What is Galaxies: Definition and 420 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. Ranku

    I What would the universe look like without dark matter?

    How different would the universe look without dark matter? Would galaxies have formed, allowing for the existence of the solar system and life?
  2. C

    Observing galaxies: area of sky would I need to survey

    Homework Statement Given that there are 10-2 Ellipticals per Mpc3 and my garden telescope can reach to 14 mag. How large an area of sky would I need to survey to find 100 Elliptical galaxies ? (assume the typical absolute magnitude for an Elliptical galaxy is -21 mag).Homework Equations...
  3. hyksos

    I Imaging galaxies receding at a velocity that exceeds c?

    There are galaxies that are so far away that metric expansion causes them to have a co-moving recessional velocity that exceeds the speed of light. However, those galaxies are also so far away that the time it took the light to reach us was itself billions of years in the passage of its journey...
  4. wolram

    B How far back can we see with light?

    https://arxiv.org/pdf/1902.10713.pdf Amazing observation, just how far back will we see?
  5. sbleuz

    I The use of 3D reconstruction techniques to build a galaxy model

    I'm learning about imaging techniques and projection/deprojection for images of galaxies at university. The big issue there is the fact that an image is a 2D representation of a 3D object (for example a galaxy), and to learn some properties of the object you need the 3D structure. What happens...
  6. M

    I Is a "collision" of galaxies noticeable on a planetary scale

    If, as suspected, the Andromeda galaxy "colides" with our own sometime in the far future, would the gravitational forces from passing stars have effect on our solar system? In accelerated models the interaction looks quite violent, but how would it be in our "uncharted backwaters of the...
  7. A

    I Dark energy and the rotation curve of spiral galaxies

    Recently I have studied that from the rotation curve of spiral galaxies, the nearly constt. behaviour of velocity of the stars situated far away from the central core suggests mass(r) ~ r ,rather than 1/√r as expected. Are there any other theory which proves the existence of dark energy ??
  8. W

    B The unusual motion of stars in galaxies

    Ive been lying in bed this morning wondering about the unusual motion of stars in galaxies.The whole disc rotating like a record at the same speed.Does it mean that the effects of gravity are simply altered by speed?Much like a wonder-wall.So the faster the speed of the object,or stars,the less...
  9. wolram

    B Active galaxies point to new physics of cosmic expansion

    https://phys.org/news/2019-01-galaxies-physics-cosmic-expansion.html can dark energy vary over the history of the universe?
  10. F

    I Infer the distance of a closest neighbor galaxy from correlation

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  11. F

    I Total number density of galaxies and problematic expression

    Hello, 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 have found the follwing relation which gives the number of galaxies ##N## with mass ##(m...
  12. T

    B Discover 4 Milky Way Arms: Older Galaxies?

    https://Earth'sky.org/space/new-map-confirms-four-milky-way-arms ...four spiral arms. The arms are where most stars in the galaxy are born. They contain most of the galaxy’s gas and dust, the raw ingredients for new stars. Two of the arms, called Perseus and Scutum-Centaurus, seem to be more...
  13. F

    Homework - A galaxy in the local universe

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  14. F

    Homework - Interparticle distance between galaxies

    Homework Statement [/B] I have the following questions as homeworks and I would like to get help. Question 1) Give the formal expression of the total number density of galaxies. Why is this expressoin problematic in practice? Question 2) In practice one uses a numerical value for the number...
  15. W

    I How many galaxies do we have red shift data for?

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  16. StillLearningToronto

    Astronomy- Initial Mass Function problem

    1. Homework Statement The Initial Mass Function (IMF) defines the distribution of stellar masses created in a star-forming event. The probability P(m)dm of forming a star with a mass between m and m + dm is given by P(m)dm = km−α dm where α is the exponent of the power law and k is a constant...
  17. A

    B Do we directly observe redshifted galaxies receding >c?

    Sorry for a basic question but I keep reading conflicting information on this. So, I know that there are distant galaxies that are being carried by the expansion of space faster than the speed of light relative to us. But were these objects actually receding >c when the light we are seeing was...
  18. StillLearningToronto

    I General Q: How do you measure a Galaxy's distance?

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  19. Sorcerer

    Other "Galaxies" a trustworthy open access peer reviewed journal?

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  20. jk22

    B Equivalent gravitational forces in other galaxies

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  21. I

    I What does L star mean in the Schechter function?

    I'm trying to understand the Schechter function, I read on Wikipedia that the L star term in the function is the 'characteristic galaxy luminosity where the power-law form of the function cuts off'. What does this mean exactly?
  22. SMBH

    I How to check if stellar/galactic spectrum extraction went well?

    I've been doing some spectrum extractions in IRAF, stellar and galactic, and I was wondering if anyone here had any ideas on how to tell if the extraction/calibration/etc went well? I have a bunch of finished projects but I'm not sure about how to tell if they're any good or not. Let me know if...
  23. pegamin

    Can gravity from other galaxies affect Earth, and how?

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  24. E

    B Does Hubble's Law imply that galaxies are accelerating away

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  25. Buckethead

    B Spiral galaxies: Multiple axies of rotation?

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  26. alantheastronomer

    Virial thereom as applied to clusters of galaxies

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  27. Buzz Bloom

    I Question regarding Evaporation of galaxies

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  28. A

    I Quasar Number Densities and Lifetimes

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  29. G

    I Are there galaxies with 2 cores?

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  30. W

    B The bizarre rotation of galaxies

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  31. wolram

    B Wobbling galaxies: New evidence for dark matter

    https://www.sciencedaily.com/releases/2017/10/171026103110.htm Astronomers have discovered that the brightest galaxies within galaxy clusters 'wobble' relative to the cluster's center of mass. This unexpected result is inconsistent with predictions made by the current standard model of dark...
  32. davidge

    B Black holes in the centres of galaxies

    Why is it believed that almost every big galaxy has a black hole on its center? Does the explanation rely on General Relativity?
  33. davidge

    I Number of galaxies in the Universe

    This may sound like a noob question, even more if it's asked by a undergraduate physics student, but here it's: How do we know about the existence of 100 billion galaxies out there (even that it's an average value)? I mean, how do we know about these exact number? Why not, say, 200 billion or...
  34. genphis

    I Galaxies moving away at the speed of light?

    I have as advised done some catching up on basic cosmology, looking at various discussion and lectures on the nature of the universe. Something that has always played on my mind, is that i remember reading and watching a documentary about the potential to achieve the velocity of light. It was...
  35. W

    B Why don't galaxies obey gravity?

    Hi all,my second question is this.Why don't galaxies obey gravity? In our solar system the closer to our sun the planet is the faster it rotates.So the outer planet takes much longer for a solar year. When we look at galaxies this gravitational law doesn't apply.The outside stars spin at the...
  36. Strange design

    I Observing Orbital Speed of stars within Galaxies

    Can someone point me to a resource that describes, or describe, how the orbital speed of stars within galaxies are observed and measured? I'm struggling with how these values can be attained with any confidence in their accuracy. Thanks :)
  37. davidge

    B How many planets can we estimate in other galaxies?

    How far are we from being capable of estimating the number of planets in other galaxies? At least, in any of the nearby galaxies that are already known to us? Would this require state-of-art equipments?
  38. J

    I Why Aren't Galaxies Formed After the Big Bang Referred to as Being Further Away?

    I’m joining this forum just to see if someone can answer this question in a way that I can understand, because it’s driving me nuts. I read about galaxies observed by the Hubble Space Telescope that were formed “shortly” after the Big Bang. For example, the article may say that they are 12...
  39. wolram

    B Is 200my enough for galaxies to form

    I thought that z= 7.5 was the maximum for galaxies to form after re ionization but this paper puts it at z=8.38, can galaxies have formed this early? arXiv:1703.02039 [pdf, other] Dust in the reionization era: ALMA observations of a z=8.38 Galaxy Nicolas Laporte, Richard S. Ellis, Frederic...
  40. Arman777

    I Are Galaxies Moving or is Space-Time Expanding?

    Are galaxies are really moving away.Or galaxies are stationary but space-time expends so galaxies move with it ? Or maybe they could be same thing ?
  41. acidmatic

    B Black holes are creating galaxies with their gravitational force

    I have never met such an idea... Can anyone comment?
  42. wolram

    B How does Ram pressure kill galaxies

    Reading this article it states that ram pressure can kill off star formation, can you tell me please what ram pressure is? https://www.sciencedaily.com/releases/2017/01/170116160620.htm
  43. A

    B Redshift of distant galaxies....

    ...does it not simply indicate that the galaxies WERE receding faster IN THE PAST?
  44. resurgance2001

    I Lyman Break drop-out technique

    Hi I have a question which is about the lyman break drop-out technique for identifying high red shift galaxies. I am basically trying to understand how the technique works. My current understanding is that the drop-out technique makes use of the fact that very high red shift galaxies...
  45. R

    B Relative speed between Galaxies

    My question is following: Is there another Galaxy in the Universe that has a relative speed to the Milky Way of 0.999c (near of light speed)? The implication of that is that our life 'observed' by extraterrestrial beings in that Galaxy might be millions of years, an eternity! This is because of...
  46. C

    I Dumb Question: How do we know galaxies are in SSCM?

    The observed galaxy rotation curve does not match the theoretical curve derived from the model that galaxies are in steady state circular motion (SSCM). The presence of dark matter would bring the SSCM curve up to the observed curve. But what observation demonstrates that galaxies are in SSCM in...
  47. Sanborn Chase

    B Spiral Galaxies: Why Do Arms Bifurcate & What's Moving In/Out?

    Why do the arms of spiral galaxies usually bifurcate? Is the visible matter in them moving in or out from the center?
  48. M

    I Exploring the Mystery of Cosmic Void Between Galaxies

    Hey guys, Regarding the cosmic void between galaxies, is it really empty? If you read articles from pop science sources mostly said "cosmic voids are devoid of matter", "there are full of dark matter", etc. Thoughts?
  49. P

    B Are all galaxies moving away from the Milky Way?

    Once again I am reading and trying to understand more about the different theories of the beginning of time/space. I don't know why I never thought about this, but they say that galaxies are moving further and further away. One day we will not be able to see them because they moved so far. My...
  50. henrco

    I Objective classification of spiral galaxies

    I have question that has been troubling me and I was hoping someone may be able to help me. I've always thought the Hubble classification of galaxies was a bit subjective. I was recently looking at some images online of galaxies and I was wondering how you might objectively observe and compare...
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