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?
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.