# Gravitational effect on planets when

Hyperreality
If the sun becomes a black hole, all the planets in the solar system will stil be orbiting the sun as usual, but would there be any alteration in their orbit trajectories? And would the gravitational influence be stronger on the planets?

If so, what is the relationship between the density of a mass and its gravitational field?

Originally posted by Hyperreality
If the sun becomes a black hole, all the planets in the solar system will stil be orbiting the sun as usual, but would there be any alteration in their orbit trajectories? And would the gravitational influence be stronger on the planets?

If so, what is the relationship between the density of a mass and its gravitational field?

No. that is what people mean when they say the planets will continue to orbit "as usual", the orbits will unnaffected except by the gravity waves and other effects that were caused while the Sun was collapsing.

Hyperreality
No. that is what people mean when they say the planets will continue to orbit "as usual", the orbits will unnaffected except by the gravity waves and other effects that were caused while the Sun was collapsing.

This can't possibly means that the density of mass is independent of the gravitational field of the body right?

And another question, is it possible for objects to be sucked into a black hole if it is not in the event horizon?

futz
At large distances from the black hole, the gravitational field doesn't care whether the mass is distributed in a sphere of finite size, or a singularity. So, it would be business as usual for the orbiting planets. Closer to the black hole, the increased density will of course result in much larger gravitational fields.

The black hole's gravitational field extends beyond the event horizon, so objects will still get pulled in. The event horizon is a mathematical construct representing the "point of no return" for an object caught in the hole's pull. Once an object moves within the event horizon, its escape velocity exceeds the speed of light, which is why nothing can escape (even light itself).

mmwave

Originally posted by LURCH
No. that is what people mean when they say the planets will continue to orbit "as usual", the orbits will unnaffected except by the gravity waves and other effects that were caused while the Sun was collapsing.

Yes, but on the way to becoming a black hole won't the sun first swell to be a red giant and probably take out the planet Mercury?
That's got to hurt.

futz

Originally posted by mmwave
Yes, but on the way to becoming a black hole won't the sun first swell to be a red giant and probably take out the planet Mercury?
That's got to hurt.

Technically, the sun will never become a black hole. It lacks sufficient mass to make this transformation.

Once it exhausts it's supply of hydrogen, it will begin to collapse. This will once again increase the pressure and temperature, which will cause the star to expand once again, becoming a red giant. The diameter of the new star will easily extend beyond the orbits of Mercury and Venus, and possibly even the Earth itself. Even if it doesn't get that big, the star will be much closer to us, and have a greatly increased luminosity. Ultimately, this will probably burn off our atmosphere, evaporate any water on the surface, and increase the surface temperature dramatically, making Earth uninhabitable. Luckily, this won't happen for many many millions of years.

A long time after that, the sun will have exhausted all of its fuel. The outer layers of gas will be expelled, leaving only the collapsed core. At this stage it is a white dwarf star.

Jonathan
Millions?!

I thought it was billions! We'd better hurry up and get to colonizing other solar systems. Btw, I heard that the sun would balloon up and swallow all the inner planets, and probably take some of each with it as it shrinks.

futz
You're right, I meant to say "many many billions". Either way, you and I will be long gone when it happens!

SEG9585
Maybe YOU will be gone when it happens :-)