Expert: Tom Whiting - 7/26/2004

Question
Why don't stars stay on zero-age main sequence while they are main sequence stars?

Any information on how I should answer this question would be greatly appreciated.

Very Respectfully,
Nathan Smith

Answer
Nathan,
Because there is a compositional change as the star
begins to fuse hydrogen to helium in the inner core
region; there is also a density redistribution change, as more and more helium nuclei are formed in, or around, the core, so the core region becomes a slightly higher density relative to the rest of the star; thus the internal core temperature slowly rises, and star becomes slightly less massive as it converts mass to energy. (All well within the boundaries of what we refer to as hydrostatic equilibrium
for a main-sequence star).
[For the sun, about a 4-5 million ton mass loss every second, using E = mc squared].

All of these conditions cause the main sequence star
to swell slightly and increase in luminosity - read that
intrinsic brightness, or a higher Absolute Magnitude-  slightly, over the 95% life time-frame on the main sequence.

(But interestingly, the slightly larger surface area coupled with the slightly increased luminosity pretty much balance, so the "surface" temperature of the star remains pretty much
the same value throughout it's 95% life cycle on
the main sequence- for the sun, about 11,000 degrees F.)
----there may be a very small change in surface brightness,
but not very much, relatively speaking----

Because of this increase in luminosity (intrinsic brightness) the star slowly moves off the ZAMS line
and very slightly upward on the H-R diagram main-sequence
band.....that's why there is some measureable thickness to the main-sequence diagonal band as each star is at a different age (and therefore a slightly different composition and slightly different luminosity) on the band.

Realize that we are only looking here at the M-S time frame,
which encompasses 95% of a star's life cycle, the remaining
5% being divided thusly....1% to go from a protostar to
the ZAMS line on the M-S band, and about 4% to evolve
from the M-S to the Red Giant/White Dwarf region of the
H-R diagram.  These 5% are very large moves -- read that
"very short time frames" relatively speaking, as compared to the small, slow, migration on the H-R diagram, during the hydrogen to helium fusion stage on the main-sequence.

(At least that is the current thinking of how a stable
star works; realize we may not yet know exactly all of what's happening, but we think the majority of our
reasoning is correct...but stay tuned; for instance, we
just recently learned that neutrino's must have a slight rest mass, thus accounting for the previously thought "missing neutrino's" coming from the sun.)
So there is still more work to be done, and more to learn;
the beauty of science in general, and astronomy in particular.

Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA