9
$\begingroup$

If I'm told correctly (eg. What Color is the Sun?) that the sun is actually white, should the photo on that page actually be white too? Is it just doctored to meet peoples expectations?:

enter image description here

Likewise for all the red photo's in a Google image search for "The Sun's surface".

$\endgroup$
  • $\begingroup$ White? I thought sun was Asian.....I mean yellow!! $\endgroup$ – Dumbledore Jan 22 '16 at 1:41
  • $\begingroup$ Actually, according to QI (a UK TV program), the sun is a black body because it reflects no light - it only emits it. Fancy that! $\endgroup$ – Richard Heyes Jan 22 '16 at 10:06
  • $\begingroup$ Thanks for sharing that. Good to know! Now, everything is contradicting with what I learnt in the 2nd grade. $\endgroup$ – Dumbledore Jan 22 '16 at 13:57
14
$\begingroup$

The term "color" is a label that humans have assigned to denote the ratio between the intensity at various wavelengths in the three different wavelength bands, or regions, that the human eye is able to perceive. These bands are centered roughly at 430, 545, and 570 nm, but are quite broad and even overlap:

vision

Human cone response, normalized to the same height. In reality, the response of the blue cones is significantly smaller, and the green is somewhat larger (from Wikipedia).

If an object emits light only at, say, 450 nm, the ratio is roughly 0.1:0.2:1 (in the order R:G:B); it then looks a special way to us, and we call it "blue", or maybe "violet". If it emits at 550 nm, or 650 nm, we call it "green" or "red". An object that emits light in a more continuous spectrum that covers the region 500–600 nm, we'd name something like orange-/brown-/olive-ish, depending on the exact spectrum.

The Sun emits photons at all wavelengths, but not in an equal amount at all wavelengths. The particular ratios between the three bands that we can see, we have labeled "white". However, when the Sun's light enters our atmosphere, some of the light is absorbed, especially at the blue wavelength. Filtering out the blue results in a spectrum that looks more orange to us. The figure below shows the Sun's "true" spectrum (in yellow), and the spectrum seen from the surface of Earth (in red):

sun

The Sun's spectrum measured outside our atmosphere (yellow) and at sea level (red) (modified image from Wikipedia, with data from Global Warming Art).

Sometimes we want to observe the Sun in a wavelength region that is invisible to humans, for instance in UV or X-rays. This can be done with a telescope and a detector that is sensitive to light in that particular region, but in order for us to see it, we represent the image with a color that we can see. The image in the top of the link you provide is taken with the European spacecraft SOHO's instrument EIT at 19.5 nm, which we call "extreme UV", bordering on soft X-rays. Since this is invisible to humans, they arbitrarily chose to represent it using green. They might as well have chosen pink or brown.

EUV

The Sun in Extreme UV, during a particularly violent solar flare (from the SOHO gallery).

Several of the photos in your second link are images taken by the Japanese space telescope Hinode, which observes both in the optical (i.e. visible by humans), X-rays, and far UV. If these are shown in orange, again it's just to make them visible to us, and you may say that they have been "doctored to meet our expectation". In this way, I like better when they choose a color such as all green, so we know it's "false color".

$\endgroup$
  • 1
    $\begingroup$ This is your answer, lela, so I'm not going to edit it. The effective temperature of the Sun is 5777 kelvins (or 5778 kelvins, depending on who one reads), not 5525 kelvin. That image at wikipedia is unsourced and is, as far as I can tell, invented. Here's a link to a much better image, also at wikipedia: commons.wikimedia.org/wiki/… . $\endgroup$ – David Hammen Jan 8 '16 at 20:28
  • $\begingroup$ @DavidHammen: Thanks, I don't know where I had my head. The picture you link to is good for showing $T_\mathrm{eff}$, but doesn't show the spectrum of the Sun at sea level. I'll just remove the part about the Planck fit, since it doesn't really have anything to do with the question. $\endgroup$ – pela Jan 9 '16 at 15:21
3
$\begingroup$

The credit for that green image includes SOHO. SOHO routinely uses that green for images of the sun in hard X-Rays at 19.5 nanometers.

Visible light only goes down to about 390nm for extreme violet.

The picture looks like it's taken in hard X-ray wavelengths.

••• Later: Sorry, I meant hard UV/soft X-Rays. Don't know where my head was when I wrote this answer. SOHO's still the green pix in hard UV source though.

$\endgroup$
  • $\begingroup$ Thanks for prompting me to investigate this properly. It turns out the image is actually UV (19.5 nm is considered "extreme UV", but on the border of "soft X-ray"). $\endgroup$ – pela Jan 7 '16 at 11:10
  • 1
    $\begingroup$ @pela You're right, hard UV. I've added a bit to the answer so people aren't left with the wrong impression. $\endgroup$ – Wayfaring Stranger Jan 7 '16 at 16:25
2
$\begingroup$

Unobscured sunlight is white by definition.


You may find this article interesting, The Yellow Sun Paradox, as to why we consider the sun yellow in casual observation.

I would add another possible physiological explanation to the three suggested in this article: The retina's blue cones saturate at lower illumination than the red and green ones, which creates a deficit of blue signal to the optical nerve under heavy stimulation, resulting in a perception of red + green = yellow.

Conversely the sensitivity of the blue cones is why very dim objects such as faint stars in the night sky usually seem to be blue.


Now, as to the images. There are three possible reasons for an image to be yellow or orange. Perhaps, as a nod to expectations, it has been colorized. Possibly it is a false color image that maps a real but non-visible spectrum (radio, microwave, IR, UV, or X-ray) onto the visible spectrum.

But most of the images you present us with in your google search are using a technique sometimes called flame gradation, where a gray-scale (black→gray→white) is mapped on to a flame-scale:

black → [purple?] → brown → red → orange → gold → yellow → white → [cyan?]

to allow a much greater range of gradation in intensity to be perceived. It also suggests rather naturally a cold→cool→warm→hot gradation.

$\endgroup$
  • $\begingroup$ I wouldn't put this question into "astronomy stack exchange" because it is more about biology/physiology. However, you seem pretty knowledgeable in this area. First, I must make the obvious statement that what we see as colors is actually the brain's "interpretation" of the information received by the retina's cones and transmitted by the optical nerve. My question is: If I moved to a planet orbiting a red star (say Gliese 581) where the light spectrum is different, would my brain eventually interpret colors differently? Easy experiment using college students. Ha ha. $\endgroup$ – Jack R. Woods Jan 18 '16 at 18:55

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.