Phil said:
I hadn't forgotten, I was just talking about hydrostatic equilibrium.
And whatever the merits of Darwin's work (his reputation in
philosophical circles is not very high just now AIUI), he was personally
a bit of a git. (The Geological Society produced a fair number of those
round about that time--if you haven't read Martin Rudwick's book, "The
Great Devonian Controversy", I highly recommend it. A classic of the
history of science, and enormous fun besides.)
Some of Darwin's supporters were worse and cartoonists had a field day!
But his basic hypothesis and the observational foundation was sound.
That's a bit of an overstatement, I think. In broad averages, the
hydrostatic solution has to work--there's nothing that's going to change
the mass of a proton or the charge on an electron, and the calculation
of luminosity on the basis of stellar mass and composition is pretty
fundamental stuff. (You can argue about the treatment of metals, but
that's a second order effect anyway.) So over timescales comparable or
longer than the thermal time constant of the Sun, I entirely agree.
I'd be wary of claiming universal constancy of output from stars in all
cases. A proportion of stars are variable and some like Cepheids and RR
Lyrae have periods that are determined by their absolute luminosity. The
solution may be OK on average but if it bounces around the equilibrium
it doesn't have to be constant. They provide excellent standard candles
for local galaxies now as Henrietta Lovett first observed. For anyone
interested:
http://www.astronomynotes.com/ismnotes/s5.htm
And even when we think we know how they behave there are still minor
twists and turns.
http://www.astronomy.com/News-Obser...tandard candle not so standard after all.aspx
And the sun itself is a bit quirky when you really look up close with
modern instruments. I can't find the latest but this will do:
http://solarscience.msfc.nasa.gov/Helioseismology.shtml
But the topic came up in regards to things like the Maunder minimum,
which was only 300 years ago. That's long compared with the acoustic
timescale, but short compared with the thermal time scale. Our
satellite data span what, 10% of the time since then? I'm not saying
that I have a good mechanism for larger variations, but who knows? Stars
do funny things sometimes.
No disagreement there and on the face of it there is now evidence that
lack of sunspots and consequent changes in UV output can alter the
position of the jetstream making Northern Europe colder in winter. Not
necessarily global cooling but locallised in highly populated areas.
There is no doubt that an active sun also fluffs up and dumps energy
into the thermosphere (mainly causing extra drag on satellites) but it
might also play a very small part in warming the Earth. The pure TSI
change on its own is too small to explain the periodic variation in
temperature so some additional feedback must occur on the Earth.
But the recent warming occurred during a period where there was good
satellite monitoring of TSI so magic hand waving will not hack it.
Jason got his Ph.D. in 1965, for doing a reasonably complete numerical
model of the Sun. I'm sure there were a fair number of people
involved--it was one of the pressing problems of the day. I haven't
seen his thesis, or read any of the other folks' stuff. I was mostly
passing on content from the class. (It was my favourite astronomy
class, closely followed by celestial mechanics.)
I rather liked the idea that more massive stars burn much faster.
Essentially since a bigger volume inside them met the conditions for
fusion and the surface area for light to escape from scales as r^2.
The measured changes do, I agree, but the very short data set we have
available doesn't prove that the Maunder minimum wasn't associated with
a century or two of lower solar output. Annual variation is too fast
and equinoctial procession is too slow to fit. Anyway, that wasn't what
I was mostly on about.
I agree there is a distinct possibility that some of what we see as
climate change on Earth is due to changes in the sun (roughly about half
of what has been observed since 1850). Certainly during the Maunder
minimum there is a real possibility that solar TSI was lower although
Keeling and Whorf offer another explantion that I personally find more
appealing - that the tidal influence of the Sun-Moon-Earth system has
certain key periodicities which seem to be reflected in climate data.
http://www.pnas.org/content/94/16/8321.abstract
Whilst I have technical reservations about parts of their analysis
(notably how they isolated the decadal variation) I think they might be
onto something. Unfortunately the non-linear coupled oceanic circulation
model folk are all in vogue to explain this at the moment.
Mostly I was subjecting Bill to mild ridicule for saying that it was
unscientific to believe stellar models, when he believes climate models,
which contain far more in the way of fudge factors and parameter fitting.
Although they do include adjustable parameters you must know as well as
I do that in astrophysics it is just the same but you don't get many
fossil fuel lobbyists complaining about relativistic jets in distant
galaxies, cold dark matter or dark energy. I find the latter much harder
to accept since it was discovered long after my involvement. YMMV
Regards,
Martin Brown
