Early and Later stars and the Kelvin-Helmoltz Mechanism

Star types today are still referred to as "early" for hotter more massive stars, and "late" for cooler less massive stars.  A holdover from when Kelvin and Helmholtz understood that hot massive stars cooled and lost mass to become smaller cooler stars. Later, in an effort to force the age of the Sun to match the age of the Earth, a nuclear interpretation of the energy of the Sun was adopted, but "early" and "late" star types are still used.

The Kelvin-Helmholtz mechanism is still understood in the standard model to occur on Jupiter, Saturn and in general brown dwarfs, in the later stage, which cooled from the earlier stage of large hot stars.

 https://en.wikipedia.org/wiki/Stellar_classification#Spectral_types

https://en.wikipedia.org/wiki/Kelvin%E2%80%93Helmholtz_mechanism

"Early" and "late" nomenclature

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 stars. 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.^[69] The terms "early" and "late" were carried over, beyond the demise of the model they were based on.

The Kelvin–Helmholtz mechanism is an astronomical process that occurs when the surface of a star or a planet cools. The cooling causes the internal pressure to drop, and the star or planet shrinks as a result. This compression, in turn, heats the core of the star/planet. This mechanism is evident on Jupiter and Saturn and on brown dwarfs whose central temperatures are not high enough to undergo hydrogen fusion. It is estimated that Jupiter radiates more energy through this mechanism than it receives from the Sun, but Saturn might not. Jupiter has been estimated to shrink at a rate of approximately 1 mm/year by this process,^[1] corresponding to an internal flux of 7.485 W/m².^[2]

The mechanism was originally proposed by Kelvin and Helmholtz in the late nineteenth century to explain the source of energy of the Sun. By the mid-nineteenth century, conservation of energy had been accepted, and one consequence of this law of physics is that the Sun must have some energy source to continue to shine. Because nuclear reactions were unknown, the main candidate for the source of solar energy was gravitational contraction.

However, it soon was recognized by Sir Arthur Eddington and others that the total amount of energy available through this mechanism only allowed the Sun to shine for millions of years rather than the billions of years that the geological and biological evidence suggested for the age of the Earth. (Kelvin himself had argued that the Earth was millions, not billions, of years old.) The true source of the Sun's energy remained uncertain until the 1930s, when it was "shown" by Hans Bethe to be nuclear fusion.

for more information:  Stellar Metamorphosis 

https://vixra.org/author/jeffrey_joseph_wolynski