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Comment by sbierwagen | Hacker News Reader

It can't. The galaxy is assumed to be roughly symmetrical, and they fill in the missing data with what we can see on our side of the galaxy. It's "best" in the sense that it's the most accurate fiction, I suppose.

Gaia is good to about 13,000 light years: https://en.wikipedia.org/wiki/File:Galaxymap.com,_map_12000_...

The Milky Way is maybe 100,000 light years in diameter. So we're only getting good distance readings on a small fraction, and nothing behind the central bulge of our galaxy. The first won't improve until we send an astrometry telescope way outside the orbit of the Earth, for better baselines, and the second is going to need a telescope sent 10,000 light years out of the galactic ecliptic.

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We can infer the general distribution of mass on the other side of the galaxy from observing the trajectory of stars, can't we?

It takes 230 million years for the Sun to make one full orbit around the Milky Way.

huh? how can sun orbit the milky way if it is within the milky way

Around the center of the Milky Way. The sun orbits the center just like the planets orbit the sun.

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)

Right, not a point source and not exactly a blob either; more like swirls. I'm not learned in this area either.

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

sometimes it feels weird reading anything non fiction because we know so little about reality

The Big Crunch has nothing to do with Sagittarius A*, the black hole at the center of our galaxy. No theories actually predict that it will consume all stars in our galaxy.

> 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.)

The same way that the Earth can orbit the centre of mass of the solar system and also be within the solar system. We say that the Earth orbits the Sun because that's where 99.86% of the mass of the solar system is located.

The Sun in turn orbits the the centre of mass of the Milky Way. But I don't think that the mass of the Milky way's central supermassive black hole dominates in the same way.

Orbit around the center. It’s like saying the Earth orbits around the solar system.

But the earth doesn't orbit around the solar system. it orbits around the sun as part of the solar system. the solar system as a unit orbits around the center of the galaxy. if you've ever seen the concept images of the Oort cloud, you could visualize that snowball looking roundish object as a visual for the solar system traveling through the galaxy.

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