5 ms·

Why are there so many stars and galaxies?

by cdelsolar 3y ago

q845712 3y ago
if it helps, that's just the stars and galaxies we're capable of observing. There's probably more :)
- qup 3y ago
  It's fun that we can't say for sure. I wonder how long before we can
  - PartiallyTyped 3y ago
    The other side to this is that given enough time, some civilisation will be oblivious to the rest of the universe. Their visible universe will just be their own galaxy.
    - alx 3y ago
      and what could they deduce from the void they have in front of them?
      - PartiallyTyped 3y ago
        Probably nothing. The problem is that at such timescales, the CMB will have shifted sooooooooo much that there’s nothing they will be able to deduce, all light has redshifted to absurd scales, and worst of all, everything will be so far away that light will never reach the galaxy.
    - BitwiseFool 3y ago
      That means they would have no way of discovering that the universe is expanding, right?
      - PartiallyTyped 3y ago
        NB: Armchair """""physicist""""" If their physics is correct, they will figure it out. We did because we verified that expansion is accelerating even though our physics are incomplete. Until the late 90s, physicists accepted a cosmological constant of 0, which meant no acceleration. Turns out that that was wrong. It may be possible that they figure it out via Quantum Mechanics because it seems that acceleration of the universe is related to the energy density of a vacuum / empty space. The problem is that they won't be able to empirically verify that I think.
  - r2_pilot 3y ago
    The physical continuing expansion of the universe eventually causes any photon to be so far redshifted as to be unobservable, so it actually goes the opposite way. One day long from now, an alien civilization might look out into their night sky and only see their galaxy, and be completely unable to tell the previous history of the universe from observing the sky.
    - Sanzig 3y ago
      There's also a possibility that a sizeable fraction of all stars exist in the intergalactic medium [1], having been ejected from the galaxies where they formed due to galactic collisions or encounters with their original galaxy's supermassive black hole. A civilization evolving around one of these stars in the far future would be totally unaware of the universe outside their own solar system due to cosmic expansion. [1] https://en.wikipedia.org/wiki/Intergalactic_star https://en.wikipedia.org/wiki/Intergalactic_star
  - asimovfan 3y ago
    I agree, things couldn't be fun if we actually knew what they were
ceejayoz 3y ago
There are probably hundreds of possible answers to that, depending on which aspect you're interested in, ranging from "God" to "gravity" to "42" to "because" to "I dunno".
deleted 3y ago
[deleted]
igleria 3y ago
at risk of being too tautologic: Because for us this is "so many". Because of that we try to compare the scale of the universe with the amount of sand on earth etc etc. At least for me, that type of information does not give me a "better" idea of the problem at hand other than "duuuuuuuuude what".
kirykl 3y ago
see Feynman on "why" https://www.youtube.com/watch?v=36GT2zI8lVA https://www.youtube.com/watch?v=36GT2zI8lVA
TechBro8615 3y ago
Probably for the same reason there are not zero.
Koshkin 3y ago
Probably because there may be an upper limit on the size of an individual celestial object (and thus on how many particles it can consist of) - for it to be anything other than a black hole, anyway. In the future, the Universe will entirely consist of black holes, whose number will be much smaller.
- dredmorbius 3y ago
  I don't know what limits there are on the size of galaxies, though I suspect it share some characteristics with that of stars. For star formation, however, the tension is between the outward flow of matter and simple photonic pressure on interstellar gas, competing against the inward pull of gravity. That is: stars form as large "dust" clouds (mostly hydrogen atoms, obviously), collapse. Once those reach a critical mass, fusion initiates within the stellar core. The rate of fusion is moderated by the pull of gravity, such that larger stars, fighting more inward mass, burn far hotter (and faster, and live comparatively short lives: a few tens of millions of years for the very largest, as compared with ~10 billion for the Sun, and potentially trillions for brown dwarfs which eke out their fuel slowly). In a curious bit of physical synchronicity, the energy density of the Sun's core (the part which is actively generating energy), in terms of watts/kg, is roughly equivalent to that of mammalian metabolism. It's just that there's a whole lot more mass to the Sun's core than there are mammals, and of course, we're comparing nuclear reactions with chemical ones (the Krebs Cycle). Back to stars: as they ignite, they start blowing the dust that's forming them away, and so limit their maximum growth potential. The least massive stars (brown dwarfs) are roughly 10x the mass of Jupiter, or about 1/100th the mass of our Sun, the most massive stars are roughly 100x the mass of the Sun, giving a total effective range of roughly 1:10,000. The upper bound, called the Eddington Mass Limit, is about 120 solar masses, and is based on total luminosity which translates to outward pressure exerted by the star against the stellar medium surrounding it: <https://en.wikipedia.org/wiki/Eddington_luminosity https://en.wikipedia.org/wiki/Eddington_luminosity> I don't know what the dynamics governing galaxy growth are, though a few ideas come to mind. Apparently the most massive galaxies are about 100x the size of the Milky Way (so we're at roughly similar ends of the scale for both stars and galaxies in that regard) in the Physics StackExchange post below. The discussion there does mention luminosity and pressure, though there's also some disagreement over just what constitutes a galaxy, and where to draw borders. <https://physics.stackexchange.com/questions/172953/what-is-the-limit-for-the-size-of-a-galaxy https://physics.stackexchange.com/questions/172953/what-is-t...>

Why are there so many stars and galaxies?

q845712 3y ago

if it helps, that's just the stars and galaxies we're capable of observing. There's probably more :)

qup 3y ago
It's fun that we can't say for sure. I wonder how long before we can
- PartiallyTyped 3y ago
  The other side to this is that given enough time, some civilisation will be oblivious to the rest of the universe. Their visible universe will just be their own galaxy.
  - alx 3y ago
    and what could they deduce from the void they have in front of them?
    - PartiallyTyped 3y ago
      Probably nothing. The problem is that at such timescales, the CMB will have shifted sooooooooo much that there’s nothing they will be able to deduce, all light has redshifted to absurd scales, and worst of all, everything will be so far away that light will never reach the galaxy.
  - BitwiseFool 3y ago
    That means they would have no way of discovering that the universe is expanding, right?
    - PartiallyTyped 3y ago
      NB: Armchair """""physicist""""" If their physics is correct, they will figure it out. We did because we verified that expansion is accelerating even though our physics are incomplete. Until the late 90s, physicists accepted a cosmological constant of 0, which meant no acceleration. Turns out that that was wrong. It may be possible that they figure it out via Quantum Mechanics because it seems that acceleration of the universe is related to the energy density of a vacuum / empty space. The problem is that they won't be able to empirically verify that I think.
- r2_pilot 3y ago
  The physical continuing expansion of the universe eventually causes any photon to be so far redshifted as to be unobservable, so it actually goes the opposite way. One day long from now, an alien civilization might look out into their night sky and only see their galaxy, and be completely unable to tell the previous history of the universe from observing the sky.
  - Sanzig 3y ago
    There's also a possibility that a sizeable fraction of all stars exist in the intergalactic medium [1], having been ejected from the galaxies where they formed due to galactic collisions or encounters with their original galaxy's supermassive black hole. A civilization evolving around one of these stars in the far future would be totally unaware of the universe outside their own solar system due to cosmic expansion. [1] https://en.wikipedia.org/wiki/Intergalactic_star https://en.wikipedia.org/wiki/Intergalactic_star
- asimovfan 3y ago
  I agree, things couldn't be fun if we actually knew what they were

ceejayoz 3y ago

There are probably hundreds of possible answers to that, depending on which aspect you're interested in, ranging from "God" to "gravity" to "42" to "because" to "I dunno".

deleted 3y ago

[deleted]

igleria 3y ago

at risk of being too tautologic: Because for us this is "so many". Because of that we try to compare the scale of the universe with the amount of sand on earth etc etc. At least for me, that type of information does not give me a "better" idea of the problem at hand other than "duuuuuuuuude what".

kirykl 3y ago

see Feynman on "why" https://www.youtube.com/watch?v=36GT2zI8lVA https://www.youtube.com/watch?v=36GT2zI8lVA

TechBro8615 3y ago

Probably for the same reason there are not zero.

Koshkin 3y ago

Probably because there may be an upper limit on the size of an individual celestial object (and thus on how many particles it can consist of) - for it to be anything other than a black hole, anyway. In the future, the Universe will entirely consist of black holes, whose number will be much smaller.

dredmorbius 3y ago
I don't know what limits there are on the size of galaxies, though I suspect it share some characteristics with that of stars. For star formation, however, the tension is between the outward flow of matter and simple photonic pressure on interstellar gas, competing against the inward pull of gravity. That is: stars form as large "dust" clouds (mostly hydrogen atoms, obviously), collapse. Once those reach a critical mass, fusion initiates within the stellar core. The rate of fusion is moderated by the pull of gravity, such that larger stars, fighting more inward mass, burn far hotter (and faster, and live comparatively short lives: a few tens of millions of years for the very largest, as compared with ~10 billion for the Sun, and potentially trillions for brown dwarfs which eke out their fuel slowly). In a curious bit of physical synchronicity, the energy density of the Sun's core (the part which is actively generating energy), in terms of watts/kg, is roughly equivalent to that of mammalian metabolism. It's just that there's a whole lot more mass to the Sun's core than there are mammals, and of course, we're comparing nuclear reactions with chemical ones (the Krebs Cycle). Back to stars: as they ignite, they start blowing the dust that's forming them away, and so limit their maximum growth potential. The least massive stars (brown dwarfs) are roughly 10x the mass of Jupiter, or about 1/100th the mass of our Sun, the most massive stars are roughly 100x the mass of the Sun, giving a total effective range of roughly 1:10,000. The upper bound, called the Eddington Mass Limit, is about 120 solar masses, and is based on total luminosity which translates to outward pressure exerted by the star against the stellar medium surrounding it: <https://en.wikipedia.org/wiki/Eddington_luminosity https://en.wikipedia.org/wiki/Eddington_luminosity> I don't know what the dynamics governing galaxy growth are, though a few ideas come to mind. Apparently the most massive galaxies are about 100x the size of the Milky Way (so we're at roughly similar ends of the scale for both stars and galaxies in that regard) in the Physics StackExchange post below. The discussion there does mention luminosity and pressure, though there's also some disagreement over just what constitutes a galaxy, and where to draw borders. <https://physics.stackexchange.com/questions/172953/what-is-the-limit-for-the-size-of-a-galaxy https://physics.stackexchange.com/questions/172953/what-is-t...>