I've read passing comments on stellar astronomy pages (such as this one from TalkOrigins) about how the Kelvin-Helmholtz mechanism turned out the be the wrong model of stellar energy production because of certain contradictions with observation, namely that one wouldn't expect a relationship between temperature and luminosity as one sees with the ~90% of stars that comprise the main sequence, which is what Hertsprung and Russell discovered in the early 20th century. However, I have been unsuccessful in finding any detailed description as to why this is so. Exactly how would stars be arranged on the Hertzsprung-Russell Diagram if they were powered by the K-H mechanism rather than by nuclear fusion. Does someone have any links that might be of use to me? If not, would I be able to get the answer I need by emailing someone? Thanks in advance.

~Zero

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"The universe we observe has precisely the properties we should expect if there is, at bottom, no design, no purpose, no evil and no good, nothing but blind pitiless indifference." - Richard Dawkins

I'd imagine they'd arrange themselves in some sort of cooling track arrangement like the main sequence with the newly formed objects at the blue end and the older, cooler objects at the red end. If this were true there would be no evolution off this track, so no giants. There would also be no dip in the number of hotter brown dwarfs (well they didn't know about this in the early 20th century). I don't know if it could explain white dwarfs either.

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Its better tae meddle wi the deil............

My guess would be to consider the increase in luminosity due to the nuclear heat source. This would enlarge the star greater than would be allowed by the K-H mech. This enlargement would produce greater total luminosity for any given temperature compared with K-H.

[Better wait for others to yeh or nah this late rambling I do. ]

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

The initial evolution of a star in radiative equilibrium while contracting on a Kelvin-Helmholtz time scale (t_KH ~ GM^2/RL ~ 10^7 years for a 1 solar mass star) is predictable without any fancy computer models. For those that are mathematically inclined, see this discussion on pp. 120-125 of this book on stellar astrophysics (you'll need a pdf viewer). In a nutshell, the star's luminosity L will scale as follows:

L ~ R^-3
and on the H-R diagram like:
L ~ (T_eff)^2.4

for stars of a particular mass M, where R is the star's radius - which decreases with time - and T_eff is the star's effective surface temperature.

Since the K-H time scale scales roughly as M^-2 (or steeper), the more massive stars would progress toward the hot-luminous corner of the H-R diagram much more quickly. Imagine zillions of tracks of L ~ (T_eff)^2.4 for stars of differing masses and "starting times" filling the H-R digram, except that for many stars and circumstances the K-H time scale is tiny in comparison to the age of the star cluster or galaxy. Thus I imagine that we would observe two main sequences - that of cooling white dwarfs and another of cooling neutron stars (~1000x smaller in size), as well a jumble of low mass stars still contracting to the white dwarf sequence.

Why the two "main" sequences of white dwarfs and neutron stars? Because the contraction would eventually lead to the electrons becoming "degenerate", and for stars with less mass than the Chandrasekhar limit, the resulting quantum statistical pressure would offset gravity and halt further contraction. I can imagine that the more massive stars would suffer a catastrophic collapse to become neutron stars (supported by neutron degeneracy) or black holes.

Mass loss due to radiatively driven winds in the final hot, luminous phase of the contracting star (excepting the lowest mass objects) would alter its evolution a bit. The lowest mass 'stars' are completely convective, and because convection is so efficent these stars would contract nearly vertically on the H-R diagram....probably until they hit the white dwarf sequence (instead of the main sequence).

All of the above presumes that the rest of nature's laws still hold, even in the proposed universe in which fusion never occurs.