Physics Stack Exchange
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Questions tagged [galaxies]

Galaxies are gravitationally bound systems of stars, interstellar gas and dark matter, often hosting a central supermassive black hole. For questions about the structure, composition, dynamics, classification, etc. of galaxies. This includes small systems of interacting galaxies (i.e. merger, or galaxy + satellite system), but for galaxy groups and clusters use [galaxy-clusters], and for questions specific to our own Milky Way galaxy use [milky-way].

Galaxies are massive systems held together by gravitational attraction, consisting of , star systems, star clusters and the ISM (), stellar remnants, cosmic dust and an important but not yet completely understood component called . Many galaxies, perhaps all, host a central super-massive .

For questions about the structure, composition, dynamics, classification, etc. of galaxies. This includes small systems of interacting galaxies (i.e. merger, or galaxy + satellite system), but for galaxy groups and clusters use , and for questions specific to our own Milky Way galaxy use .

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Do solar systems typically spin in the same direction as their galaxy?

Is the net angular momentum vector of our solar system pointing in roughly the same direction as the Milky Way galaxy's net angular momentum vector? If yes or no, is that common for most stars in the galaxy?
Alex
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Why the galaxies form 2D planes (or spiral-like) instead of 3D balls (or spherical-like)?

Question: As we know, (1) the macroscopic spatial dimension of our universe is 3 dimension, and (2) gravity attracts massive objects together and the gravitational force is isotropic without directional preferences. Why do we have the spiral 2D…
wonderich
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How would we tell antimatter galaxies apart?

Given that antimatter galaxies are theoretically possible, how would they be distinguishable from regular matter galaxies? That is, antimatter is equal in atomic weight and all properties, except for the opposite reverse charge of the particles,…
Soren
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Why don't galaxies orbit each other?

Planets orbit around stars, satellites orbit around planets, even stars orbit each other. So the question is: Why don't galaxies orbit each other in general, as it's rarely observed? Is it considered that 'dark energy' is responsible for this…
user98038
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Why aren't there spherical galaxies?

According to the Wikipedia page on Galaxy Types, there are four main kinds of galaxies: Spirals - as the name implies, these look like huge spinning spirals with curved "arms" branching out Ellipticals - look like a big disk of stars and other…
haneefmubarak
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Does a star need to be inside a galaxy?

Must a star belong to a galaxy, or could it be completely isolated? In case it can be isolated (not belong to a galaxy), could it have a planet orbiting around it?
cooper
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Would we even notice the merger with the Andromeda Galaxy?

I have read this question: When galaxies collide it is not that their stars crash into each other, because their individual cross-sections are extremely small when compared to the space between them. This is dealt with in qualitative terms on the…
Árpád Szendrei
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What are galactic speeds measured against?

The Earth moves through space at 67,000 MPH. The Milky Way travels through a local group at 2,237,000 MPH. Wouldn't you need a fixed point to be able to measure velocity against? After all, compared to the total speed of our Milky Way, the Earth…
Sean Long
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What is the evidence for a supermassive black hole at the center of Milky Way?

Black holes cannot be seen because they do not emit visible light or any electromagnetic radiation. Then how do astronomers infer their existence? I think it's now almost established in the scientific community that black holes do exist and…
Solidification
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Why isn't the center of the galaxy "younger" than the outer parts?

I understand that time is relative for all but as I understand it, time flows at a slower rate for objects that are either moving faster or objects that are near larger masses than for those that are slower or further from mass. So, the illustrative…
Yevgeny Simkin
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What causes our Sun to oscillate around its mean Galactic orbit?

According to this answer on Astronomy.SE, The Sun executes oscillations around its mean orbit in the Galaxy, periodically crossing the Galactic plane. I borrowed this illustration (not to scale!) from https://www.nbcnews.com/id/wbna40787453 to show…
Earth is a Spoon
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How does one measure the mass of a galaxy? And other such large quantities?

Trained as a pure mathematician, I see claims about the mass of a galaxy and other such huge measurements that are arrived at experimentally, and I just have to scratch my head. I know this is a bit of a vague question--but does anyone have a good…
Jamie Banks
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How do spiral arms form?

Why aren't all spinning galaxies shaped as discs as my young mind would expect? I understand how the innermost parts of a galaxy spin faster than the outer parts, and that could explain why some galaxies are more spiraled than others based on age.…
David Ball
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A star or a galaxy?

When we look into the beautiful sky in the night, exclaiming how beautiful these shining stars are. My question is how could we tell, whether any of these shining "point" is a star or a galaxy? If indeed many of these are shining galaxies, then…
Daniel
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How far apart are galaxies on average? If galaxies were the size of peas, how many would be in a cubic meter?

The actual number: How far apart are galaxies on average? An attempt to visualize such a thing: If galaxies were the size of peas, how many would be in a cubic meter?
user12345
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