What is Galaxy: Definition and 518 Discussions

A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million (108) stars to giants with one hundred trillion (1014) stars, each orbiting its galaxy's center of mass.
Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Many galaxies are thought to have supermassive black holes at their centers. The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. As of March 2016, GN-z11 is the oldest and most distant galaxy observed. It has a comoving distance of 32 billion light-years from Earth, and is seen as it existed just 400 million years after the Big Bang.
In 2021, data from NASA's New Horizons space probe was used to revise the previous estimate of 2 trillion galaxies down to roughly 200 billion galaxies (2×1011). This followed a 2016 estimate that there were two trillion (2×1012) or more galaxies in the observable universe, overall, as many as an estimated 1×1024 stars (more stars than all the grains of sand on planet Earth). Most of the galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3,000 to 300,000 light years) and are separated by distances on the order of millions of parsecs (or megaparsecs). For comparison, the Milky Way has a diameter of at least 30,000 parsecs (100,000 ly) and is separated from the Andromeda Galaxy, its nearest large neighbor, by 780,000 parsecs (2.5 million ly.)
The space between galaxies is filled with a tenuous gas (the intergalactic medium) having an average density of less than one atom per cubic meter. The majority of galaxies are gravitationally organized into groups, clusters, and superclusters. The Milky Way is part of the Local Group, which it dominates along with Andromeda Galaxy. The group is part of the Virgo Supercluster. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids. Both the Local Group and the Virgo Supercluster are contained in a much larger cosmic structure named Laniakea.

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  1. Lotto

    What is the mass of the Galaxy's core?

    It is clear that the speed is constant because dark matter hasa gravitational effect on stars, so when a star is further from the core, gravitational force of it is smaller, but the net gravitational force of dark matter is bigger. So the net force acting on each star has to be the same. So...
  2. Onyx

    B Creating Metric Describing Large Disk

    How can I create a metric describing the space outside a large disk, like an elliptical galaxy? In cylindrical coordinates, ##\phi## would be the angle restricted the the plane, as ##\rho## would be the radius restricted to the plane. I think that if ##z## is suppressed to create an embedding...
  3. A

    A Advice needed for exposure time computation for galaxies

    I am trying to compute the exposure time needed for an extended object (galaxy), in python. I have found the following formulas: Exposure time app # press the help button at the bottom of the calculator for the formulas used Exposure time calc Let's take for example the M33 galaxy. It has...
  4. Ranku

    I Mass measurement of galaxies with non-gravitational means

    What are the non-gravitational ways of determining mass of galaxies, and how accurate are they?
  5. S

    B Supernova in Pinwheel Galaxy!

  6. K

    I Deur Gravitational self-interaction Doesn't Explain Galaxy Rotation Curves

    Deur Gravitational self-interaction Doesn't Explain Galaxy Rotation Curves this paper A. N. Lasenby, M. P. Hobson, W. E. V. Barker, "Gravitomagnetism and galaxy rotation curves: a cautionary tale" arXiv:2303.06115 (March 10, 2023). Directly comments on Deur's theory of self-interaction...
  7. F

    B Map of M87 Galaxy & Universe: Find & Explore Here!

    Does anyone know where I can find the map animation of the galaxy / nearby galaxy cluster that illustrate the geometry between Earth and M87 at the 29th minute in the Netflix show ? Alternatively if that's not a public source, can anyone recommend a publicly available "map of the universe", I'd...
  8. bakerjay

    A Data on galaxy rotation curves vs visible matter

    I'm after some raw data for testing theories of dark matter in galaxies. Basically what I want is table showing visible mass vs total mass within different radii (or, observed rotational velocity vs expected rotational velocity without dark matter). Plus error percentages. And ideally, for...
  9. FactChecker

    B Are the stars that we see in constellations within the Milky Way galaxy?

    Are the stars that we see in constellations within the Milky Way galaxy? Should I assume that they are close and within the Milky Way?
  10. mfb

    I New IceCube results November 3 (neutrinos from galaxy NGC 1068)

    Announcement Thursday 1 pm CDT, that's 18:00 UTC. This post was made 1:20 UTC. Conversion help. It's possible they picked up some neutrinos associated with some event observed by other means, or maybe we get an update on extremely high energy neutrinos.
  11. Barbequeman

    Surface density of stars in a Galaxy

    a.) The scale length of the disk is the length over which the surface density of stars decreases by a factor of e. In this case, the surface density decreases by a factor of 10 over a distance of 9 kpc, so the scale length is 9 kpc. The surface density of stars at a radius of r from the center...
  12. AotrsCommander

    Structure of the Galaxy: 6 Questions Confirmed

    Summary: I am looking to confirm a few things on the structure of the galaxy as we know it to make sure I have a credible enough understanding for a sci-fi project I'm working on. Background to question: For my sins, I make sci-fi starships and ground forces wargames models as part of my day...
  13. S

    B So what's your go-to method to locate M31 (Andromeda Galaxy)?

    Here's the image that a lot of pop science articles use: Evidently, a lot of articles say to look for the Great Square of Pegasus, but it seems to me to not be a very noticeable asterism. I use the Cassiopeia's W deeper-side (i.e., right) arrowhead and count 3 lengths of the depth of the...
  14. pinball1970

    I Distant Galaxy study, dark matter and the CMBR

    The article is pop but there is a paper Pop article https://www.vice.com/en/article/g5vvjw/the-universes-oldest-light-reveals-unprecedented-dark-matter-patterns Paper. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.129.061301 The language is a little bit click bate, I just wanted...
  15. J

    B Exploring the Possibilities: A Pre-13.8 Billion Year Old Galaxy

    What would happen if they found a galaxy that dated to older than 13.8 billion years old? How would this affect cosmology?
  16. narrator

    B Temporal differences across a galaxy?

    Hi. I'm a science enthusiast. I haven't been here in a long while. This is something I've wondered about for quite some time. I'm wondering if the temporal differences across something so vast as our milky way galaxy might pose any implications for how our galaxy holds together (ours and other...
  17. C

    B Which Way Do Planets in Our Galaxy Spin?

    Do all the planets in all the solar systems in our Galaxy spin in the same direction ? Clockwise or anti clockwise depending on the viewers POV.
  18. A

    B Our Sun's position in the Milky Way Galaxy

    Is this sentence correct? Our Sun lies in one of the spiral arms of the Milky Way, about 2/3 of the way out from the center of the galaxy.
  19. A

    B How does the Milky Way galaxy move in the local Group?

    How does the Milky Way galaxy move in the local Group? Is there a circular motion around the center of the local Group like the sun moves around the center of the galaxy?
  20. D

    B Accelerating to a stop in the Milky Way Galaxy

    If the sun is traveling around the center of the Milky Way at a speed of 536,865 mph and we are tagging along with it, couldn’t a spacecraft travel to another solar system by stopping? Would it be possible currently to slow down from that speed to 0 mph? And lastly, why does physics make my...
  21. M

    B Alcyoneus Radio Galaxy: Myth or Reality? Discover the Truth Here!

    I saw on another forum mention of Alcyoneus radio galaxy and have been google searching but can't find any information on it from any reputable astronomy related websites. Is it real or just fake news?
  22. Buckethead

    B Dark matter in galaxy IC1101

    I've been reading up a little on IC1101, the largest known galaxy and there is not a lot of info on it, but it seems to have an unusually large mass to light ratio according to this popular article (see half way down): According to this article at 20kpc it's got 12:1 mass to light, at 200kpc...
  23. A

    I Question about the expanding Universe and the Andromeda Galaxy

    Hi If the universe is expanding, then why does the Andromeda Galaxy approach us? Does not it contradict Hubble's law?
  24. Astronuc

    I Ultraluminous X-ray source in the galaxy NGC 55

    https://phys.org/news/2021-11-ultraluminous-x-ray-source-galaxy-ngc.html Spectral and Temporal Properties of Ultra-luminous X-ray Source NGC 55 ULX1 https://arxiv.org/abs/2111.06637 Same paper - https://academic.oup.com/mnras/advance-article-abstract/doi/10.1093/mnras/stab3307/6428406 From...
  25. J

    The Genesis Project plans to use robots to seed the galaxy with life. Will this be possible in the future?

  26. F

    A Photometric Galaxy Clustering Error and Poisson Noise

    The error on photometric galaxy clustering under the form of covariance which is actually a standard deviation expression for a fixed multipole ##\ell## : ## \sigma_{C, i j}^{A B}(\ell)=\Delta C_{i j}^{A B}(\ell)=\sqrt{\frac{2}{(2 \ell+1) f_{\mathrm{sky}} \Delta \ell}}\left[C_{i j}^{A...
  27. S

    B Time Dilation in a Galaxy: Calculating Mass Effects

    Hello everyone - The gravitational force near the edge of the galaxy at point A (see attached image) can be calculated by assuming that all the galactic mass is located in the center of the galaxy. - In order to calculate the gravitational force in the middle of the galaxy (point B) we take...
  28. elcaro

    I Can magnetic fields explain anomalous galaxy rotation curves?

    Magnetic fields as an alternative explanation for the rotation curves of spiral galaxies ABSTRACT THE flat rotation curves of spiral galaxies are usually regarded as the most convincing evidence for dark matter. The assumption that gravity alone is responsible for the motion of gas beyond the...
  29. Bolhuso

    I Galaxy redshift and thermodynamics

    I just finished rereading the great "A Brief History of Time". To me, what stands out the most in this book, is its ability to keep raising questions while you read it. This thought came up. It's been stuck in my mind for days, so I will humbly post it here to get some feedback. Please forgive...
  30. M

    Could the Star Wars galaxy possibly be a void galaxy?

    A void galaxy is a galaxy that exists in a cosmological void like the Bootes void. Most galaxies are clustered into filaments. Here is a link to the Wikipedia page for a void galaxy https://en.m.wikipedia.org/wiki/Void_galaxy Anyway, in Star Wars Legends, the Star Wars galaxy has 7 companion...
  31. T

    B Some visuals for Size of Solar System, Galaxy, Universe

    I was cleaning out my email archive after being notified I was out of storage. I decided to keep these. Enjoy. (original at: https://dingo.care2.com/cards/flash/5409/galaxy.swf) This one would be impossible on a phone and slow on a laptop, use a fullsize computer if available. I couldn't...
  32. shahbaznihal

    A 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 122 in chapter 14 where the author talks about the Correlation function. I am able to reproduce...
  33. A

    Milky Way Velocity: How Fast Does Our Galaxy Move?

    Do we know at which speed the whole milky way is moving around in the universe? Around the next cluster I suppose?
  34. K

    I Will stars on the other side of the galaxy affect gravity here?

    Does intervening mass between two stars decrease the gravitational attraction between these two stars? Is gravity a local phenomenon in that local mass interacts with the surrounding gravitational field caused by the local star and distant stars?
  35. S

    I Calculating Time Dilation & Galaxy Rotation Curve

    Hello, What I understood from multiple answers on different threads is that the effect of the time dilation is too small to explain the galaxy rotation curve. I was advised to do some calculations in order to see it myself. And this is what I would like to do but I need some help. - What is...
  36. Ranku

    I Matter leaving a spinning galaxy

    By what means can a galaxy's spin increase whereby matter is expelled from the outer reaches of the galaxy?
  37. T

    I Fast Radio Bursts (FBR) detected in our own Galaxy

    FRBs apparently from "...a “run-of-the-mill” magnetar. Astronomers labeled it SGR 1935+2154..." About 30,000 light years away. https://news.mit.edu/2020/ultrabright-radio-flashes-detection-1104 The research paper is behind a paywall, but Abstract at...
  38. T

    B Finding the present day distance to a galaxy

    Hi everyone! Does using the distance modulus—m-M=5log(r/10)—on type 1a supernovae only provide the distance to the host galaxy at the time of the supernova? If so, how would I mathematically determine the present day distance to a galaxy by taking into account both the expansion of the universe...
  39. Michael_L

    I How to find the distance from a star to the center of the Milky Way Galaxy?

    The distance of the sun to the Galactic Center is about 7.4–8.7 kiloparsecs. If I use the known data of stars in various stellar catalogues, how can I calculate (approximately) a distance of a given star from the Center? What kind of data I need to look for? can it be calculated from Right...
  40. Sagittarius A-Star

    B Explaining Galaxy Rotation Speed Discrepancies with GR & Gravitic Fields

    The speed of the ends of the galaxies is higher than what it should be. Current solution: This could be explained by hypothetical "dark matter", which was not found up to now, or by a MOND theory (MOdified Newtonian Dynamics). Can this be explained instead with rotational frame-dragging...
  41. Spinnor

    B The Hubble Telescope Images a Magnificent Galaxy With “Flocculent” Spiral Arms

    Had to Google Flocculent, adjective having or resembling tufts of wool. "the first snows of winter lay thick and flocculent" having a loosely clumped texture. "a brown flocculent precipitate"...
  42. astroYEEET

    Mass, luminosity and magnitude of a galaxy

    So i first calculated the angular area of the galaxy. a=22.6'' and a/b=0.85 => b = 26.588''( btw I do not know why a/b =0.85 since b is the semi minor axis). Then the area of the elipse is a*b*π =1887.745''. Then using the first equation we get an apparent magnitude of m=16.511. Using the second...
  43. T

    I A cold, massive, rotating disk galaxy 1.5 billion years after the BB

    Neeleman, M., Prochaska, J.X., Kanekar, N. et al. A cold, massive, rotating disk galaxy 1.5 billion years after the Big Bang. Nature 581, 269–272 (2020). https://doi.org/10.1038/s41586-020-2276-y Abstract Massive disk galaxies like the Milky Way are expected to form at late times in traditional...
  44. Adec

    Comp Sci Galaxy simulation with the velocity Verlet algorithm

    To simulate the trayectories of solar systems around a black hole (i.e. a galaxy) I have 3 classes in C++: cSystem, cBlackHole and cGalaxy. cSystem assigns initial values of position, velocity, etc to a solar system. cBlackHole does the same but just for the black hole. And cGalaxy mixes...
  45. pinball1970

    I Gravitational lensing used to observe a distant galaxy

    The alert is here, https://www.sciencealert.com/supermassive-black-hole-jets-have-been-seen-slamming-into-gas-in-the-early-universe The abstract from the paper here, https://iopscience.iop.org/article/10.3847/2041-8213/ab7b7e The effects of jets from a black hole within the galaxy including...
  46. G

    Special places in the galaxy

    In Sf, we can regularly see planets and other places that are very different from what we see on Earth, or even in the solar system. How plausible are the following ones? - Dense asteroid storm. Maybe a new planet is forming, or a giant comet recently shattered? Can such thing exist in a...
  47. Spinnor

    B Simulation of a Galaxy, most detailed large-scale simulation to date

    From, https://www.universetoday.com/143977/watch-a-simulation-of-a-galaxy-from-the-big-bang-until-the-present-day/ "This was made possible by the Hazel Hen supercomputer in Stuttgart, where 16,000 cores worked together for more than a year – the longest and most resource-intensive simulation to...
  48. S

    B Galaxy size and civilizations

    We do not currently have the ability to detect a Kardeshev 1.0 civilization at Alpha Centuari. Here is a paper showing Alpha Centuari has infrared emission close to a K1.5 civilization. Emissions from a K1.0 would be completely lost in the noise. There is some evidence that Kardeshev 2.0...
  49. F

    Measuring the Transverse Velocity of a Galaxy

    See the attached figure. I understand that we look for the apparent transverse velocity v , for example through v = d/t (d - distance, t - time). The distance to the galactic nucleus is known as D. Though I am not sure how to read off the time from this figure.
  50. D

    I Time travel to the Andromeda Galaxy

    Hi, I recently learned that if you can travel at the speed of light, or nearly, you can reach the Andromeda Galaxy within 30 years, due to time dilation and bypassing 2.5 million years on Earth. Is this true?