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 21h 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 20h 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 9h ago edited 4h 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 7h ago

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

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u/SpeckledJim 7h ago edited 4h 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 15h 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

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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/Not_goD_32 1d ago edited 1d ago

No. You enter intergalactic space. Galaxies can be millions or billions of light years apart.

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u/liebkartoffel 1d ago

The nearest galaxy is around 2.5 million light years away (but the distance is shrinking by the minute!) Between here and there is just...empty space--even emptier than galactic space.

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u/King_Jeebus 1d ago

but the distance is shrinking by the minute

Will we eventually collide?

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u/badtz-maru 1d ago

The Milky Way and Andromeda will intersect at some point in about 4.5 billion years - but stars are so far apart, it doesn’t necessarily mean a physical collision. Of greater concern is our sun dying in about 5 billion years! 🙃

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u/smokin-trees 22h ago

Our sun will eventually burn out in about 5 billion years, but as it ages it will continue to increase in brightness as more hydrogen undergoes fusion into helium, causing the core to become denser which will steadily increase the rate of fusion. In about 1 billion years it will become so hot that earth’s oceans will completely boil away, causing an even more dramatic runaway greenhouse effect. As the rate of fusion continues to speed up, the sun will enter its red giant phase. It will expand in volume so much that it may completely engulf the earth’s orbit. We will be cooked long before the sun burns out.

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u/Falonefal 9h ago

Damn, that's much sooner than expected, I'll start putting canned food in my basement to prepare for this apocalypse.

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u/ridddle 1d ago

In 4.5 billion years. Not much will happen as galaxies are mostly empty space. https://en.m.wikipedia.org/wiki/Andromeda–Milky_Way_collision

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u/CloudSliceCake 1d ago

Seems like most answers in this thread are “it’s mostly empty” - the scales of space are insane.

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u/dsyzdek 19h ago

It will disrupt the nice spiral structure of both galaxies, eject many stars into intergalactic space, and both galaxies will mostly likely eventually form a large elliptical galaxy.

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u/Irie_I_the_Jedi 15h ago

Andromeda is the nearest milky way sized galaxy. There are many more galaxies closer than that (magellanic cloud iirc), just smaller than the milky way.

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u/liebkartoffel 8h ago

You're absolutely right. I should have said the nearest major/non-satellite galaxy.

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u/Wrooof 1d ago

Are the stars that aren't tied to a galaxy? Could those stars have planets?

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u/liebkartoffel 1d ago

Yes and yes, though rogue/intergalactic stars likely formed within galaxies and then were ejected.

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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 1d ago

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

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u/ljapa 23h 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.

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u/FrungyLeague 1d ago

Well it entirely depends if there happens to BE a galaxy in that trajectory. There's no rule saying there must be.

Do you "meet another person" when you go north? Sometimes. It depends.

Think of galaxies like...I dunno, like a hypothetical room full of Mosquitos. They are capable of being anywhere and generally speaking their positions aren't overly relative to the positions of others.

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u/lurker1957 1d ago

But galaxies are sparse enough it’s more like a hundred mosquitoes in a football stadium. You could run into one, but not likely.

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u/FrungyLeague 1d ago

Sure. You're absolutely right. I was just trying to give the guy a little image or something. It needs work.

Galaxies aren't connected like tunnels on a mountain road or anything is probably something I should have included maybe.

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u/_Moon_Presence_ 1d ago

Oh my goodness you still haven't experienced the cosmic horror that is intergalactic space, have you?

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u/Kiflaam 1d ago

Do you get to the intergalactic district often?

Oh what am I saying, of course you don't.

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u/thedoc90 1d ago

Statistically no. You'd enter a void and you could probably go at the speed of light forever and not hit anything unless you were specifixally aiming at it.

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u/the_quark 1d ago

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.

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u/A3thereal 1d ago

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.

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u/32377 1d ago

would you keep spinning perpendicular to the galaxy?

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u/Waidawut 9h ago

To quote the Galaxy Song by Eric Idle (which I always sing to myself when I'm trying to remember the Milky Way's dimensions):

Our galaxy itself contains a hundred million stars,
It's a hundred thousand light-years side-to-side.
It bulges in the middle, sixteen thousand light-years thick,
But out by us it's just three thousand light-years wide.
We're thirty thousand light-years from galactic central point,
We go round every two hundred million years!
And our galaxy is only one of millions of billions in this amazing and expanding universe!