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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How the Hertzsprung-Russell Diagram allows us to calculate distance to stars

I understand how to interpret a H-R diagram, in the sense that I know that the upper right top corner is occupied by cool stars, but they are very luminous so they must be big; and the bottom left ...
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Can a super-duper massive black hole eventually eat an entire galaxy?

Might there be some roaming black galaxies out there eating up other galaxies?
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Why does dark matter form a halo, Unlike normal matter? [duplicate]

Hi I am curious to know why dark matter forms a halo? Or why doesn't normal matter form a halo.What is the difference between the two My level is amateur
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Why do galaxies have a super massive black hole at their center?

I know this has now been a common understanding that every galaxy has a supermassive black hole at their centers, But how does this understanding came into existence? Even though these massive black ...
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What models do physicists use in predicting the movement of stars in a galaxy that has lead to a wide-spread conviction that Dark Matter exists?

My (possibly poor) understanding of the argument for Dark Matter's existence is that stars in a large galaxy move more slowly than "they should" (presumably due to either some simplified model of the ...
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Coriolis force effect on spiral galaxies

I know the Coriolis force explains the counterclockwise rotation of the hurricanes hitting the U.S. I wonder if a Coriolis effect also determines (or at least influences) the direction of rotation ...
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Can we see galaxies that tell us what the Milky Way will be like in future time?

When we are looking at any galaxy we look usually at past time. As in case of Andromeda Galaxy we are looking at 2.5 million years back. In the same way do we have galaxy which will show future ...
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187 views

Is it possible for the Oort cloud to account for the Dark Matter issue?

I'm no cosmotolicist, so forgive me if I'm completely off the mark here. A few years back, I read that there is not enough matter in the universe to account for the gravitational forces at play, ...
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Is it possible to estimate the number of galaxies in the universe?

According to this article the observable universe may contain 2 trillion galaxies. Assuming we know the large-scale curvature of the universe thanks to standard model of cosmology, can we estimate ...
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What is “first passage” in orbital mechanics?

Sometimes people talk about the "first passage" of an object falling in toward or beginning an orbit around a more massive object. I'm specifically thinking about this phrase in the context of ...
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Why are all galaxies roughly the same size?

what determines the stable mass of galaxies, stars, star clusters or even galaxy clusters? Is there some obvious way to determine the classes of stable mass clusters from the initial conditions of the ...
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How do we know the actual position of the Andromeda galaxy, if we are seeing 2.5 million years in the past?

Scientists estimate that Andromeda and The Milky Way are going to collide in about 2.5 million years, how accurate is that calculation?
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How does dark matter distribution help to identify it's composition

A question regarding this recently released data, and the paper https://arxiv.org/abs/1708.01538 that details it's distribution: Abstract: We use 26 million galaxies from the Dark Energy Survey (...
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Why is the specific frequency of globular clusters in cD galaxies so high?

I understand that cD galaxies, very large and bright galaxies, have more globular clusters than other galaxies. For example, by calculating the specific frequency for a cD galaxy, the number is ...
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Is there 'gravitational force of repulsion'?

According to hubble's law of universal expansion, the velocity of a galaxy moving away from ours is directly proportional to the distance between the two. Now velocity is increasing in direction away ...
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Does the Milky way have relativistic mass against galaxies which are moving away from it at high speeds?

According to the article cited behind, and to a post in Astronomy SE , there are galaxies moving away from Milky Way faster than light, even at speeds of 2.3c . According to this article Can two ...
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Why do galaxies over time become more refined? [duplicate]

Why do galaxies over time become more refined, ordered and defined instead of more random and disordered?
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Can the centers of galaxies act as particle accelerators?

At the centers of galaxies, the amount of dark matter is much higher than that of ordinary matter. So the centers can effectively gravitationally attract and accelerate ordinary matter- without much ...
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Does the orbital velocity of stars in a galaxy equal the rotational velocity of the SMBH at the centre?

Pretty straightforward, couldn't find anything on Google, was hoping someone might know. Thanks in advance.
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Why isn't Dark Matter distributed among ordinary matter?

DM: dark matter; OM: ordinary matter I know that DM has a distribution which is mostly concentrated in the center of galaxies. This was mainly deduced by accounting for gravitational effects on the ...
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When galaxies collide, what happens to dark vs. light matter?

I have read that galactic collisions have been used as evidence demonstrating the effects of dark matter. In particular, that luminous matter slows down and interacts while dark matter shoots through, ...
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Why are stars spherical whereas (some) galaxies are disks?

I read here that galaxies become disks if there is a lot of gas in them, since their angular momentum is conserved while their energy decreases due to collisions of the gas particle. I have two ...
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Do galaxies have a halo of neutrinos and cosmic microwave background?

If virial arguments as in "How can dark matter collapse without collisions or radiation?" allow concluding that dark matter could collapse to galactic halos purely gravitationally, then is this true ...
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Current constraints on Dark Matter self-interaction from galactic profiles

The self-interaction of dark matter may be small but it cannot be negligible if it is able to dissipate energy to relax into galactic clumps (necessary to explain galaxy rotation curves). According ...
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Why sun has a velocity with respect to the dark matter halo?

I was reading that Sun has a velocity with respect to the local dark matter halo of about 244 km/s. Both sun and the dark matter halo revolve around the Milky way. Since the local dark matter halo ...
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Is the expansion of space universal or local?; does matter impede the expansion of space?

Michael Strauss in his recent book "Welcome to the Universe" with authors Tyson & Gott describes that space is NOT expanding within galaxies but rather between galaxies. So then the expansion is ...
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How do we know the stars orbiting Sgr A* are orbiting a supermassive black hole and not just the center of mass of the Milky Way galaxy?

It is my understanding the best evidence we have for Sgr A* being the black hole at the center of our galaxy is the incredible velocities of the stars orbiting around it. But wouldn't the stars ...
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What is a “scale length”, and how do I calculate it for galaxies?

I'm trying to work through, understand, and apply concepts regarding mass models of galaxies. Looking at the Hernquist model, I'm finding the equation $$Φ(r)=−\frac{GM}{r+a},$$ where a is the scale ...
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What is the percentage of dark matter in the galaxy? [closed]

Assuming the mass of the stars and gas within NGC 5055 is $8\times10^{10} M_{sun}$. What is the percentage of the dark matter in it?
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From a planet in one of our satellite dwarf galaxies, would one be able to see the Milky Way?

Say there was a lifeform not unlike ours native to a planet somewhere in one of our nearest satellite galaxies, say Sagittarius, during their nighttime would they be able to see the Milky Way brightly?...
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Thin Disk Potential-Density Relation

Suppose that there is an infinitesimally thin disk in the plane $z=0$ with surface density $\Sigma(x,y)$. How do you show that the potential $\phi(x,y)$ in the disk satisfies $$\Sigma(x',y') = \frac{...
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Orbit of stars in a galaxy

Do all stars orbit around the galactic center? If yes what makes them orbit around the center , what object creates such a massive force which makes stars millions of light years far away from the ...
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1answer
238 views

Distribution of orbital velocities in a disk galaxy for N-body simulation?

I'd like to write an N-body simulation in which I collide two disk galaxies. To give you an idea of the accuracy I'm trying to achieve, I'm aiming to make this my screensaver at 30fps on my work ...
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1answer
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Direct Dark Matter Detection: relative velocity between WIMPs & Nuclei

In direct dark matter detection it is said that the relative velocity between the WIMPs, which form a DM halo, and the target nuclei on earth is of order $100 \frac{\text{km}}{\text{s}}$. How does one ...
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In large scale cosmological structure, are filaments, walls, sheets and nodes different terms for the same things?

As the question says - are these all essentially synonyms referring to the same phenomenon (described with different words depending on the specific superficial appearance)? Or do they convey a real ...
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Can a photon that is emitted from a denser part of the universe to a less dense part appear redshifted?

Galaxy one is located in a dense area of the universe and galaxy two is located in a less dense part of the universe. Would galaxy one appear red-shifted to galaxy two? Is the mass density at our ...
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Criterion for galaxy mergers, negative E?

I'm writing about galaxy mergers and came across this section in a book describing criteria for a merger. I don't understand how there can be a negative $\hat{E}$ yet this is the region where mergers ...
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On the trajectory of the Andromeda-Milky Way collision

The Andromeda-Milky Way collision is going to happen in approximately 4 billion years. What trajectory would the Andromeda galaxy follow on its path to collision with the Milky Way? How could this be ...
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Flat rotation curves and gravitational potential

I have been reading about spiral galaxies rotation curves, and I have a question I would like to clarify. For example, many of them have flat rotation curves after some characteristic distance $r>...
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1answer
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A paradox about distant galaxies [duplicate]

When we observe a galaxy farther than 13 billion light years away, we see that galaxy as it was 13 billion years ago. But back then, that galaxy was much closer to us ,if indeed we live in an ...
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2answers
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Is the Maffei 1 galaxy gravitationally bound to the Milkway Galaxy?

I watched a Kurzgesagt video explaining how the limit human exploration is the local group, because outside galaxies are accelerating away from us faster than the speed of light, (or faster than we ...
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How is it that we can detect CMB radiation but not the first stars and galaxies despite CMB originating from before the first stars were born?

The CMB originates from when the Universe became transparent, around 380,000 years after the Big Bang, and stars were born around 100 million years later. I know the first stars and galaxies aren't ...
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Could black hole mergers be the source of density waves that show up as galaxies' spiral arms?

Consider the following mechanism: a black hole occasionally falls into the supermassive black hole in the centre of a galaxy. This causes gravitational waves to be emitted that take e.g. 100,000 years ...
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Where are all the Population III galaxies?

We can now see back to 400 million years after the Big Bang. Yet we still haven't seen any Population III galaxies. Why is this? How do metal-rich galaxies evolve in just 400 million years?
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What is the evidence that distant galaxies are moving away from us with speeds greater than $c$, due to space expansion?

I came up with this query after @Rob Jeffries's answer to a previous question of mine. So, is there any evidence that distant galaxies are moving away from us with speeds greater than $c$, due to ...
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Is the gravitational effect of distant galaxies lost forever?

Hubble's law is usually expressed by the equation $$v = H_0D$$ According to this equation, the space between us and very distant galaxies, is expanding with a speed greater than the speed of light ...
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Gravitation and the structure of galaxies

When seeing artist's impressions of galaxies they always look somewhat like this: _ Even in school we have been taught that such a type of galaxy consists of spiral arms. According to Kepler's laws ...
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Where is the mass and gravitational influence of dark matter? [duplicate]

I'll have to ask an elementary level question about orbital motion and the mass/gravity that influences it. In solar systems, the large mass of the sun pulls on the planets and we see the velocity of ...
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Are we closer to an accurate 3 D image of our galaxy using the Gaia billion star data set?

From the BBC: Report on Gaia Mission Data 14th September 2016 Astronomers working on the Gaia space telescope have released a first tranche of data recording the position and brightness of over a ...
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What happens at the edge of a galaxy

What happens at the edge (around the optical radius) of a galaxy when it has a flat rotation curve? After some length scale: does the velocity start to decrease or is there a phase-transition-like ...