Starlight and Fine Tuning.

Is the following argument valid?

"parent star color"
if redder: photosynthetic response would be insufficient
if bluer: photosynthetic response would be insufficient

I was under the impression star type was based on surface temperature
and composition, and that all stars basically emitted most of the
visible spectrum.

In article . com,
"Malnutritious" writes:
"parent star color"
if redder: photosynthetic response would be insufficient

Photosynthesis in earthly plants requires some red light and some
blue light. _Very_ red stars put out little blue light, and
photosynthesis as we know it would be difficult or impossible.
However, all but the very coolest stars are hotter than 3000 K, and
that's hot enough to put out about one tenth the blue relative to red
as compared to the Sun. That's probably enough for photosynthesis as
we know it, though no doubt with reduced efficiency.

Even for stars cooler than 3000 K, it's conceivable that an entirely
different photosynthetic path could exist.

if bluer: photosynthetic response would be insufficient

No, stars hotter than the Sun put out more blue light relative to
red, but there is still plenty of red light.

I was under the impression star type was based on surface temperature
and composition,

Spectral types depend mostly on temperature unless the composition is
quite unusual.

and that all stars basically emitted most of the
visible spectrum.

Nearly all stars have surface temperatures between 3000 and 10000 K
(corresponding to spectral types M5 to A0). As you say, these stars
emit over the entire visible spectrum. For comparison, 3000 K is
quite a bit hotter than an incandescent light bulb filament.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
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