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I always see galaxies in "disk" shape in pictures. It is like, there is a rotational movement on a plane and the system is balanced by the pulling-in gravitational force which prevents the stars to leap out of the galaxy. What is the physical reason behind this? I have never seen a star in disk shape. A star is nothing but a much much smaller non-solid mass revolving around an axis, just like the galaxies. Does size matter? What makes the difference? Why doesn't a typical galaxy form in a spherical shape rather than a disk shape?

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Please note that even if stars and galaxies are not solid there is a enormeusly big difference: the star is a compact body of plasma, with pressure, while the galaxy is a big void of very sparse atoms with some clouds and very small stars, so it is nothing like compact. –  Envite Dec 12 '13 at 23:11
Many duplicates on Physics SE - this may be the best one physics.stackexchange.com/questions/93830/… –  Rob Jeffries 2 days ago

2 Answers 2

Spiral galaxies may be the most commonly depicted probably because we find ourselves in one, however, galaxies come in many sizes and shapes. Wikipedia's page on Galaxy morphological classification explains the most common classifications.

Answering the question on the title:

The basic answer to your question is Conservation of Angular Momentum. Astronomers have been interested in this question for a long time and are still writing papers on the subject. Even if you assume that at the time of the big bang there was no net angular momentum in the universe, one would expect that there were local fluctuations. When galaxies initially formed by gravitational attraction these net local amounts remained. Now angular momentum is conserved so when the initial collection of stars and gas in one galactic volume start to collapse the net angular momentum has to stay the same. Since angular momentum $\vec{L} =m\vec{v}\times\vec{r}$, for a given $v$ its easier to conserve $L$ if the $r$ is large. In the orthogonal direction (along the axis of the accidental angular momentum) there is no such hindrance to collapse and so the collection turns out to be disk shaped. By the way there has been several studies on the net angular momentum of a large collection of galaxies coming from the Sloan Digital Sky Survey and the answer is pretty close to but not quite zero. Stay tuned.

Extracted from an University of Phoenix Department of Physics article

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I would argue one has to also refer to shocks which cancel out momentum of the gas perpendicular to the plane; the conservation of angular momentum does not explain alone discs galaxies because elliptical galaxies also conserve angular momentum. –  chris Mar 15 '14 at 21:06
@chris I agree, this doesn't answer the question at all. The presence of gas and dissipational processes is mandatory. Many elliptical galaxies rotate. –  Rob Jeffries yesterday

Galaxies are disc shaped because they are gas rich and dynamically young. Stars are also gas rich but they are dynamically old so they have had time to rid themselves of their discs. Young protostars (which are dynamically young) are surrounded by proto-stellar discs. The reason many young gas rich objects are disc shaped has to do the fact that circular orbits are compatible with no orbit crossing, hence no shocks. In some sense, many young astronomical objects are disc shaped because they contain(ed) gas which can radiate away non circular motion. But a disc is not the most likely state of a gravitational system: given time, instabilities or viscous processes it will tend towards a more likely compact state, where mass flows inwards and angular momentum outwards. This is why proto-stellar discs become stars. Galaxies on the other hand have not had time to turn themselves into gigantic black holes, or been given the opportunity to do so via torquing with their environment. When two gas poor disc galaxies collide they produce an elliptical which is not disc like. When two gas rich disc galaxies collide they produce a disc like galaxy with a bulge.

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