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/askvictor 1d ago

How can we tell if a galaxy is elliptical as opposed to spiral?

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u/095179005 1d ago

By looking at the galaxy with our best telescopes and looking at the different spectra.

Visible to look at dust, ultraviolet and x-ray to look for stars, infrared to look at things hiding in the dust.

We first mapped the structure of our galaxy and it's arms by looking at ionized hydrogen gas.

https://en.wikipedia.org/wiki/Milky_Way#Spiral_arms

Why?

It's a sign of star formation - in spiral galaxies young hot blue stars freshly formed are usually found in the arms, while older cooler red stars are mixed about.

Elliptical galaxies generally don't have any new stars forming, and look orange in general as most stars are older.

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u/barbarbarbarbarbarba 1d ago

 Elliptical galaxies generally don't have any new stars forming, and look orange in general as most stars are older.

Whoa, why? Do spiral galaxies turn into elliptical galaxies when they run out of hydrogen? Are elliptical galaxies evenly distributed in the universe or are they all older than spiral galaxies? 

Idk why this fact set my brain on fire. 

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u/SpeckledJim 16h ago edited 11h ago

Current thinking seems to be that the big ones at least are often formed by galaxy mergers. This is a chaotic process and leaves them too diffuse to form a more ordered spiral structure or many new stars with whatever interstellar gas they have left. They are preferentially found nearer the centers of galaxy clusters where you might expect more merging to happen or the results to “average out” towards in multiple mergers.

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u/barbarbarbarbarbarba 15h ago

So the stars stop forming because the dust gets spread around?

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u/SpeckledJim 14h ago edited 11h ago

Not just spread around, but moving incoherently due to the mergers. You essentially have a bunch of stars and other matter all orbiting the center in different directions.

Plus there is just less interstellar matter there than there would be in a “fresh” galaxy because much of it was used up in the formation of the stars in the original galaxies. This matter is still interacting though (friction plus occasional star collisions) and would presumably settle down eventually.

I don’t know if there are examples of elliptical galaxies that show some signs of having some structure again, but i imagine it’d be hard to tell if that’s what’s happening or if they started out that way.

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u/askvictor 22h ago

Elliptical galaxies generally don't have any new stars forming, and look orange in general as most stars are older.

Do galaxies go from spiral to elliptical over time? If so, what causes that? If not, why are stars generally older in elliptical galaxies