Neutron stars and strange spinning objects
What stops a star collapsing to a point under the influence of gravity? For a star burning nuclear fuel it is the pressure of the radiation emitted from its burning core. Initially, when a star forms, it burns hydrogen, forming helium and other heavier elements. When the hydrogen runs out, the star burns helium forming still heavier elements, etc, etc. This process halts when iron is formed in the core of the star. There is nothing left to burn; the radiation pressure preventing the star collapsing disappears, and the core of the star collapses resulting in a massive explosion, a supernova which can be so bright as millions of normal stars. The arrow on the photograph of M51 points to such a supernova.
The core of a supernova collapses into a superdense gas of neutrons: a neutron star. If the sun was to form a neutron star, it would be just a few kilometers accross!
Since angular momentum is conserved in a collapse, the rotation speed increases enormously. A neutron star may rotate 600 - 800 times a second. The sun, for example, rotates around once every month.
How could we see such a small, dark, object? Matter that is accelerated onto the star emits radio waves that are swept accross the universe like a lighthouse beam. We observe the beam as short pulses with the same frequency as the rotation of the star. This type of object is a pulsar. The picture above shows the crab nebula formed by a supernova which has a pulsar at its heart.