Sundry supplement

The more hotter and more massive the star, the shorter its main sequence and the time it takes to consume its primary fuel.

O class are hottest and shortest lived, bluish white and ultraviolet stars. They burn for only a few million years. They are as rare as 1 in 3 million in the solar region.

B class, luminous blue stars making up 1 in 800 of all stars. They are also short lived and static, clustering up with O class stars in their nurseries of molecular hydrogen and carbon monoxide.

A class, white or bluish white stars to the naked eye, 1 in 160 stars. They emit strong hydrogen and ionised metal spectral lines.

F class, white to yellow shining, 1 in 32 stars. They show spectral lines of hydrogen and neutral and ionized metals.

G class, our sun, like near neighbour Alpha Centauri A, is yellow in appearence. Surprisingly, 1 in 13 stars are yellow. Our Sol is estimated to be nearly halfway through its 10 billion year supply of hydrogen fuel. It emits spectral lines of hydrogen, ionised and neutral metals and carbon hydrogen (CH) molecules. The hydrogen fuel will run out in 5 to 6 billion years. Sol will use helium as fuel. Just as nuclear fusion of hydrogen produces helium, nuclear fusion of helium produces carbon. The sun swells as it uses a lower energy level of fuel each time, on its way to the lowest state of iron, reddening and brightening into a giant. After growing out to the orbit of Jupiter, ejecting mass, it will collapse into a brown dwarf mainly made of iron.

Below that is the K class, more orange looking than Sol. 1 in 8 stars is a K class. Their spectrum shows little or no hydrogen, plenty of neutral metals, a few molecular lines such as titanium oxide.

Below that is the most common, the M class of star. 80% of all stars are red or brown or black dwarfs. Like all other classes, there can be giant and supergiant versions. Dwarves in this class have all kinds of molecular lines, such as titanium and vanadium oxide, in their spectrum as well as elements, but no hydrogen lines.