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].

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Why is the Andromeda galaxy coming closer to Milky way?

If both these galaxies are on collision path because of their gravity but how could that be? They are 2.5 million light-years apart now but I imagine they are still n collision path for a long time ...
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Galaxy is 8 times closer 13 billion years ago? [duplicate]

Based on below, How can I calculate the distance of a point from Earth now vs 13 billion years ago? The excerpt below claims it is 8 times further now. Can someone provide relevant equations ? I ...
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Calculate distance of galaxy 13 billion years ago [duplicate]

How can I calculate the distance of a point from Earth now vs 13 billion years ago? The excerpt below claims it is 8 times further now. Can someone provide relevant equations ?
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What would happen if our galaxy and an identical anti-matter galaxy annihilated each other?

What would happen if every bit of matter in our galaxy annihilated with an anti-matter equivalent at the same time? Such as that stars completely annihilated with identical antimatter stars, planets ...
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2answers
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If the distribution of dark matter is not uniform, why it is not attracted towards center of galaxies?

The dark matter hypothesis was formed due to difference in theoretical and observed speed of the outer parts of galaxies. Therefore, there is more dark matter in the outer parts of the galaxy than in ...
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Why are galaxies so far apart?

With the Big Bang, one would expect the distribution of mass to be somewhat uniform, but we have galaxies that have billions of stars and there are huge distances between galaxies. Why is there is so ...
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1answer
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How do I decompose my overall rotation curve for M31 into a rotation curve for the disk and halo?

How do I decompose my grand rotation curve into rotation curves for the disk and halo, like the image below? The picture is from this link pages.pomona.edu/~tmoore/RotCurve The data points are ...
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Questions about Dark Matter and Galactic Rotation

First, does the dark matter halo and galactic rotation of a galaxy depend on it's size? So bigger galaxies, bigger dark matter halo. Secondly, is the amount of dark matter for a galaxy universal? So ...
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Required Preconditions for Galactic Formation

In recent weeks, I have been working on building simulations to look at the formation of galaxies from dust clouds. It's a relatively crude simulation at this point as I needed a working model to ...
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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 ...
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1answer
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Why don't the galaxies collide due to gravity?

Why doesn't the galaxies collide due to gravitational attraction and why doesn't all the matter in the universe stick together due to gravitational attraction?
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1answer
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If Earth is mostly fluid, why does it not stretch as it rotates? [duplicate]

Discs are naturally occurring shapes in the world, because any soft object experiencing rotation on a single axis will stretch itself into a disc shape such as pizzas and galaxies due to centrifugal ...
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Can one galaxy have a higher dark matter ratio than another?

Dark matter do not interact with ordinary matter and light, it has mass and is affected by gravity. If the distribution of dark matter and ordinary matter is evenly spread at the early universe then I ...
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Galactic Birkeland currents

Does Milky Way or any other galaxy produce Birkeland currents such as seen on Earth in auroras? If yes, how strong would they be? According to this questions answer, galaxies do have magnetic fields. ...
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Is it possible that galaxies formed from incredibly fast neutron star collisions?

In the recent Kurzgesagt youtube video there is mention of how neutron star collisions are the likely candidates for creating the heavier elements in the universe. It is also possible that it created ...
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1answer
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View from inside clashing galaxies

The below image from Hubble show two clashing galaxies. Is there a simulation (or whatever) as how the sky would look withing those galaxies? For example, would the night sky be noticeably brighter ...
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1answer
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How scientists look for anti-matter?

If matter and anti-matter look exactly the same, then how can scientists look for anti matter from a distance (other galaxies)?
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Is dark matter inside galaxies different from dark matter in intergalactic space?

I just read a text about astronomy and when talking about dark matter the author says: [...], the dark matter responsible for the orbits of the stars in the Milky Way is probably different from the ...
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Where can I find the star distribution data of our galaxy?

I am doing some research on dark matter and for that I need data of star distribution in our Galaxy. Is there any mathematical law that predicts the distribution pattern of stars in our Galaxy?
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Schwarzschild Radius of a Galaxy

If an ultra compact/dense Galaxy has a Schwarzschild radius same as it is own radius, how can it be observed from the outside of the Galaxy?
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2answers
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Can an observer perceive a single source of light as multiple sources because of curved spacetime?

Is it possible that some of the stars and galaxies we observe here on Earth are actually replicates of other stars and galaxies we are observing from different angles because of curved spacetime? Let'...
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28 views

Generally how much faster are stars on the outskirts of galaxies orbiting than they should be?

This assumes there is no mysterious matter causing the extra speed. Are they orbiting say 10% faster than expected etc?
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How will Andromeda collide with Milky Way in spite of Hubble's law? [duplicate]

According to Edwin Hubble our Universe is expanding because he noticed that other galaxies are moving away from us. But then how Andromeda galaxy will collide with Milky Way in ~4.5 billion years, as ...
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1answer
176 views

Formula for the bias of galaxies

From the article "Large-Scale Galaxy Bias", I try to deduce the equation that my teacher told me which links 2 quantities: the global number density of galaxies the local number density of galaxies ...
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1answer
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Database of Galactic Mass?

I'm currently working on a paper investigating any relationship that might exist between the mass of the AGN, or more specifically the relativistic mass at its center, and the mass of the host galaxy. ...
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Spiral galaxy stability

Is there a limit of baryonic masses $M_{B}$ ($M_{B}=M_{\star}+M_{g}$), beyond which a spiral galaxy is no longer rotationally supported? Like for example: Could spiral galaxies of baryonic masses $M_{...
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Texts on Hoag's Object

Are there any good texts online that someone can recommend on Hoag's Object? It is a very weird galaxy and I would like to learn more.
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1answer
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Why are galalxies spiral? [duplicate]

In every picture we see that galaxies are spiral, why so? are there any other shapes possible?
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Why is the distribution of dark matter in a Galaxy different from the distribution of normal matter?

The dynamics of a galaxy are driven by gravity. And dark matter experiences the same gravitational forces as normal matter. The effects of other forces are very small in comparison. So why is the ...
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1answer
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Are wet mergers good for star formation?

In my astrophysics lecture my professor said wet mergers are bad for star formation since it often results in gas being fed to the black hole. In the wiki however it says the opposite. What is the ...
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How well supported by observations is the hypothesis that concentrations of dark matter are essential to galaxy formation?

Much work regarding the formation and early evolution of galaxies seems to assume that galaxies form based on clumps dark matter. The following observations indicate that some galaxies started with ...
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Gravitational redshift and mass decrease

If there are two distant gravitationally unbound galaxies and the observer galaxy loses its mass twice faster than the distant galaxy. So gravitational blueshift decreases in the observer galaxy. ...
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1answer
125 views

Is a galaxy made up of dark matter really a galaxy?

So I am watching a video discuss Dragonfly 44. The galaxy is made up almost entirely of dark matter and is 100x dimmer than the Milky Way galaxy. Wouldn't this mean it is just a large clump of ...
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1answer
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Could six copies of the universe exist only interacting by gravity? [closed]

The ratio of dark matter to matter is roughly in proportions $1:5.4$ This suggests that if there were about 6 Universes placed in the same space that were completely invisible to each other, except ...
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How do Oort's constants relate to shear and vorticity?

I am told to show that Oort's constants represent shear and vorticity in the velocity field of an orbiting star. I know that Oort's constants are defined as: $A = \frac{1}{2}(\frac{v_{\bot,0}}{R_{0}}...
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Gravitational potential energy of an isothermal sphere

I am trying to calculate the gravitational potential energy, W, defined as: $W = -\frac{1}{2}\int\rho(r)\Phi(r)d^{3}r$ for an isothermal sphere. I am given that the density profile varies with r as:...
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2answers
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Why can't I see whole galaxies with the naked eye?

I have read these questions: At what distance could you see andromeda with the naked eye? Do all the individual stars that we can see in the night sky belong to Milky Way? I look at the night sky ...
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Jeans equation for a fully isotropic velocity dispersion tensor

Given a known spherically symmetric gravitational potential, $\Phi(r)$, I need to calculate the value of the velocity dispersion tensor $\sigma_{rr}$. To calculate $\sigma_{rr}$ I am using Jean's ...
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How to derive the gravitational potential on an infinitely thin disc of finite radius?

I am stuck on a problem that requires me to calculate the gravitational potential on an infinitely thin disc of finite radius $R$ with mass $M$. This infinitely thin disc models a galaxy. Once I have ...
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Would a galaxy be visible from outside, but nearby?

We all know the typical sci-fi image of a guy standing on the ship deck and able to see a full galaxy. If you somehow were able to stand a few lightyears away from a galaxy would you be able to see it ...
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1answer
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Could dark matter be normal matter that is beyond the observational horizon?

As I understand it, matter that is too distant for its light to reach us would not be observable, and the CMB is the limit of what is observable from our location within the universe. Could what is ...
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2answers
8k views

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?
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1answer
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Traveling to another galaxy impossible for ever?

If galaxies are moving apart from each other faster than the speed of light, does that mean that it's impossible to travel to another galaxy, regardless of what technology get discovered? Because you ...
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1answer
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Effects of dark matter

I was wondering, when sims of galaxies show that we need dark matter to account for the movement of outer stars, are models taking into account relativistic effects on gravity? The tips of spiral ...
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0answers
40 views

Does dark matter experience drag?

Is dark matter dragged around the galaxy by gravity as stars and groups of stars orbit the galaxy, and, if so, is its shape/distribution deformed? Similarly, would the Oort cloud be deformed by being ...
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1answer
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Relative size of galaxy calculation

So this is a past exam problem, however I am confused to a question which is related to this graph and the questions states: Question: On the graph, one galaxy is labelled A. Determine the size of ...
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1answer
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Doubts about the formation of the bullet cluster

The bullet cluster is formed by the collision of two clusters of galaxies. After the collision, the stars and galaxies in those two clusters passed through each other. But the intergalactic gas clouds ...
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2answers
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Could the cosmic fluid be better described as a kind of van der Waals gas, or a kind of foam, instead of a dust gas?

In cosmology, it is usually assumed that the cosmological fluid made of galaxies could be described as a gas of "particles" without any pressure (the dust gas), of density $\rho_{\text{matter}} \...
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0answers
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Total number density of galaxies and problematic expression in practise

I am asked to give the formal expression of the total number density of galaxies and explain why is this expression problematic in practice. From what I saw from my research and into my lectures, I ...
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2answers
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How can a galaxy not be disk shaped [closed]

If a galaxy is the accretion disk for a supermassive black hole, how can it be a shape other than disk?

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