APOD: The Reddening of M71 (2014 Dec 10)

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Expand view Topic review: APOD: The Reddening of M71 (2014 Dec 10)

Re: APOD: The Reddening of M71 (2014 Dec 10)

by geckzilla » Sat Dec 13, 2014 12:27 am

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

Re: APOD: The Reddening of M71 (2014 Dec 10)

by Chris Peterson » Fri Dec 12, 2014 11:32 pm

Ron-Astro Pharmacist wrote: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.

Re: APOD: The Reddening of M71 (2014 Dec 10)

by Chris Peterson » Fri Dec 12, 2014 11:24 pm

Ann wrote: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).

Re: APOD: The Reddening of M71 (2014 Dec 10)

by Ron-Astro Pharmacist » Fri Dec 12, 2014 11:23 pm

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?

Re: APOD: The Reddening of M71 (2014 Dec 10)

by Ann » Fri Dec 12, 2014 7:14 pm

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

Re: APOD: The Reddening of M71 (2014 Dec 10)

by Chris Peterson » Fri Dec 12, 2014 6:34 pm

Ann wrote: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.

Re: APOD: The Reddening of M71 (2014 Dec 10)

by rstevenson » Fri Dec 12, 2014 6:20 pm

"It's not easy being green."

Rob

Re: APOD: The Reddening of M71 (2014 Dec 10)

by Ann » Fri Dec 12, 2014 6:19 pm

BMAONE23 wrote:
Kermit in Space
Exactly! :mrgreen: :D 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

Re: APOD: The Reddening of M71 (2014 Dec 10)

by BMAONE23 » Fri Dec 12, 2014 6:00 pm

Re: APOD: The Reddening of M71 (2014 Dec 10)

by Ann » Fri Dec 12, 2014 5:38 pm

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

Re: APOD: The Reddening of M71 (2014 Dec 10)

by Chris Peterson » Fri Dec 12, 2014 5:07 pm

Ron-Astro Pharmacist wrote: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.

Re: APOD: The Reddening of M71 (2014 Dec 10)

by Ron-Astro Pharmacist » Fri Dec 12, 2014 4:57 pm

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?

Re: APOD: The Reddening of M71 (2014 Dec 10)

by Ann » Fri Dec 12, 2014 2:23 pm

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

Re: APOD: The Reddening of M71 (2014 Dec 10)

by rstevenson » Fri Dec 12, 2014 12:39 pm

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.
omegacencenter_hst.jpg
Rob

Re: APOD: The Reddening of M71 (2014 Dec 10)

by Chris Peterson » Fri Dec 12, 2014 12:52 am

Ron-Astro Pharmacist wrote: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:
Of course, you could further boost the saturation in a photo editor if you wanted even more color. But still no greens.

Re: APOD: The Reddening of M71 (2014 Dec 10)

by geckzilla » Fri Dec 12, 2014 12:48 am

Ann wrote:I think that the two last spectra (those from galaxies) have been blueshifted instead of redshifted.
Hm, I don't think so.

Re: APOD: The Reddening of M71 (2014 Dec 10)

by Ann » Fri Dec 12, 2014 12:45 am

geckzilla wrote:Attempt at a drawing.
reddning_vs_redshift.png

Great image, Geck, but I think that the two last spectra (those from galaxies) have been blueshifted instead of redshifted.

Otherwise the illustration is indeed extremely instructive and illuminating.

Ann

Re: APOD: The Reddening of M71 (2014 Dec 10)

by Chris Peterson » Fri Dec 12, 2014 12:45 am

MarkBour wrote:I agree with geckzilla, that a picture could be helpful here. I think I get what you said, but I will look for one that portrays it. (Perhaps it would be labelled a spectral chart?
I think it would be difficult to show in an astronomical image. But a spectral chart is doable. Here is the spectrum of M66, with some simulations applied. The top chart shows the actual spectrum, and an example of what we might see through dust. Note the prominent H-alpha spike at 656 nm. The bottom chart shows the same thing, but now we've moved the galaxy 500 million ly away (from its actual 36 million ly distance). It now has a redshift of z=0.037, which pushes the H-alpha line up to an observed 680 nm.

In either case, the dust creates reddening by attenuating the blue end of the spectrum more than the red end. But the dust never changes the position of any of the emission lines.
ngc3627.png

Re: APOD: The Reddening of M71 (2014 Dec 10)

by Ann » Fri Dec 12, 2014 12:41 am

Ron-Astro Pharmacist wrote:Or even to assigned the stars with a color coded distance estimate. I suppose, when Gaia data become available, this might be done to be able to look at objects to get a visual clue for those stars of close proximity.

http://www.spaceflight101.com/gaia-mission-updates.html
Gaia will produce tremendously interesting results, if things work out as they are supposed to, but unfortunately Gaia will not tell us much about the color and temperature of huge numbers of stars. Gaia will photograph the stars through two filters, a red and a blue one, but that means that all stars that are dust-reddened will be seen by Gaia as red and cool. This means that Gaia will not be able to see the true nature of for example NGC 3603, a compact young cluster full of hot blue stars. This splendid picture has been processed by stephen 63 using Hubble images taken through no fewer than six different filers, to produce an image where the stars look properly blue. To Gaia the stars will look red, because they are reddened by dust to an apparent color redder than the Sun.

Ann

Re: APOD: The Reddening of M71 (2014 Dec 10)

by geckzilla » Fri Dec 12, 2014 12:38 am

Attempt at a drawing.
reddning_vs_redshift.png

Re: APOD: The Reddening of M71 (2014 Dec 10)

by geckzilla » Thu Dec 11, 2014 9:04 pm

MarkBour wrote: I agree with geckzilla, that a picture could be helpful here. I think I get what you said, but I will look for one that portrays it. (Perhaps it would be labelled a spectral chart? Maybe there is one in Dr. Nemiroff's course materials...)
I have been thinking of a way to illustrate the concept. I understand it well enough that I could create a number of images but the problem is containing what is a dynamic concept into a single, easy to understand image. I would hate to make it even more confusing when the fact is, as Chris said, that there really is no confusing dust reddening with redshifting. Dust reddening is something we can easily observe up close and in nearby galaxies but at the distances where redshifting becomes very apparent and there is no mistaking it. The wavelengths are NOT attenuated or scattered. They are clearly shifted because we still receive their signal. In fact they could be both reddened by dust and redshifted at the same time and we are able to see distant dust in distant galaxies. That is why infrared light is so important in astronomy because all of our favorite wavelengths get pushed into this red zone. If the Universe was really full of a fine dust, we'd never even see blue light (as shifted into redder light). Plus, the dust would have to be in an impossibly smooth distribution. What would make the dust do that instead of being thicker in some places and thinner in others? The idea is easily broken down even by an amateur such as myself.

Re: APOD: The Reddening of M71 (2014 Dec 10)

by DavidLeodis » Thu Dec 11, 2014 8:36 pm

I like the image and also that Bob Franke has given a lot of information about it through the "this color composite view" link. I find it adds greatly to the interest of an APOD when at least the date and location an image was acquired is available (at least through a link) and here Bob also gives information on such as the exposures. :)

Re: APOD: The Reddening of M71 (2014 Dec 10)

by Ron-Astro Pharmacist » Thu Dec 11, 2014 6:27 pm

Or even to assigned the stars with a color coded distance estimate. I suppose, when Gaia data become available, this might be done to be able to look at objects to get a visual clue for those stars of close proximity.

http://www.spaceflight101.com/gaia-mission-updates.html

Re: APOD: The Reddening of M71 (2014 Dec 10)

by Ron-Astro Pharmacist » Thu Dec 11, 2014 5:08 pm

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.
Christmas tree.jpg
Christmas tree.jpg (3.58 KiB) Viewed 21956 times

Re: APOD: The Reddening of M71 (2014 Dec 10)

by Chris Peterson » Thu Dec 11, 2014 4:55 pm

MarkBour wrote:So, one starts with a spectrum of light, and dust-reddening reduces the amount coming through, preferentially reducing the higher frequencies, whereas redshifting of distant galaxies moves the whole spectrum over?
Exactly. Indeed, redshift can actually make a source appear more blue, depending on its original spectral content (for instance, if invisible UV shifts into visible blue).

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