Sun was too hot a million years ago!?

[sunfish]

Sorry, Jinx, multicellular plants and animals - you, your pet dog, and the maple tree outside, for example - are most definitely eukaryotes. From Merriam-Webster Online:

eukaryote: an organism composed of one or more cells containing visibly evident nuclei and organelles

[sunfish]

@Jinx

Simple explanation: Spectral classification is a system for classifying stars according to surface temperature (Ref: "Complete Idiot’s Guide to Astronomy (c) 2001) The Hertzsprung-Russell (H-R) Diagram is based on this with “O” are the hottest stars at about 35000 K and “M” stars are coolest at about 3000 K. It is well known that the sun is an average star lying in the “main sequence” on the H-R diagram. It was hotter when younger; it will be cooler as it ages. There is agreement on this across many references. (The sun is “G” class about 6000 K at the surface.)

Also, I think you’re misreading the Hertzprung-Russell diagram. Stars do not pass through all the spectral classifications as they age. Their initial mass and composition is going to determine where they fall in the spectral classification, and they will go through the particular life cycle for that class star. The Sun is a G-type star, always has been, always will be.

dtilque’s link mentions the following tidbit:

Ordered from highest temperature to lowest, the seven main stellar types are O, B, A, F, G, K, and M. Astronomers use one of several mnemonics to remember the order of the classification scheme. O, B, and A type stars are often referred to as early spectral types, while cool stars (G, K, and M) are known as late type stars. The nomenclature is rooted in long-obsolete ideas about stellar evolution, but the terminology remains. (emphasis mine)

It seems that The Complete Idiot’s Guide to Astronomy may need some updating, if it still refers to this idea.

[Grey]

@Jinx

As L and T, I could not find info on this at this time. However, it’s hard to imagine how a brown dwarf can have a spectral class when it is defined as “a failed star where forces of energy and gravity rose to equilibrium before the core temp could rise sufficiently to trigger nuclear fusion in the core.” (Ref: ibid.)

I think it comes from the lithium not being “burned” up in the stars corona. Let me check…

@AAS 196th Meeting

Dismissed at the time by many researchers as a freak unrelated to normal stars or brown dwarfs, GD 165B has instead proven to be the prototype of a new spectral class, called L dwarfs'', which is just cooler than class M and encompasses both low-mass stars and warm brown dwarfs. Likewise the methane dwarf Gl 229B, heralded as the first bona fide brown dwarf discovery in the immediate vicinity of the Sun, has proven to be the prototype of a spectral class just cooler than L, a class referred to as T dwarfs’’.

Link

Well maybe not, time to dig back into this stuff.

[sunfish]

@sunfish

Also, I think you’re misreading the Hertzprung-Russell diagram. Stars do not pass through all the spectral classifications as they age. Their initial mass and composition is going to determine where they fall in the spectral classification, and they will go through the particular life cycle for that class star. The Sun is a G-type star, always has been, always will be.

And now to correct myself, in part:

The Sun will remain a G-type star until such time as it begins to run out of hydrogen fuel at its core. Enhanced fusion of hydrogen at intermediate levels in the Sun will force the outer layers to expand, ultimately producing a red giant star with a surface cooler than the original G-type star. At this point the Sun will have left the main sequence, on its way to dying out.

However, initial mass does determine where a star will fall on the main sequence and what its life cycle will be like as a certain type of star in the main sequence. This page on stellar evolution has some really nice explanations to clarify issues.