Why does the sea rush to shore?

Because the heat and pressure caused at the center of the Sun due to is gravitational self-attraction are sufficient to cause thermonuclear reactions that release energy that works it way out to the solar photosphere, which is incandescent with a temperature of 5800 Kelvins and radiates the energy off into space. Which is a good thing, because you live off of that energy.




Oh, wait.... This is just one of those song lyrics threads, isn't it?

Never mind.

Yeah, but wouldn't the two sentences you wrote to begin with make a great line in the lyrics?

I await Brotoro's explanation of the second line. Not because I don't know the answer myself, but because I'm in awe of Brot's ability to formulate the answer clearly and fairly concisely. Though to be fair, I don't know anyone who gives a crap about the exact temperature, and I'm vaguely suspicious of anyone who says 'Kelvins' when 'Kelvin' would do.

The sun is a mass of incandescent gas, a gigantic nuclear furnace.

What's that line from HIGHLANDER again, Chris? I never get it quite right.

Where hydrogen is converted into helium, at a temperature of millions of degrees.

It's a hot spot,
It's a gas!

Hydrogen and helium
In a big, bright, glowing mass.

It's a star,
It's a star,
So Janet got an autograph!

I hereby vow to try and make it so this evening.


If I remember.

Niiiice.

[youtube]http://www.youtube.com/watch?v=PfPEvKsme-c[/youtube]

"Kelvins" is the correct usage for the unit of measure on the absolute temperature scale. One says "degrees Celsius" or "degrees Fahrenheit," but one says "Kelvins" (without "degrees").

The temperature of the Sun's photosphere is important because it determines the wavelength that the Sun's energy emission is most intense, which happens to be in the middle of the visual band. Important for shining.

[edited by Sorah]

OK, I buy that - almost. "Degrees Kelvin" is still hardly unknown, but I accept that "kelvins" makes more sense for an absolute measure. Note that CGPM 13 resolution 3 is pretty clear that it's "kelvins", not "Kelvins".



We both know that "happens to be" is wrong, of course. Cute. I still don't know anyone, other than you, who gives a crap about the actual numerical value. I'm well aware that it has affected me, as well as all other life on Earth.

Well, on consideration I probably do know some people who care about the numeric value, but I haven't talked to most of them in years, and the topic never came up previously. My old B5 viewing group consisted of about half of the graduate members of the University of Arizona astronomy program. Quite the party. You'd have fit right in.

The color temperature of the Sun (versus other light sources) is also of interest to photographers -- or at least it was back in the days of film photography. Color photographic emulsions are balanced in terms of the color temperature (measured in Kelvins) of the light source.

Also, my statement about the maximum intensity of the Sun's energy output "happening" to be in the center of the visual band is not as slam dunk as one might think ("We evolved under the Sun, so of course we would evolve eyes that are sensitive to where the Sun has it most intense output"). You also have to consider that the opacity of the atmosphere is such that it has a narrow "window" that matches the visual spectrum, allowing visible light to easily reach the Earth's surface, AND that the opacity of salt water (and the fluids inside our eyeballs) also has a narrow opacity window that allows the visual spectrum through. The range of the visual spectrum may have more to do with water and air than it has to do with the Sun's spectral output.

Opacity of atmosphere:

 
Opacity of water:

No, it's not. But it also isn't nearly as randomly miraculous as your previous statement seems to imply. And while it isn't a "slam dunk", it still doesn't seem all that surprising, either. The biggest "coincidence" there is that the window in atmospheric absorption matches the window in salt water absorption. Wel, yeah, we're on a water planet. And so our atmosphere has many of the same elements and molecules as the water that covers much of its surface. And so the frequencies absorbed by molecules in our atmosphere are, umm, not unrelated to the frequencies absorbed by molecules in our oceans. Because they are, in many cases, the same molecules. Sometimes they're in the ocean, and sometimes they evaporate into the atmosphere. And often they eventually return to the oceans. Their relative concentrations and absorption coefficients may vary, depending on the circumstances. But the point is, if there's a spectral band where none of the major constituents of sea water happen to absorb light - is it really very surprising that none of the major constituents of the atmosphere absorb it either? Considering that they do, in fact, share many of the same constituents?

The second biggest "coincidence" you mention (parenthetically)seems even less surprising to me. Yes, the fluids within human eyeballs have a similar gap in their opacity. So what? One would think that that's because (a) they're starting from the same basic molecular building blocks - stuff found in our seas and atmosphere, an (b) there's the added benefit of evolution here - organisms that develop vision well-tuned to area they inhabit will have an advantage over those who don't. So, yeah, eyeballs are well-adapted to the functions they need to serve. What a coincidence.

Which is not to say that there aren't some surprising "coincidences" in the evolution of life on Earth. (Insert some form of anthropic principle here, to taste.) But this particular case still doesn't seem like much of a coincidence to me.

My point was that the wavelength at which the Sun radiates its maximum intensity is entirely unrelated to where the narrow opacity windows found in air and water happen to be located. The complex chemical reactions of life as we know it require some liquid medium for all the fun chemicals to be dissolved in, and although other solvents may potentially work for other forms of life, water is far more abundant in the interstellar medium than methane or ammonia or sulphur or what have you. So I think it likely to find life based on liquid water around a variety of stars with a variety of temperatures (and, therefore, a range of wavelengths of peak energy emission). And while our Sun's peak emissions fall nicely into the center of the opacity gap of water and a Nitrogen/Oxygen atmosphere, this would not be the case for stars of other temperatures. So we have more efficient eyesight than I expect you would find of life around, for example, red stars with spectral type M (which vastly outnumber Sun-type stars in our galaxy). The fact that the Sun's photospheric temperature of 5800 Kelvins places it peak output so nicely in the water/air opacity gap is important for that reason. No miracles were implied.

OK, cool, I missed that part. Thanks.

Mike is bored by the history of intelligent life in the cosmos.