Ok, let's go there, and not just to the surface....

What color is the Solar core and why is it a given? [ Requires that we use our eye, intenisty is reduced properly, we have an exposed view of it, etc. No one is vaporized in this gedankenview. ]

Hint: Think about Eroica's favorite Irish physicist?

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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.

I read in a popular science book by a respected German science journalist (some 25 years ago) that the core of the Sun is absolutely and utterly pitch black.

The reason is that at the temperature the emission of the Sun's core in the visible wavelengths is nearly nil, as the emission maximum lies far, far away in the gamma range.

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Non sunt multiplicanda entia praeter necessitatem.

Oooh, hmmmmm, uhhh. A muddy color is in the works. Is there a Solar Phsicist in the house?

The photon production in the core is primarily gamma rays as a result of the proton-proton reaction. I think there are several different energies of these produced in the chain. There are other high energy photons produced by the CNO process which constitutes only about 1% of the energy production. I don't know what might take place within the core that would produce much lower energy photons that would allow the production of photons that would fall in the optical range. So, my color view is starting to turn black.

But, I want to look at the core's "surface" and consider that the random walk of the photons is indeed random. Thus, photons of all energies can be found here since they are meandering in all directions, though more outward than inward. It is not until the temperature drops enough, when hydrogen can accept a second electron, that photons get trapped (ie the convective zone). Thus the inner sections allow radiative transfer, including the core.

There is also Doppler shifts due to the kinetic energy levels of the hydrogen in the core, though I won't bother calculating them.

So, thanks mainly to this sunny, photonic Volksmarsch, I will argue against a black core, for now. [It would still be pretty colorful if you are right, admittedly. ]

For the sake of this question, and given the colorful spirit it was asked, let us assume a more blackbody result for the energy production of the core, at least along the outer boundary. You may further assume a 15 million degree temperature here.

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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.

I suspect Tyndall might be your favorite Irish physicist, George...

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- Learn a lot teaching others.

There had to be something wrong with the pitch black Sun, as I found out here - a hot black body radiator emits more energy in absolute terms at any wavelength than a colder one. So although the maximum of a black body radiating at 15 million K (if we accept that temperature as that of the surface of a "naked" Solar core) is in the x-ray and gamma, there is still copious energy at the visible range. Enough to blind you, I should say.

Were he still alive, I'd chalk the jourmalist one down for bad astronomy. And me, too, for accepting it.

As for the colour: In the visible spectrum, the intensity curve of a 15 MK black body rises monotonously from red to blue. I suspect we'll see a pure white object becaue of the ferocious intensity of the radiation. Filtered down with a strong neutral grey filter, the colour might have a bluish tinge, as the shorter wavelengths predominate. So I'll suspect we'll have a blue-white object - a lot like in "white dwarf", which according to The Once and Future Sun starts out at about 120000 K. I doubt the difference between 120000 and 15000000 million K makes much of a difference as far as the appearance in the optical spectrum is concerned.

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Non sunt multiplicanda entia praeter necessitatem.

By the way, formulaterp, do you recognise something in the header of this website?

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Non sunt multiplicanda entia praeter necessitatem.

True, thanks to you.

That is true for blackbodies: more temperature moves the peak energy to the shorter wavelengths and increases the energy level at all other wavelengths, too. But, I suspect the core would not be a true blackbody due to its special conditions. Yet, the random walk should allow the core to take in and release lower energy photons, too.


I'll accept the blue-white answer, though it should be much more blue than you think.

Now the why. You are so close, too, but it may involve some good guessing (which is why I through Tyndall in there) or some math.

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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.

Hmm, that does look vaguely familiar. Oh by the way, I knew the answer to your last question, but I didn't want to cheat

George - If you want math, I am afraid you have to get it elsewhere. My ineptitude there was my prime reason not to study physics.

As for guessing - hmmm, Tyndall effect. I don't know. Scattering? In the densely packed core of the Sun?

As for black body - I thought it is the point of the core of the Sun that emits all its energy via a perfect thermal spectrum. I don't see what should stop the Sun's core from emitting one. All the distubances (emission and absorption lines) happen in the convective zone and the photosphere at much lower temperatures.

Anyway, I'll leave it here. Surely, someone will provide the solution as to the why.

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Non sunt multiplicanda entia praeter necessitatem.

That's great - I had completely forgotten this post.
And thanks for the sportsmanship. I doubt you would have been caught.

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Non sunt multiplicanda entia praeter necessitatem.

Yes. You got the color and half of the why; you only have a fourth to go.

Tyndall also missed the math that stood-out in an energy-temperature experiment he did, IIRC.

You may be right. However, the core is a gigantic gamma ray making machine. I don't see how it could produce a perfect bb profile. But, in the optical band, I am assuming it comes close, possibly due to the surrounding photon flux the moves into the core then back out.

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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.

Hint: Tyndall did experiments that allowed another prominent physicist, Stefan, to realize the relationship of radiant energy loss varies to the fourth power of the absolute temperature.

Arneb has already corectly guessed Tyndall scattering as part of the answer. [I think it was Tyndall who was first to claim the sky's color was due to scattering; hint, hint.]

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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.

As a WAG, is the Stefan-Boltzman Law involved in the answer?

E = sigma X T4

But it yields the total amount of radiation being emitted.

Weins Law gives the wavelength at which most of the radiation would be emitted.

lambda max = 2897 mm / T(K)

Dave Mitsky

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De gustibus non est disputandum.
Never attribute to malice that which can be adequately explained by stupidity.

It is a logical guess that took a while, so it is a LAG; what kept you?

Yes, but remember what region of the spectrum we are limited to in the question.

It is amazing how well that works, and I did see the math once on how the derivative of Planck's nasty equation was done to derive this.

So, you have 15+ million degrees, Rayleigh Scattering and Stefan-Boltzman (or Planck). What is the connection for the color determination?

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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.

At 15 million degrees, the black body equation is dominated by the nu^3 term in the visible range. Blue light has a frequency of about 7x10^14 hz, and red light has a frequency of around 4x10^14hz. So there would be about (7/4)^3 = 5.4 times as much blue light as red light.

Bingo. Also, the spectral energy version is the fifth power of wavelength. At temperatures in this regime and higher, the ratio of the production of blue photons to red photons is very close (< 1%) to that of Rayleigh-Tyndall scattering. [I wonder why, but am afraid to ask? ]

Thus, the rich blue sky we see from the top of a mountain would also be the optical color of anything close to a bb profile if it is at such a high temperature (assuming proper attenuation).

However, this does not mean hotter stars would appear bluer because of the saturation issues which would cause blue stars to look white. Consider the color of white dwarfs; are they blue or white? [rhetorical as my question time is up.]

Your go Gilese 581c .


[Added: it was interesting to learn that it was Tyndall that led us into the fourth power relationship of scattering light and had the data that led us to the fourth power temperature relationship to energy (as realized by Stefan).]

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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.

OK, here's my question:

What percentage of the speed of light was the original Orion project supposed to reach? Why couldn't such a spacecraft go much faster?

I think it was 1 %

and why couldn't a space craft go much faster ?

I guess because all other forms of propulsion like anti-matter and wormholes were based on theoretical or science fiction concepts, rather than being sceintifically achievable in the near future