Short answers, pulled from the story.
A star is a luminous spheroid of plasma held together by self-gravity. These objects shine because thermonuclear fusion converts hydrogen into helium within their cores, releasing energy that radiates into outer space.
Ancient Egyptian astronomers produced the oldest accurately dated star chart in 1534 BC. Babylonian astronomers compiled early catalogues during the late 2nd millennium BC under the Kassite Period.
A protostar forms at the core when hydrostatic equilibrium is reached after approximately 10 million years for Sun-like stars. The period of gravitational contraction lasts up to 100 million years for red dwarfs before reaching the main sequence.
Stars with at least 8 solar masses expand into red giants then fuse heavier elements until iron production begins. Iron nuclei are more tightly bound so fusion beyond this point does not release energy and the core suddenly collapses causing a supernova explosion.
The modern scheme orders classifications by temperature ranging from type O which are very hot to M which allow molecules to form. Each letter has ten subdivisions numbered zero to nine indicating decreasing temperature levels and rare types include L and T classes that classify coldest low-mass stars.