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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Can Black hole eat white hole or white hole eat black hole? [duplicate]

No light can enter black holes, but is it possible for a white hole to enter a black hole? Or vice versa? Could there be black holes inside the white holes? Or were there black holes inside them that ...
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Cosmological expansion around ropes between galaxies

The FLRW cosmology is a fair approximation of cosmological expansion for space between galaxies and clusters, however within galaxies themselves, is known to not hold, and in fact observational ...
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About the non-intuitive announcement at 12 May 2022 of the EHT team that spin axis of Sgr A* Black Hole facing Earth?

What is this all about?: At 12 May 2022 at the ESO official announcement live streaming event the EHT representatives claimed that the Sgr A* BH accretion disc spin axis is sort of facing Earth's ...
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How can we extract the Doppler shift of supernovae?

My understanding is that supernovae are used as standard candles, whose spectral lines indicate the recession velocity of the host galaxy. But the material from the supernova is ejected at a ...
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Does magnetism play a role in the formation of galaxies?

Forgive my ignorance as I know next to nothing about physics. From my layperson's understanding, galaxies are formed primarily by the interaction of gravitational forces of stars and planets, however, ...
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Could the center of the galaxy explode as a result of a chain reaction of stars going supernova?

I was reading "Ringworld" by Larry Niven. I'll spare you most of the details of the story, and say that one of the parts of the plot is that the center of the Milky Galaxy blows up, because ...
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Why is the ratio of dark matter to normal matter larger in galaxies than the cosmic average?

There seems to be a discrepancy between the ratio of dark matter to normal matter in the Universe (about 5 to 1 according to $\Lambda$-CDM) and the ratio of the average dark matter halo mass to the ...
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What is the reason for density fluctuations in the primary matter of the early Universe? [duplicate]

After a period of inflation and nucleosynthesis, small density fluctuations appeared in the primary gas, which became the "seeds" of the first stars and galaxies. What led to the appearance ...
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How many full orbits around the galactic center our Earth has done so far since its creation?

I have read that the estimated age of our Milky Way galaxy is 13.61 billion years which is by using our current size and status of our galaxy about 59.17 Galactic years which each galactic year ...
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Do counter rotating galaxies have dark matter?

Have counter rotating dark matter galaxies been observed? Counter rotating galaxies, you may already know, are galaxies where some stars or arms rotate in one direction and other stars or arms rotate ...
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In what type of trajectory do the Magellanic clouds move through our galaxy?

In what type of trajectory do the Magellanic clouds move through our galaxy? Can be estimated is it a elliptical, parabolic or hyperbolic trajectory? Although it may be a problem due to dark matter ...
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How do simulations of galaxies or star systems account for the propagation delay of gravity?

I've made simple $N$-body gravity simulations as a programming and physics exercise in the past. But they've never accounted for speed-of-light time delay. The way I'm thinking about this is ...
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What is the effect of a supermassive black hole at the centre of the galaxy on this galaxy's spectrum?

After analysing the spectrum of the spiral galaxy (featuring flat rotation curve) we are asked to draw a sketch showing the effect on the galaxy spectrum if the galaxy were to have a supermassive ...
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What is the smallest scale at which the effect of dark matter can be observed? [duplicate]

Dark matter is primarily postulated in the context of large scale things e.g. spiral galaxies, in order to provide additional forces where vanilla gravity doesn't seem sufficiently strong to explain ...
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Galactic Dynamics: spherical coordinates velocity integral help!

I have been studying galactic dynamics and the following is an extract from Binney and Tremmaine's 'Galactic Dynamics' book. I have been having some trouble to understand how in the (4.37) integrals, ...
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Why are galaxies much closer spaced (relative to their size) than stars?

I have read this question: So the average spacing is somewhere in the range of 10 - 100 times the size of the biggest galaxies. The peas I had for lunch today were (at a guess - I didn't measure them!...
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Scale for Hénon-Heiles Potential

I was reading about the Hénon-Heiles Potential and I read that it describes "non-linear motion of a star around a galactic center with the motion restricted to a plane" (Wikipedia Link). So ...
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Questions on observations of faster than light recession of galaxies [duplicate]

I've been reading several threads here and online articles trying to absorb the current understanding of observations of far galaxies receding faster than the speed of light, it is said because the ...
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Gravitational binding energy as alternative to dark matter?

Pondering this question: Casimir effect and negative mass and, in particular, the response of John Rennie "as the mass of any bound system is slightly less than the mass of its parts" I ...
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Is the center of mass of the visible galactic arms located close to the center of mass of the whole galaxy?

Is the center of mass of the visible galactic arms located close to the center of mass of the whole galaxy?
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Galaxy filaments data access

I'm a PhD student in applied mathematics seeking to analyze geometric structure of galaxy filaments and walls, specifically the curvature and branching nature of the filaments and walls. I'm curious ...
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Is it possible in principle to observe all galaxies in the observable universe?

Are there fundamental physical limitations that prevent the observation and cataloging of all (or almost all) galaxies in the observable universe? If there are no physical limitations, then what ...
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Does gravity affect dark matter?

On this photo the red color is ionized gas and a blue color is a fark matter. We see the shockwave on gas but the clouds of dark matter move through each other without any interactions. The dark ...
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Why does the classic textbook by Goldstein say that distant galaxies are inertial reference frames?

Why does pp 2 of the classic textbook by Goldstein say that distant galaxies is inertial reference system in the context of the mechanics of astronomical bodies? How could we identify an inertial ...
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Is Dark Matter in Motion?

What is known about the motion of dark matter, especially in galaxies? It seems as though a particular distribution of dark matter might be required to cause the very flat galactic rotation curves ...
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Would light sails be a practical means of travel in the center of Andromeda? [closed]

I've read that at Andromeda's center, stars are only 0.008 lightyears apart from each other. Is this a short enough distance for light sails to be a practical means of space travel, or is this still ...
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Do more distant galaxies move faster with respect to the cosmic background radiation rest frame?

Aparently our galaxy is moving approximately 1/500th lightspeed with respect to cosmic background radiation. While this is certainly fast, it stands to reason that it could had been a lot faster, so ...
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What is the "temperature" of the galaxy?

The temperature of a gas is a measure of internal energy density. If the atoms have a high kinetic energy, the temperature is high. Can we do the same thing with the galaxy, where the stars are like ...
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Is it possible something in the universe is not orbiting anything?

I thought I heard that astronomical bodies could be ejected from a galaxy. If a body was ejected from a galaxy, could it have been ejected faster than the escape velocity of the galaxy? If that is ...
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How do bars in barred spiral galaxies form?

There is this same question in astronomy stackexchange. But its only answer is unfortunately a rather unsatisfying quote which I could not even find within the provided link. Also, that question is ...
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Why does $10^{-31}$K change the orbits of the stars, but 2.7K does not?

Many people discussing the rotation curves of the stars in galaxies explain that the rotation curves are influenced by a cosmological acceleration of about $1.2 \cdot 10^{-10}\,\rm m/s^2$, and that ...
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How do galaxies move through space-time?

If we assume that galaxies only consist of planets, stars, and space, then how do galaxies move through space? How do you move empty space, or is it that space remains same, but the planets, stars, ...
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3 votes
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Is the size of far away galaxies smaller or larger than expected?

When items are further away, they look smaller. If we know the size of something nearby, then we can calculate the size of something a known distance far away. So if there is a galaxy whose size (...
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What are the Galactic Orbits?

For some reason, I can't get an answer in Google. I know the Galactic Orbits within the galactic plane are about circular, with constant angular velocity (which violates Kepler's laws, which is the ...
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What are other potentials like the Henon-Heiles Potential? [closed]

The Henon-Heiles Potential, as I have learned, is a two-dimensional nonlinear axi-symmetric potential of galactic systems. I wanted to know if there are any other potentials that also describe ...
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How did astronomers at Edwin Hubble time decide the size of the Milky Way Galaxy and settled the Great Debate?

I have been reading some books about Great Debate that whether the Milky Way Galaxy was the entire universe and Edwin Hubble settled the debate by identifying some Cepheid variable star (V1) in ...
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2 votes
2 answers
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How to interpret line of sight velocity distributions and dispersions of a galaxy?

What I did: I am analysing some simulation snapshots of the Milky Way and it is modelled as a DM halo – bulge – disc system. I produced some mean line of sight velocity (LOSV) and dispersion of mean ...
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Does barred in spirals galaxies like ours have the same speed (spin) as the center black hole?

In the case of old galaxies (elliptical) the black hole, like one in M87 have 2 jets. This look like polarisation pushing from central BH. In barred spiral galaxies, the bar have to cross the central ...
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Rotation of galaxies and gravitomagnetism [duplicate]

Can we explain the bigger speed of rotation of the galaxies from gravitomagnetic field acting from the "inner layers" of the galaxy to the "outer layers" of the galaxy?
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Effect of Newton's law to the orbits inside a galaxy like Milky Way

Does Newton's law describe the orbital motion of stars in a galaxy? Is it true that the closer a star is to the center, the lower is the gravity, so the lower the speed of the star?
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What would be the likelihood that when the Milky Way formed, it would be composed entirely of sun-like stars? [closed]

So, what would be the likelihood that when the Milky Way formed, it would be composed entirely of sun-like stars? What was the configuration of the gas, the history of the primordial stars, for this ...
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Why do they say universe will become cold with expansion?

The Universe is expanding and they say it will eventually become cold, and new star formation will stop. But galaxies are only moving away from each other, and each galaxy itself stays intact and the ...
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Finding the dominant frequency of a stellar orbit

I was reading this paper on astro-ph (Sanders et al. 2020, "Models of Distorted and Evolving Dark Matter Haloes"). It's about modelling the gravitational potential in a cosmological ...
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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. ...
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1 answer
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Can you explain what is Heliocentric velocity

In this photo, What is the meaning of heliocentric velocity ? Is the velocity at which the Sun's motion was corrected or before the Sun's motion was corrected?
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19 votes
3 answers
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If gravity is attractive, why doesn't the Milky Way contract?

Einstein introduced the cosmological constant because without it, one cannot get a static universe - gravity would cause the universe to contract. Given that, why is the Milky Way not contracting? For ...
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Age determination of stars

In the report 'New observations of the most distant galaxies close in on cosmic dawn', an astronomer states: 'Using a well-understood age indicator based on the amount of hydrogen absorption seen in ...
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Galaxy statistics calculation in Saslaw's book

I am trying to follow a calculation from the book of William C. Saslaw, The Distribution of the Galaxies: Gravitational Clustering in Cosmology. The calculation is shown on the pages following page ...
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Galaxies: Negative pressure, positive feedback

As part of research trying to develop a cosmological model that can be static (scale-factor), but in dynamic equilibrium - a way is being considered whereby dense collapsing regions of matter 'bounce' ...
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Compute the variance of spectroscopic Shot Noise in cosmology context

I would like to know the right expression for the expression of variance of Shot noise in spectroscopic probe. Sometimes, I saw $$\sigma_{SN,sp}^{2} = 1/n_{sp}$$ with $n_{sp}$ the average density of ...
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