Stars look deceptively alike to the unaided eye, except for brightness. Only on occasions a hint of blue or red betray there are underlying differences, while a telescope widens the range but only so much. It is the spectroscope - an instrument capable of breaking down light in its different color components - the right tool to unveil an amazing diversity.
It was only natural for structured persons as scientists are, to start the game of classification as soon as the marriage of spectroscope and telescope was settled enough to acquire quality data. Starting with the most brilliant stars, dimmer ones followed suit as technology allowed. This led to a somewhat arbitrary spectral-type assignment labeled with a single letter at first, with a second subtype letter added later and/or number as required.
For memory impaired astronomy students, the helper "Oh Be A Fine Girl (Guy) Kiss Me" proposal was coined to stand for the O-B-A-F-G-K main color classification sequence.
What's in a letter
Arbitrary as it may seem at first, the underlying color classification rationale has to do with surface temperature, which in turn is determined by a combination of initial size, composition, and current age. This ranges from the hot bluish (searing 60,000 deg. K or more) to the redder and comparatively cooler (just several thousands deg. K) ones. To understand how size and age bear on temperature, a sketchy tale of what makes a star shine is in order.
The very first star generation resulted from gravitational collapse of primeval hydrogen, still by far the most abundant element populating the Universe.
Later generation ones are also overwhelmingly composed of hydrogen, to which heavier elements cooked in previous supernovas were added in trace amounts as narrated in a previous article. As it was explained then, collapse leads to compression, to heating and ultimately to fusion ignition.
Depending on initial size - that is, the size of the original collapsing hydrogen cloud - the resulting star may follow very different destinies.
In a simplified way, stars feature different regions as we travel from surface to its center.
Apart from a tenuous, extremely hot atmosphere mostly invisible (UV and X-ray active), there is a thick layer of hot broiling hydrogen from where most starlight comes, the chromosphere. Far below is the actual burning zone, a thin layer where a perpetual downpour of overlying hydrogen meets the required temperature and pressure conditions for ignition. Below the fusion layer in turn, lies a pile up of leftover ashes, a mostly helium bearing core.
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