why are pictures of stars distorted?

The Hubble is an example of a Cassegrain telescope assembly (look it up), of the Ritchey-Chretien design, involving two hyperbolic mirrors. The main mirror reflects incident light back up the telescope 'tube', until it hits a secondary mirror, held above the main mirror surface, which then reflects the light back down the tube through a circular hole in the main mirror to the cameras and other instruments behind the main mirror.

The problem is the fact that the secondary mirror has to be held somehow above the main mirror. In some telescope designs, notably the Schmidt-Cassegrain type, the secondary mirror is just a silvered or aluminised area on the back of a lens which sits in front of the main mirror. However, in the Hubble design, the secondary mirror is a separate piece of glass held in place by four 'arms'. These four arms correspond to the four spikes that you see on stellar images.

The process involved, despite what some other people have said, is NOT scattering, but rather diffraction, and is unavoidable with this sort of design.

That is caused by light scattered from the secondary mirror supports in the telescope. Telescope designs with no secondary mirror, or that support the secondary mirror with a clear glass plate do not show this effect.

Those are called "diffraction spikes" and are caused by light refracting around the supports that hold the secondary mirror in position.

There is some light scatter by the vanes that support the secondary mirror.