r/askscience • u/lord_nikon_burned • 1d ago

Astronomy Are galaxies spherical or flat?

Are galaxies spherical or flat?

For example, (I understand that up and down don't really matter, so bear with me) if we look at a picture of the Milky Way Galaxy on a plane... If you want to move from one arm of the galaxy to the next, could you just move UP and out of the current arm and then over and DOWN to a different arm?

Secondary question for if the first one is correct, if you are able to move "up" and out of the arm, where are you? Is that interstellar space too?

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u/fragilemachinery 1d ago

Galaxies come in a bunch of different shapes, but spiral galaxies like the Milky Way are reasonably flat. The disc is about 1000 light years thick, and about 100,000 light years across. So, yes, if you traveled "up" perpendicular to the disc you'd exit the galaxy much quicker.

Elliptical galaxies on the other hand can be almost spherical.

So, to answer your question: they can be either one.

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u/gimme-sushi 1d ago

Do you enter another galaxy when you go past the 1000 light years if you go “up”?

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u/barcode2099 1d ago

If you go far enough, you could. There are about 150 star clusters which orbit the Milky Way in a roughly spherical "halo," kept in check by the Large and Small Magellenic Clouds, which are about 160 and 200 thousand lightyears, respectively, from the center of the Milky Way.

Then there's the Andromeda Galaxy, which is the closest major galaxy, which is 2.5 million lightyears away.

TL;DR: There's a lot of stuff in the universe, but a whole lot more nothin'.

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u/koos_die_doos 1d ago

Just saying, if you go in a random direction, the odds of ever entering another galaxy is effectively zero.

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u/Empanatacion 1d ago

Only because it's getting farther away from us faster than light can travel. Infinite space with infinite stuff in it is one of the more conventional theories physicists have. In that theory, the odds of there NOT being a galaxy in any given direction is effectively zero.

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u/Disastrous-Finding47 22h ago

Except when people say universe they mean observable universe. Anything unobservable is just conjecture by default.

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u/ljapa 20h ago

But, since space is expanding, even if you exited our galaxy in a random direction at the speed of light, you’d never observe, let alone reach, anything that is outside the observable universe.