Black holes have much gravitation to hold all stars and nebulas but why they are aligned in disc type shapes rather than spheres because gravitation is everywhere around black holes. Even on upper and lower sides. Even if rotation is the cause why cant they move above black holes in smaller circles.
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1$\begingroup$ possible duplicate of Why the galaxies forms 2D plane (or spiral-like) instead of 3D ball (or spherical-like)? $\endgroup$– joshphysicsCommented Feb 20, 2014 at 18:50
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$\begingroup$ Also related: physics.stackexchange.com/q/8502/2451 , physics.stackexchange.com/q/12140/2451 , physics.stackexchange.com/q/26083/2451 and links therein. $\endgroup$– Qmechanic ♦Commented Feb 20, 2014 at 19:22
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TL;DR They can orbit in any direction but they usually stick to the disk.
This is an excellent question.
It happens to be for the same reason that the earth and all of the planets all orbit the sun in the same direction on the same plane, I.E. the solar system is a disk as well.
First imagine a large ball of gas in space that is roughly spherical. It is all spinning around an axis at uniform angular velocity. We let gravity act on this spinning sphere of gas. Everything is attracted to the centre, but the equator is spinning and so opposes this attraction (it keeps missing the centre) but the poles are moving quite slowly so fall inward.
Letting this evolve and a small amount of damping with collisions this will end up as an accretion disk, of gas and dust, orbiting a dense core, the sun in the case of the solar system and the super-massive black hole (and dark matter and light matter) in the case of the galaxy. This dust then forms planets in the case of the solar system and solar systems in the case of galaxies.
Even if rotation is the cause why cant they move above black holes in smaller circles.
If we look closely at the centre of the galaxy at the super-massive black hole we do see many stars orbiting it in many different directions and in different planes, this sphere around the super-massive black hole does exist and is called the galactic halo.
