APOD: The Reddening of M71 (2014 Dec 10)

[Chris Peterson]

Just as some astrophotographers enhance a nebula's color is it possible to enhance the intrinsic color of stars? Say to get a Christmas tree-like light effect.

Yes (but there won't be any green lights). The standard way is to shoot an image of the sky that is slightly out of focus. That shows the star colors very nicely. Here's an example posted by Ann a few years ago:

Click to view full size image

Of course, you could further boost the saturation in a photo editor if you wanted even more color. But still no greens.

[rstevenson]

Just as some astrophotographers enhance a nebula's color is it possible to enhance the intrinsic color of stars? Say to get a Christmas tree-like light effect.

How about this Hubble image of the center of globular cluster Omega Centauri?
It was the APOD for Sept. 14, 2009.

Rob

[Ann]

The colors of the Omega Centauri picture are exaggerated, but the colors are also very instructive.

Click to view full size image

M55: Color Magnitude Diagram
B.J. Mochejska, J. Kaluzny (CAMK), 1m Swope Telescope

This is a typical globular cluster color-magnitude diagram. Compare it with the colors and luminosities of the stars of Omega Centauri. Note that the brightest stars are also very red. They are at the tip of the red giant branch. Soon these very red and bright stars will become unstable and start shedding their outer layers. They will turn into short-lived planetary nebulas and then into tiny, searingly hot but cooling white dwarfs. Young white dwarfs are not white in color but blue, and you can see a few tiny blue dots that may well be white dwarfs scattered in the rich Omega Centauri field of stars.

The much larger-looking, relatively bright blue stars are blue horizontal branch stars. They are evolved stars that have previously been red giants. They started out as main sequence stars, then they exhausted their core hydrogen and swelled into cool red giants. But as their core temperatures rose, they became hot enough to get their core helium fusion going. If helium-fusing stars are very metal-poor, they will become hot and blue like the blue horizontal branch stars in globular clusters. If they are more metal-rich, they will become yellow-orange helium-fusing giants like, say, Dubhe in the Big Dipper.

The helium-fusing giants, whether they are blue horizontal branch stars like the ones in globulars or yellow-orange ones like Dubhe in the Big Dipper, will eventually exhaust their core helium, too. When that happens they will swell and become bigger, cooler and brighter than ever before. And then they will become unstable and shed their outer layers and turn into planetary nebulas and white dwarfs.

There are yellow-orange stars in Omega Centauri, too. They are red giants that have exhausted their core hydrogen, but they haven't yet reached the tip of the red giant branch. Therefore they are both yellower and fainter than the biggest, reddest red giants.

Note the large number of white-looking, relatively faint (but not extremely faint) stars in the cluster. They are Sun-like stars. Also note the huge numbers of extremely faint red stars. They are small red dwarfs.

Ann

[Ron-Astro Pharmacist]

Thank you all for your replies.

Ann – Though Gaia won't tell about color I was, as often, "shooting for the stars" too maybe use Gaia's distance data to label stars with colors for distance to be able to see which are closer together. It would probably take way too much work to label individual stars in a large start field to color code their distances.

Chris - I like the bent Christmas tree effect from the blurred out image. Wonder what's special about green? Most of the other colors are covered. Thanks Google - here's the "green" flash.

http://blogs.discovermagazine.com/badas ... een-stars/

http://blogs.discovermagazine.com/badas ... -in-space/

Rob – The colored stars really stand out against the more blasé background stars. That was the image I was hoping to see. A Christmas APOD someday?

Again, thanks for replies. My sense of humor is probably too warped for most. Maybe I should change my signature again?

[Chris Peterson]

Wonder what's special about green?

Stars are blackbody sources. They have a continuous spectral output with a peak energy at a part of the spectrum determined by their temperature. The Sun could actually be considered a green star, since that's the color of its peak output. But a blackbody curve extends on both sides of the peak, and that means that we see a mix of wavelengths for a body of any given temperature. And the nature of our eyes- the way we construct color- prevents us from ever seeing green. As we heat something up, it goes from red to orange to white to unsaturated blue. There's never a combination of wavelengths that we see as green.

We do see green astronomical objects, of course. But those are produced by atomic emission lines, not by thermal sources like stars.

[Ann]

Ron-Astro Pharmacist wrote:
Wonder what's special about green?

Well, basically our Sun is green! But it wouldn't do us any good to see it as such. So we see the Sun as white. (Yes, we do, because the yellowish direct light from the Sun plus the scattered blue light from the cloudless sky equals white.) Therefore, any other star which is actually green, like the Sun, will look white to our eyes, because our eyes are wired to see green stars as white.

Ann

[BMAONE23]

The Voorwerp
http://apod.nasa.gov/apod/ap110210.html
Kermit in Space

Click to view full size image

[Ann]

BMAONE23 wrote:
Kermit in Space

Exactly! But the green color of Hanny's Vorwerp (as I think it is called, and I'm too lazy to google) is due to a narrowband emission line. So it's not a green star, but a green nebula.

Ann

[rstevenson]

"It's not easy being green."

Rob

[Chris Peterson]

Well, basically our Sun is green! But it wouldn't do us any good to see it as such. So we see the Sun as white. (Yes, we do, because the yellowish direct light from the Sun plus the scattered blue light from the cloudless sky equals white.) Therefore, any other star which is actually green, like the Sun, will look white to our eyes, because our eyes are wired to see green stars as white.

It goes beyond how our vision evolved and our perceptual processing, however. It would require a very complex visual system to actually see our star as green. Because it isn't. It has its peak energy output in the part of the spectrum we perceive as green, but that isn't the same as actually being green. Most of the energy is still at wavelengths we don't see as green.

It's more about physics than it is about perception. As a blackbody heats up, it initially produces almost all of its visual output at the red end of the spectrum. And that's what we see... saturated red. As it heats up more, the peak pushes towards shorter wavelengths, but there is still a lot of energy at longer wavelengths. So as the peak pushes towards orange and yellow, we see those as less saturated, because the longer visible wavelengths are still mixed in. And although it's sharper, there are some shorter wavelengths, too. By the time the peak is in the green, there is a lot of energy all across the visible spectrum, so we see something like a warm white. As it gets still hotter, we still see a very desaturated color, but the long wavelength tail finally gets low enough that we're seeing significantly less red and orange in the mix, which imparts a blue cast to the color.

[Ann]

There is something called the CIE Chromaticity Diagram, where the color green is quite dominant. I have read somewhere that if the sensitivity and night color vision of our eyes were much better than it is, then we would be able to see Sun-like stars as green.

But I think that if we had that sort of night vision, the daylight Sun would burn our eyes to a crisp.

Ann

[Ron-Astro Pharmacist]

Fascinating that an International Commission on Illumination and an objective specification on color quality exists. Thanks Ann and Chris for your further illumination.

http://en.wikipedia.org/wiki/CIE_1931_color_space

http://en.wikipedia.org/wiki/Chromaticity

Instead of the eye having developed a two-dimensional surface mechanism from which to absorb the various wavelengths, one could envision it having depth; a three-dimensional organ with the various wavelength penetrating to different depths. I really don't know how the sensors of particle accelerators are designed but could you imagine if a mechanical eye might one day be built to sense a wider range of wavelengths? What would the sky look like?

Maybe I'll find out tomorrow on APOD?

[Chris Peterson]

There is something called the CIE Chromaticity Diagram, where the color green is quite dominant. I have read somewhere that if the sensitivity and night color vision of our eyes were much better than it is, then we would be able to see Sun-like stars as green.

But I think that if we had that sort of night vision, the daylight Sun would burn our eyes to a crisp.

No, our vision would have to be radically different- more spectral (lots of different narrow wavelength sensors) and not trichromic. Sensitivity isn't the issue. The problem is that a star like the Sun may have its peak output in the green, but the green region of the spectrum still represents only a small part of the total energy. That's different for the red and blue ends (cool and hot).

[Chris Peterson]

Instead of the eye having developed a two-dimensional surface mechanism from which to absorb the various wavelengths, one could envision it having depth; a three-dimensional organ with the various wavelength penetrating to different depths. I really don't know how the sensors of particle accelerators are designed but could you imagine if a mechanical eye might one day be built to sense a wider range of wavelengths?

That's pretty much how the Foveon sensor used on a few digital cameras works. Different energy photons penetrate to different depths. They just pick off a red, green, and blue signal, but in principle you could have much finer spectral resolution.

[geckzilla]

I was reading recently about humans being able to perceive infrared light under certain conditions. Of course, it doesn't look like any new color.
http://www.pnas.org/content/early/2014/11/25/1410162111