4 ms·

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 any

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...>

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...>