Editing Stellar classification (section)

==="Early" and "late" nomenclature<span class="anchor" id="Early and late nomenclature"></span>===
Stars are often referred to as ''early'' or ''late'' types. "Early" is a synonym for ''hotter'', while "late" is a synonym for ''cooler''.

Depending on the context, "early" and "late" may be absolute or relative terms. "Early" as an absolute term would therefore refer to O or B, and possibly A stars. As a relative reference it relates to stars hotter than others, such as "early K" being perhaps K0, K1, K2 and K3.

"Late" is used in the same way, with an unqualified use of the term indicating stars with spectral types such as K and M, but it can also be used for stars that are cool relative to other stars, as in using "late G" to refer to G7, G8, and G9.

In the relative sense, "early" means a lower Arabic numeral following the class letter, and "late" means a higher number.

This obscure terminology is a hold-over from a late nineteenth century model of [[stellar evolution]], which supposed that stars were powered by gravitational contraction via the [[Kelvin–Helmholtz mechanism]], which is now known to not apply to [[main-sequence star]]s. If that were true, then stars would start their lives as very hot "early-type" stars and then gradually cool down into "late-type" stars. This mechanism provided ages of the [[Sun]] that were much smaller than what is observed in the [[geologic record]], and was rendered obsolete by the discovery that stars are powered by [[nuclear fusion]].<ref>{{cite encyclopedia |url=http://www.daviddarling.info/encyclopedia/L/latetype.html |title=late-type star |first=David |last=Darling |encyclopedia=The Internet Encyclopedia of Science |access-date=14 October 2007}}</ref> The terms "early" and "late" were carried over, beyond the demise of the model they were based on.