Galaxies Definition and 424 Threads

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. 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?
  2. 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?
  3. 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...
  4. 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...
  5. G

    Stargazing What telescopes are good for seeing planets and galaxies?

    Hello, I am currently considering on buying a telescope to do some of my own research and for sight seeing. Any recommendations? I would prefer to have a telescope that requires me to use code or at least a computer so I can familiarize myself with the mechanics.
  6. V

    I Database of Galaxies and their parameters

    HI, I am looking to find a table of galaxies, their masses, types, black hole mass and dark matter content. Does anyone know if such a place exists anywhere on the internet? I'm trained in physics but is not a physicist - more a hobbyist. Any suggestion that you might have will be greatly...
  7. O

    B The news media on 2 trillion galaxies: Confused, or am I?

    Hello. So the last few days there have been articles about how our universe may now contain around 2 trillion galaxies, up from 100-200 billion galaxies that was previously determined. But unfortunately I can't make complete sense of the articles I've read, including the one on NASA's site...
  8. P

    Stargazing How can the telescopes take photos of distant galaxies?

    So how can the photos of distant galaxies have no nearer stars in front of them? Or photos of nebulae? I feel like there are so many stars.that it would be impossible to take a photo of a distant object without a star in front of the photo blocking the way. Or is it luck that there are no stars...
  9. C

    B Found: The Least Luminous Galaxy Ever Seen

    Lying 280,000 light-years from Earth, the Virgo I galaxy emits only 180 times as much light as the Sun, half as much as the previous faint-galaxy champ. The galaxy's proximity to the Milky Way suggests that our Galaxy has hundreds of other satellite galaxies. New Scientist has the exclusive...
  10. UHECRon

    A Photon densities within galtactic disc compared to CMB

    I am interested in what the average photon densities are within galactic environments due to non CMB sources. In extra galactic environments, the CMB certainly dominates, however as you enter more dense and luminous areas (e.g. , the galactic disc) broad spectrum photon densities from local...
  11. T

    I Why are stars grouped in galaxies?

    I have been thinking at this for a couple of days now: why are stars grouped in the massive collections that we call galaxies? I can assume that in the very early Universe, matter was grouped in these areas, that matter interacted thus resulting in the formation of stars. Then, the...
  12. ebos

    I Where galaxies are now or is there a universal 'now'?

    No matter what direction I point my super duper telescope, I will observe galaxies that are barely 500 million years old in every direction some of them being located 12+ billion light years away. So 2 galaxies on opposite side of the Earth should be twice 12+ billion light years apart. In other...
  13. R

    What is the predicted mass of the Coma cluster using a simple model?

    Homework Statement An estimate of the mass of the Coma cluster can be made from the velocity of the galaxies relative to the centre of mass of the cluster; the velocities are around 2000 km s-1. The stars that form the cluster have been together for a long time. Show that a simple model will...
  14. wolram

    B Is the Milky Way unique or do other galaxies have these voids in their centers?

    In this paper https://www.sciencedaily.com/releases/2016/08/160801210354.htm it states that the Milky Way has a huge void at its center, i can not think of any other galaxy that shows such a void, so is our Milky Way unique
  15. C

    B The Milky Way's Dark Twin Revealed

    A galaxy as massive as the Milky Way emits only 1% as much light. Link: Nature.
  16. WCOLtd

    I Rotation curves of galaxies as a function of isolation.

    Which galaxy has the largest angular momentum of all galaxies observed? What does its rotation curve look like? Which galaxy has the smallest angular momentum of all galaxies observed? What does its rotation curve look like? Which two galaxies are closest to one another? What do the rotation...
  17. J

    Apparent Flux and number of stars

    The stars in our Galaxy have luminosities ranging from $L_{\text{min}}$ to $L_{\text{max}}$. Suppose that the number of stars per unit volume with luminosities in the range of $L$, $L+dL$ is $n(L)dL$. The total number of stars per unit volume if clearly $$n = \int_{L_{min}}^{L_{max}} n(L)dL.$$...
  18. K

    I Redshifts of galaxies and the expansion of the universe

    (Sorry for my poor English.) I was watching a PBS video on expansion of the universe and the guy says the wavelength of a photon emitted in a supernova becomes larger as it travels to the Earth. Is it because the photon lose energy (to space)? If so, is that energy contributing to the expansion...
  19. K

    B Are massive gravitons what ripple when galaxies collide?

    Is a sea of massive gravitons what ripples when galaxy clusters collide? Accepted at APS Physical Review D: https://journals.aps.org/prd/accepted/8b07dQ8dFbd1051b06704364802ee6ba42eea6f5e Precursor: Bigravitons as dark matter and gravitational waves We consider the possibility that the...
  20. K

    A Charged black holes and charge balance in galaxies

    Greetings, There seems to be a supposition in astronomy that charged black holes are uncommon. Is this assertion well supported? According to a paper I found by Briet and Hobill, if you look at gravitational lensing alone charged black holes look just like regular black holes, but with a mass...
  21. Buckethead

    B Expansion of space and momentums of galaxies

    It is my understanding from reading a few threads that remote clusters don't have any "speed" relative to us, they are simply moving away. Does this mean they have no momentum? And if that is true, does that imply that they have no mass (in this context, not in a local context). And if that is...
  22. A

    I The "real" reason why some galaxies are flat

    I'm doing research on why some galaxies and other phenomenon like Saturn's rings are flat. There seems to be two answers. Here they are in shortened form: 1) When the galaxy is forming, there is some net angular momentum in some direction.Over time, all particles with an angular momentum...
  23. wutronic

    B Are galaxies part of constellations?

    reading up on the local group and in the wiki it says galaxies have constellations: https://en.wikipedia.org/wiki/Local_Group does this mean they are just in that area, or they are visible from Earth as part of the constellation? I thought Andromida and possibly only Canis major dwarf were...
  24. J

    B Can we identify the centre of the Universe?

    Hi I have two novice question If the universe is expanding and it is all moving apart, how is it that galaxies are colliding? If our galaxy and all others are moving away from some singularity central point. And as we see other galaxies moving away from us at a speed relative to there distance...
  25. D

    I Measuring Speed of Light from Galaxies & Shapiro Time Delay

    Usually we measure the speed of light using light generated here on Earth or coming from the Sun or other stars, but have we ever tried to measure the speed of the light coming from distant galaxies? As in directing that light into an apparatus and measure its average velocity over a round-trip...
  26. H

    B Entropy is greater in galaxies or pre atomic era

    1. I understand an expanding gas has increasing entropy and at a cosmic scale the universe is an expanding gas...sort of. 2. back before the universe was cool enough to form atoms it would seem to be very disordered, ie a high temperature universe of a plasma made of nuclei and elementary...
  27. secur

    A Do Large Galaxies Exist Without Central Supermassive Black Holes?

    I'm wondering about large, regular galaxies that lack SMBH's. Here are some possible examples I found. The general question is, what large galaxies have no central SMBH? ----------------- A2261-BCG: From the Astrophysical Journal, 756:159, 2012 September 10, "A BRIGHTEST CLUSTER GALAXY WITH AN...
  28. M

    Stargazing Receding Galaxies and the Range of Gravity

    Hello, Sorry if this is in the incorrect forum. I've come across a discussion that has made the following argument, but I'm unable reconcile the views. I've searched within and without and have come across no answer that addresses this specific claim. To begin, from my understanding gravity...
  29. CrackerMcGinger

    B How exactly do SMBHs in the center of galaxies form

    I've been thinking for awhile now, and I still don't understand how these types of black holes form, since they can't form by conventional means.
  30. Einstein's Cat

    Gravitational Lensing of Fast Receding Galaxies: Does Speed of Light Matter?

    I am aware that the greater a body's velocity the greater its relativistic mass. As a result, I assume that the faster a galaxy is receeding from us, the greater its gravitational lensing affect is. My question is this: does the extent of a galaxies gravitational lensing continue to increase at...
  31. H

    Measuring the Distance Between Two Galaxies

    Homework Statement The diameter of our disk-shaped galaxy, the Milky Way, is about 1.8E+5 light-years (ly). The distance to the Andromeda galaxy, which is the spiral galaxy nearest to the Milky Way, is about 1.8E+6 ly. If a scale model represents the Milky Way and Andromeda galaxies as dinner...
  32. C

    Is There Evidence for Redshift Drift in High-z Galaxies?

    I am interested in locating information of the evidence of z evolution that has been collected of the spectra of relatively high z sources over the previous 75(35) years. Before I get too far into the question, I thought I would try to find what research might have been done on the subject...
  33. Astronuc

    Are Dual Supermassive Black Holes in Galaxies Rare and Insignificant?

    Apparently rare - only 12 galaxies are known to exist with two black holes in their midst, according to Dr. Julie Comerford. Comerford reported a recent observation of a double black hole galaxy at the American Astronomical Society's annual meeting in Kissimmee, Florida. One of the BHs is...
  34. V

    Length Contraction & Galaxies: Time Dilation Adjustments

    Since galaxies are moving away from us, shouldn't they be contracted in length than they would be if they were at rest (wrt us)? In other words, are we observing increasingly shrunken galaxies as we look deeper into space? When measuring supernova light curves, we do adjust for time dilation...
  35. E

    Does time dilation occur between comoving galaxies?

    As an astronomer observes a galaxy, which is receding, Does time dilation occur between the two comoving galaxies? If so, which (rf) is (t) longer in, as observed here on Earth (Our local Milkyway galaxy or the remote galaxy)? Thank you.
  36. T

    Ancient Galaxies twice the size of the Milky Way

    I saw this news about galaxies twice the size of the milky way observed a relativity short time after the BB and wondered if this is still consistent with the BB theory or its timing? http://news.yahoo.com/ancient-monster-galaxies-scientists-perplexed-122353576.html I also wanted to ask if...
  37. C

    Good 1st year grad level texts in galaxies and cosmology

    I'm looking for some appropriate 1st year, or possibly 2nd year, level grad texts on galaxies and cosmology. The grad course at my university requires Cosmological Physics by Peacock and Galaxy Formation by Longair. I've noticed many old cosmology course websites require Weinberg or Dodelson or...
  38. E

    Weight of object in outer space away from galaxies

    Homework Statement Suppose an object were sent far out in space away from galaxies, stars, or other bodies. How would its weight change? Homework Equations w=mg where w and g are vectors The Attempt at a Solution Since mass is an intrinsic property and doesn't depend on external sources, then...
  39. J

    A question about the flatness of galaxies

    Good day to you all, I'm very sorry since there are countless of topics that have discussed this question before. Call me dumb, but I've read almost every single one of them and I'm still having a bit of a hard time understanding why most galaxies have their planets, stars, dust, gas etc. all...
  40. Earnest Guest

    Why do so many galaxies in a cluster have near zero velocity

    I'm looking at a velocity chart of the Coma Cluster: And the question occurred to me: why are there so many galaxies that have a zero velocity (relative to the core of the Coma Cluster which is roughly 7,000 km s-1)? At a distance of a Mpc or two you would expect to find galaxies at the peak...
  41. S

    Galaxies Colliding: A Rare Phenomenon in Space

    have we ever seen galaxies collide?
  42. Armando Valle

    How do we know galaxies are exactly where we see them?

    i have a doubt, how do we know far away (light years) galaxies and planets are on the right spot we see them? if light takes a lot of time to reach the Earth it means we are seeing the past of the galaxy/planet. Example: If Earth is becoming extinct and we need to find an exoplanet 30,000 light...
  43. RyanH42

    Is dark matter distrubition always near galaxies?

    Is there any dark matter where no baryonic matter (which here the galaxies).I mean are we observe dark matter only around the matter ? Or dark matter can be anywhere (In empty space which there's no baryonic matter around)
  44. Dry

    Expansion is a compounded error based on a wrong assumption?

    I accept that what I write below will be unpopular and argued with vehemently, however I think it should be considered with an open mind and recognised for the sense that it has. Originally the concept of an expanding universe (and therefore by extrapolation the concept of the Big Bang that...
  45. RyanH42

    Why dark matter distribution is so different in galaxies?

    I learned that dark matter distrubition is homogeneius and isotrophic in cosmic scales. I searched some galaxies dark matter distrubition.And I am actually suprised. Why cause every galaxy has a different ratio of dark matter baryonic matter distrubition. In cosmic scales baryonic...
  46. T

    How can we see galaxies behind other galaxies?

    How can we see galaxies behind other galaxies?
  47. L

    Origin of Galaxies and Super Massive Black Holes

    I just watched a popular science program entitled Super Massive Black holes that proposed the idea that galaxies were formed long ago from hydrogen gas clouds whose centers collapsed to become super massive black holes that then generated star formation and ultimately the formation of galaxies...
  48. marcus

    800+ ultra-diffuse galaxies in Coma cluster

    The galaxies are unusually depleted of ordinary matter---estimated 99% dark matter---and star-formation in them has stopped (they've run out of gas or it has been drawn out of them by other galaxies in the cluster.) http://arxiv.org/abs/1506.01712 Approximately A Thousand Ultra Diffuse Galaxies...
  49. C

    814 Ghostly Galaxies in the Coma Cluster

    The Subaru Telescope has spotted hundreds of "ultra-diffuse" galaxies in this famous cluster. Link: ScienceNOW
  50. C

    Hubble redshift and calculation of galactic rotation curves

    I am interested in whether it is necessary to account for the effects of the Hubble Redshift in determining the rotation velocities of galaxies exhibiting keplerian motion and, in particular, whether the associated spatial expansion of the Universe is known to result in spectral shifts that...
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