# Black holes and infinity

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## Main Question or Discussion Point

Good morning. Not sure if I am doing this right, but I just wanted to ask a question.

## Answers and Replies

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Also, I'm not looking for homework help. Just an understanding.

I am assuming that a black hole has an infinite mass in any direction, from a direct line from any observer.

PeroK
Homework Helper
Gold Member
I am assuming that a black hole has an infinite mass in any direction, from a direct line from any observer.
A black hole has a finite mass. This is generally the mass of the star that collapsed to form it.

So the mass doesn't increase over time, and if it's the same mass of the star that created it, how does it have an event horizon or the same gravitational "footprint" as the star that created it?

PeroK
Homework Helper
Gold Member
So the mass doesn't increase over time, and if it's the same mass of the star that created it, how does it have an event horizon or the same gravitational "footprint" as the star that created it?
It can of course increase in mass over time by objects crashing into it. But, that can happen to a star as well. In any case, the mass remains finite.

The gravitational attraction of a black hole is the same as the star that created it. Except that the star's mass has a physical extent: the Sun is about 1.5 million km across. So, you can only get so close to the centre of the Sun before you crash into the surface. But, if the Sun were a black hole you could get closer than that and the gravity would get stronger the closer you got. And at a certain radius - it would be about 3km for the Sun - you would reach the event horizon.

In other words, the gravitational difference between a star and a black hole is in the region closer than the surface of the star. Outside of that, the gravity is the same for both.

Hey thanks! I was focused on the wrong thing and didn't take the radius into account.

berkeman and PeroK