i want to know why galaxies are spiral in nature.. let us say there is some sort of intense mass (black hole?) at the centre of our milky way galaxy. the intense gravitaional pull is keeping evey solar system to revolve around it. thats clear, because that is what is happening in our solar systm too. but the planet goes round around the sun (ellipticaly, to be correct). but i recently saw pics of galaxies and they were spiral..? if the gravitational pull is keeping every solar system to revlove around the centre of galaxy, what different force is giving them the spiral shape? i mean, spirals are created in viscous material (water, air) but the universe is all vacuum..? then how are galaxies spiral in nature?
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2$\begingroup$ See here physics.stackexchange.com/q/25128 $\endgroup$– PaulCommented May 21, 2015 at 19:25
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2$\begingroup$ As a shortcut to reading up on the details: not all galaxies are spiral. Elliptical galaxies can be rather featureless. $\endgroup$– CuriousOneCommented May 21, 2015 at 19:36
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2$\begingroup$ Related: physics.stackexchange.com/q/93830/2451 $\endgroup$– Qmechanic ♦Commented May 21, 2015 at 19:40
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Not all galaxies are spiral, there are elliptic and irregular shaped ones also. Search for them on Google and images abound of non spiral shaped ones.
A more interesting question is: why are galaxies, in general, laid out in a flat plane, like a dinner plate? According to studies dating back over 40 years ago, spiral galaxies are gravitationally unstable and should evolve in time into bar shaped rotating objects . They would however, stay spiral shaped if they were embedded in a halo of spherically distributed material and this was one of the early indications of the possible existence of dark matter.
The spiral shaped arms are the result of density waves passing through the galaxies and are not permanent features.
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$\begingroup$ okay..i got your point..just some more queries...you remember that eintein's general relativity is based on the fact that speed of light is contant in every frame..and that light requires no medium..that's how it was proposed that space is empty..now we have proofs for dark matter.....so, why dont we simply say that this dark matter is infact that "luminiferous aether" that was once rejected by the concept of relativity..? $\endgroup$ Commented May 22, 2015 at 11:54
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$\begingroup$ From Wiki: "the aether had to be a fluid in order to fill space, but one that was millions of times more rigid than steel in order to support the high frequencies of light waves. It also had to be massless and without viscosity, otherwise it would visibly affect the orbits of planets. Additionally it appeared it had to be completely transparent, non-dispersive, incompressible, and continuous at a very small scale." Dark matter, if indeed it does exist, as I personally don't know of any definitive proof to date, just needs gravity to work. $\endgroup$– user81619Commented May 22, 2015 at 12:08
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$\begingroup$ even if dark matter exists (let us just say that old aether is the new dark matter), it would mean that light do need medium to travel...wouldnt that mean relativity is wrong..? $\endgroup$ Commented May 22, 2015 at 12:24
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$\begingroup$ Look at it this way. Every day, for the last 100 years, thousands of scientists have been testing general relativity, in one way or another. If any one of them proves that it's wrong, even slightly, he/she gets a Nobel Prize, no question about that. So far, nobody has. I'm sorry, I don't follow how dark matter (if it exists, I don't know anything except something strange is going on with galaxies and DM is our best guess), has anything to do with how light travels in empty space. Actually, if it did, that would be great, we might be able to see dark matter. $\endgroup$– user81619Commented May 22, 2015 at 13:01