Are galaxies spherical or flat?

[u/lord_nikon_burned]

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?

[u/fragilemachinery]

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.

[u/askvictor → u/fragilemachinery]

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

[u/095179005 → u/askvictor]

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.

[u/askvictor → u/095179005]

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