What is Galaxy: Definition and 522 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. 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...
  2. S

    B Are There Any Kardeshev 2.0 Civilizations in the Milky Way?

    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...
  3. 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.
  4. 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?
  5. S

    I Are the red dots in the Whirlpool Galaxy individual red supergiants?

    I was looking at a photo of this galaxy, and noticed a bunch of red dots in it. Could it be that these dots are individual red supergiants? (I used the photo at the Wikipedia article for reference.)
  6. D

    B Why does the Andromeda galaxy appear so tiny from our perspective?

    Hi, and thanks for reading. I asked this question in another forum too. If we agree that Andromeda is 2.5 million light years away, and its diameter around 220,000 light years, I don't see how it just appears as a regular point of light like any other star in the unaided eye. Something that so...
  7. K

    I Question about Galactic Rotation curves in the Milky Way galaxy

    The graph in Wikipedia, article Milky Way, section Galactic Rotation, shows the actual rotation speeds in blue and the calculated speeds due to observed mass in red. (The graph is to the right of the article.) At about 3 kpc the actual speed is about 205 km/s. To account for the decrease in...
  8. M

    Do you have to be beyond smart to be an engineer in the Star Wars galaxy?

    I could never be an engineer here on Earth. I’m too dumb. My math skills and spatial intelligence suck. I wonder. If I lived in the Star Wars galaxy, would it even be harder to be an engineer since everything is so much more advanced? I imagine engineering schools in the Star Ward galaxy only...
  9. H

    I Any direct measure of Universe expansion via galaxy subtended angles?

    As indicated in the title and summary, I'm wondering if there is any large scale astronomical effort to assess directly the universal spatial expansion assumed by the Doppler interpretation of the redshift/distance relationship, by measuring individual galaxy subtended angles over time. The...
  10. M

    What avenue of physics can a scientist in the Star Wars galaxy study?

    What I’m looking for is some new avenue of physics for a scientist in the Star Wars galaxy to be studying. They probably know what dark matter and dark energy are since Starkiller Base is powered by it. I’m sure they know about how the universe began, why it’s expanding and what it’s fate is...
  11. M

    B What is responsible for the spinning of a galaxy?

    If you fill a sink with water and then let it drain, the water swirls due to the rotation of the Earth influencing it. During star formation, the primordial accredisk disk spins due to it's galaxy's rotation as I understand it. This being the case, what force made / makes the galaxy spin to...
  12. DuckAmuck

    I Universe Expansion: Does Speed of Motion Matter?

    So the universe is expanding, and galaxies are getting farther apart from one another on average. Does this motion count the same as ordinary motion, in that if a galaxy is being expanded away from us at 0.5c, that clocks in that galaxy would appear to tick slower at 0.866 the rate of clocks here?
  13. G

    I Dark Matter not present in Galaxy when distance is Increased

    I thought this article was interesting (and here). It made me think, if a transformation is sufficient to eliminate or create Dark Matter... Then the mystery of the existence of Dark Matter would become a mystery of why all of the other distance measurements are wrong in a universe without Dark...
  14. Buzz Bloom

    B Galaxy Database with distances from Earth

    Summary: I am assuming that there is a database somewhere listing all of the galaxies which astronomers have found. If that is correct, my question is: What fraction of these galaxies in the database have a distance from Earth value associated with it, other than a distance based only on the...
  15. A

    Estimating the mass of our galaxy, and the number of stars in our galaxy

    A previous thread outlined the problem with a correct answer, however I don't understand where they got the formulas from. Here are the steps I've taken so far: 1. Convert 2*10^8 years to seconds = 6.3072*10^15 seconds (period,T) 2. The previous thread then went on to say you plug period and...
  16. Nathan Warford

    B Radio source Hercules A and galaxy 3C 348

    I have found plenty of information about the radio source Hercules A. Its large radio jets are 1.5 million light years long from end to end, the supermassive black hole that produces Hercules A is 3 to 4 billion solar masses, and the black hole exists in the center of galaxy 3C 348. However, I...
  17. B

    A How does the electromagnetic force influence a galaxy (if at all)?

    Like what are the chances that dark matter can be described with electromagnetism instead?
  18. F

    B How to find inverse steradian from arcmin^-2 for density of galaxy

    Hello, I am using a code on EUCLID future mission. The original author of this code has set a value for the density of galaxy equal to : ng = 354543085.80106884 I think this is expressed in inverse steradian. I think that EUCLID mission has a 30 arcmin^-2 value for density of galaxies...
  19. James Beedy

    Recessional Velocity of a galaxy

    I need to calculate the recessional velocity of a galaxy 500 Mpc away. I need to express my answer in KM, distance in speed, age of the universe in seconds and years.
  20. K

    How to measure the Sun's acceleration relative to the galaxy?

    How would the acceleration of our sun relative to our galaxy be determined? A spaceship in orbit around our sun, with engines turned off, is in free fall. Accelerometers will read zero. Turning the engines on, the accelerometers will read the acceleration due to the thrust from the engines...
  21. C

    About initial mass function and mean mass in stellar cluster

    Homework Statement Assuming a Salpeter IMF with upper and lower mass limits of 0.1 and 20 M⊙ respectively, calculate: (i) the mass point at which half the mass formed in a stellar cluster lies in more massive systems and half in less massive systems. ii) the mass point at which half the...
  22. 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...
  23. K

    I How is Dark Matter distributed around our galaxy?

    Wikipedia describes the distribution of Dark Matter as a halo around our galaxy. Are there any laws or equations on the distribution of dark matter? From what I have read, dark matter does not clump into stars, since it cannot radiate energy. It cannot be seen or directly observed. Does it repel...
  24. 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...
  25. wolram

    B A second galaxy with no dark matter

    https://arxiv.org/abs/1901.05973 It seems like there is some thing unexplained here, what is going on?
  26. D

    B Galaxy rotational curves vs time

    Hello to everyone, I'm trying to find some data about the relation between galaxy age and rotational curve... until now without success. Are there any teams working on this? Are there any studies in this direction? Thanks!
  27. S

    B Galaxy Collisions & Expansion: What's Going On?

    Hi, this is just a question I've had for years and have not been able to figure out because of conflicting information. I have read that space is expanding and that it's speeding up, but also have heard that in about 5 billion years our galaxy and Andromeda will collide. How is this possible...
  28. F

    B Why most stars seem to rotate the same direction in a galaxy?

    After much questioning and soul-searching I got my answers of why galaxies rotate, and for that I greatly thank the gurus in the forum (so I'll not forget: conservation of initial angular momentum, non-isotropic material scattering, slowdown of infalling materials and non-isotropic infalling of...
  29. F

    I Compute the total stellar mass from a galaxy with Red Giants

    Hello, Suppose that all stars in this galaxy were born in a single major-merger burst event about 10 Gyr ago If the luminosity in the B band (absolute magnitude in B-band is equal to -21.22) is dominated by stars of in the RG branch, with masses ##m \sim 1\,\text{L}_{\odot}## (within ##\sim...
  30. F

    From the original burst, fraction of stellar mass still on the Main Sequence

    <Moderator's note: Moved from a technical forum and thus no template.> Suppose that all stars in this galaxy were born in a single major-merger burst event about 10 Gyr ago. From this original burst, I want to compute the fraction of stellar mass still surviving as stars in the main sequence ...
  31. F

    I Infer the distance of a closest neighbor galaxy from correlation

    I am trying to estimate the distance of closest galaxy neighbor knowing the expression of number of neighbors into a volume ##\text{d}V##, the mean density ##n_\text{gal}## and the correlation function, i.e with this expression : ##\text{d}N=n_{\text{gal}}\,\text{d}V\,(1+\xi(r))## with...
  32. F

    Stargazing Telescope recommendations for watching a galaxy?

    I'm close to 60 years old now, and when I was a 12-14 years old kid my father had a small 50mm telescope -- I still remember how amazing the word "Tasco" looked back then! Back then I could watch the moon and Jupiter, although I was upset that Jupiter kept moving out of sight all the time. How I...
  33. Ennio

    I Gravitational field in the galaxy and Schwarzschild´s time dilation

    Is it possible to estimate the gravitational force of the center of a Galaxy (it could be Andromeda or the Milky way) to any point (such as a planet) of its Orbit? Furthermore is there such as Schwarzschild solution that calculates the time dilation of any point of an external Galaxy (e.g...
  34. F

    B Does galaxy formation conserve mass and angular momentum?

    Last week I posted in General Physics some questions about what happens in a collapsing gas cloud, and I was advised that total angular momentum is conserved. I thought of asking for extra clarification here, as that seems really amazing -- I apologize for asking the same thing twice. I use a...
  35. Simon Peach

    B The age of the Milky Way galaxy

    It seem incredulous to me that the Milky Way was formed just 13.775 million years after the start of the universe. If this is correct was it in the form that it is today?
  36. P

    B Exploring Time Dilation and the Galaxy: Effects of Gravity and Angular Velocity

    I'm just wondering about time dilation in regards to both special and general relativity within our galaxy. Does time move slower or faster relative to us within the inner parts of the galaxy? I know at the horizon of the black hole time is effectively not moving. This implies that locations...
  37. Andy_K

    B Galaxy Expansion or Attraction?

    First of all, sorry for my naive question here which likely doesn't make sense. The universe is currently expanding with galaxies receding from each other at increasingly faster pace. Is it possible that the universe is a closed hypersphere system, where galaxies are actually not actively...
  38. F

    Homework - A galaxy in the local universe

    Homework Statement I have the following questions as homeworks and I would like to get help. Here's some informations given to help us to answer : Photometry : U=11.60 B=11.16 V=10.20 Redshift : z= 0.00780 Central velocity dispersion : ##\sigma_{v}## = 210 km/s Introduction : The...
  39. 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...
  40. F

    I Understanding Gunn-Peterson through the Reionization epoch

    Into an extra-galactic physics course, I don't understand well the "Gunn - Peterson" effect on the spectrum of high redshift quasars (z greater than 3 or 4, up to the current limit of z = 6-7 I think) The observed fact is that there is an almost total absorption for the photons of the blue part...
  41. C

    B Treating a galaxy as a quantum system

    If a wave function could be assigned to a whole galaxy, would its mass spread along the wave? Could this account for the anomalies in our calculations for galactic spin?
  42. C

    Is There a Model to Define Earth's Position in the Milky Way?

    I was hoping someone knows of a model that can define a point on Earth at any given moment that is pointing in the direction of the Earth's path around the Milky Way. I realize that the point would always be changing.
  43. Prof Sabi

    I How on Earth This is possible? (distance to a far galaxy)

    https://amp.scroll.in/article/883720/how-we-proved-that-einsteins-theory-of-general-relativity-holds-true-even-beyond-the-solar-system Read the Section in this link: Cosmic alignment The galaxy we investigated has the catchy name ESO 325-G004 – let’s call it E325. Located some 450m light years...
  44. T

    B Did Astronomers Really Think the Milky Way Was the Only Galaxy in 1900?

    I read that in 1900 astronomers thought that our own Milky Way Galaxy was the only galaxy in the whole universe. This puzzles me because I presume they knew about the pinwheel structure of our galaxy and there were telescopes back in 1900( Yerkes for instance)which were large enough that could...
  45. Ranku

    B Density of matter at a galactic center

    Why is the density of matter greater near the galactic center?
  46. Lukeblackhill

    B Is the Milky Way the same as 'The Galaxy'?

    Good Morning, I've got a doubt originated from a statement of Berkeley Physics Course, Vol.1 - Mechanics, pg. 107, where in a footnote we read, "The systems are called galaxies, that one which contains our own sun is known as the Galaxy. The Milky Way is part of our galaxy". I've always...
  47. M

    I How do we measure the velocity of stars in a galaxy?

    I have read somewhere that the parallax method can be used to measure the distance to stars up to 400 light years away. I did a quick calculation and estimated that it means that our telescopes can resolve an angular movement of 8.15x10-3 arc seconds. Taking the distance between the sun and the...
  48. phinds

    I Galaxy with no dark matter? (NGC1052-DF2)

    Just noticed this article. Wonder if anyone here has further info. Sounds very interesting. http://www.bbc.com/news/science-environment-43543195
  49. N

    I Diameter of a Galaxy: Calculate Distance/Arc Min?

    My last thread got lots of hits and was a success so I thought I would start another discussion based on the Galaxy I was speaking about, In my previous thread we found the distance to the galaxy, now I want to take this further and work at the diameter of the galaxy. If two edges of this galaxy...
  50. N

    I What is the recession speed of a galaxy based on the wavelengths of two stars?

    I have been having a conversation with a few friends, and we have been discussing the speed of stars in a galaxy base of the wave lengths. We have 2 stars measuring wave lengths 494.6nm and 494.2nm. At rest the wavelength is 486.1nm. From this we managed to deduce that the speed of both stars...
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