Comment by gunian
3 days ago
is there some sort of gravitational body in the middle that makes everything orbit in galaxies? it must be massive right
3 days ago
is there some sort of gravitational body in the middle that makes everything orbit in galaxies? it must be massive right
One of today's lucky ten thousand. Most galaxies have a black hole at the center that mass at least a hundred thousand times more than our sun: https://en.wikipedia.org/wiki/Supermassive_black_hole
> mass at least a hundred thousand times more than our sun
The sun is 99.86% of the mass of solar system. So if you orbit the centre of mass of the solar system, you orbit the sun, more or less. Give or take a small correction for Jupiter.
But ... there are a lot more than a hundred thousand stars in the milky way. So if I guess right, the ratio of central mass vs the rest would be very different for the Milky way? It's more of a blob.
Even at "The current best estimate of its mass is 4.2 million solar masses" it does not dominate? https://en.wikipedia.org/wiki/Sagittarius_A*
It's not a singular source of gravity at the center though, it's the collection of all mass in the galaxy interacting with each other as well. Like a daisy chain of gravity, which explains why it looks like a spiral instead of an evenly distributed circle.
(I think anyway, I just made it up, I'm not learned in this area, just a HN shitposter)
1 reply →
does that mean all galaxies will eventually be consumed by the black holes at their center?
No. Does this mean that solar systems will eventually be consumed by the stars at their centre, planets falling out of their orbits due to gravity? It does not. Gravity doesn't work like that.
The planets may be consumed, when the star runs out of fuel and swells a lot, and such is the Earth's fate. But that scenario is not one that happens to black holes.
That’s one of the theories, called Big Crunch: https://en.wikipedia.org/wiki/Big_Crunch
There are other (more probable) theories about the end of the universe, and if you’re up to it you can read more here: https://en.wikipedia.org/wiki/Ultimate_fate_of_the_universe
I’m in favor of the Big Chill, since I like the concept of entropy as introduced by the second law of thermodynamics: https://en.wikipedia.org/wiki/Heat_death_of_the_universe
2 replies →
> is there some sort of gravitational body in the middle that makes everything orbit in galaxies?
No. The Sun's orbit is determined by the total mass of stars, gas, and dark matter interior to the orbit. This is mostly due to the stars (we're not far enough out from the center for dark matter to be the dominant component) and is on the order of several tens of billions of solar masses.
(There is a supermassive black hole at the center of our galaxy, but its mass is only about 4 million solar masses, so it's negligibly small compared to the mass of all the stars.)