Sun that it would collapse into a Sun-mass black hole (pretending no energy is lost in this collapse). The Earth wouldn't notice a thing and would continue to orbit a black hole the mass of our Sun.

All stellar-collapse black holes (the ones formed in supernova) are roughly the mass of the original star (minus whatever energy and mass didn't fall into the black hole during the supernova). Nothing outside of the black hole feels any greater pull than before when there was a star in its place.

A black hole like the one at the center of our galaxy is getting bigger because stars and other objects are falling into it. As it grows in size and mass the things orbiting it do feel a greater and greater pull. This doesn't cause them to fall in though, it causes their orbital radius to shrink and their orbital velocity to increase. If this were to continue then eventually the black hole would grow in size and mass enough to envelope the things orbiting it.

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.