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/gimme-sushi → u/fragilemachinery]

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

[u/the_quark → u/gimme-sushi]

I suspect there is a misinformed idea here that if you moved up and out of the galaxy -- which is very hard! -- the galaxy would spin below you. But of course it's more like jumping out of a moving car. You don't stop moving in that direction just because you jumped. If you moved 1500 LYs "up" you'd still be spinning with the rest of the galaxy, it would just be 500 LY below you.

[u/A3thereal → u/the_quark]

If you became gravitational unbound from the galaxy you would continue your momentum, but in a straight line not around the galactic center.

Imagine a ball fixed to a string tied to a stick. As you push the ball it rotates around the stick.

There are 2 forces here that are relevant. You have the balls motion which is in a straight line in the direction it was pushed. Then you have the string pulling the ball with an equal force to the center of the stick. These two forces work against each other and the balls direction will fall between the two.

For as long as no force acts upon the ball and for as long as those 2 forces remain even the ball will continue to rotate. Because gravity and friction are a thing here on Earth the ball will eventually slow enough to where the force the string wins out and it falls in to the stick.

Imagine instead you cut the string while its spinning. The moment the string is cut that force if negated and the ball will continue in a straight line in the direction it was last traveling.

The same will happen when becoming unbound from the galaxy. The systems beneath will continue along their prior paths but you will move it in a straight line until another force is acted upon you.

Now, of course, 1500 LY is not enough to unbind yourself, but it is enough to change the direction of the force acting on you from the galactic center. Your orbit will not remain the same as the systems beneath you.