Short answers, pulled from the story.
A star is a luminous spheroid of plasma held together by its own self-gravity. Stars forming in the present Milky Way are composed of about 71 percent hydrogen and 27 percent helium by mass, with a small fraction of heavier elements. The nearest star to Earth is the Sun.
The observable universe contains more stars than all the grains of sand on Earth, distributed across more than 2 trillion galaxies. Only about 4,000 stars are visible to the naked eye, and all of them lie within the Milky Way. A typical large galaxy like the Milky Way holds hundreds of billions of stars.
A star shines for most of its life through the thermonuclear fusion of hydrogen into helium in its core. In the Sun, with a 16-million-kelvin core, this happens via the proton-proton chain reaction, while more massive stars use the carbon-nitrogen-oxygen cycle. Each reaction releases only a small amount of energy, but enormous numbers of them sustain the star's radiation.
At the end of its life a star's fusion ceases and its core becomes a stellar remnant: a white dwarf, a neutron star, or a black hole if it is massive enough. Low-mass stars like the Sun shed their outer layers as a planetary nebula and leave a white dwarf. Massive stars collapse and explode as supernovae, leaving neutron stars or black holes.
The earliest star catalogues were compiled by Babylonian astronomers in the late 2nd millennium BC, and Hipparchus catalogued 1,020 stars in the 2nd century BC. Chinese astronomers were the first to observe and record a supernova in 185 AD, now known as SN 185. The brightest stellar event in recorded history, SN 1006, was observed in 1006.
Stars are given a single-letter spectral classification from O to M, in order of decreasing surface temperature. Class O stars are 33,000 K or hotter, like Zeta Ophiuchi, while class M stars are 2,600 to 3,850 K, like Proxima Centauri. The Sun is a main-sequence G2V yellow dwarf of intermediate temperature and ordinary size.
A star's lifespan is set mainly by its initial mass and the rate at which it burns fuel. The Sun is expected to live 10 billion years, while the most massive stars last only a few million years. Red dwarfs burn so slowly they can endure for about a trillion years, with the most extreme reaching roughly 12 trillion.