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View Full Version : So....about green stars....



lupendark
2011-Apr-04, 01:25 AM
We have all heard and read about red stars, blue stars as well as several other shades, so why do we never hear about stars that radiate in the green portion of the visible spectrum?

Nick Theodorakis
2011-Apr-04, 02:14 AM
They are stars that radiate in green part of the spectrum; in fact, our sun is one of them. We just don't perceive them as green due to the mix of other wavelengths emitted.

See:
http://blogs.discovermagazine.com/badastronomy/2008/07/29/why-are-there-no-green-stars/
http://www.universetoday.com/25152/are-there-green-stars/

Nick

EDG
2011-Apr-04, 02:15 AM
We do. They're white ;)

A star radiating most of its light in the green part of the visible spectrum is also radiating a lot of light in the red and blue parts on either side too, but that all balances out to make white starlight. You'd have to somehow prevent the star from radiating in the red/blue parts of the spectrum for it to appear green.

A red star is radiating most of its light in the red part of the spectrum, with some light in the orange and infrared parts. However, since we can't see the infrared light, the star looks red to us.

WayneFrancis
2011-Apr-04, 05:56 AM
I'm not sure if you know this but white is not actually a colour. There is no frequency of light that is white. All stars emit light in a bit of a bell curve. Our sun peaks in the green range of frequency. Our eyes are most sensitive to green light
http://upload.wikimedia.org/wikipedia/commons/thumb/e/e9/Eyesensitivity.png/287px-Eyesensitivity.png

Take a bit of blue, take a bit of red and take a bit of green and put them close enough together and we see not those individual colours but white. Go ahead get REAL close to your monitor and see that white turn into a bunch of coloured pixels.

This isn't by chance either. It is expected that many forms of life would adapt to seeing in the band of light that is most abundant. Evolution works that way. You don't see any life out there with gamma ray vision because there is little to see in the gamma ray band :)

Shaula
2011-Apr-04, 06:34 AM
ISTR there are only two stars in the sky that are said to have a green tint. But most people would class them as white. Collins Gem guide said so, it must be true!

Jeff Root
2011-Apr-04, 07:52 AM
I think it is pointless to say that white isn't a color. Compare any
two different materials that are white. They will be different colors.
Compare any two stars that look white. They will have different
spectra.

I'd argue that black is a color, too, although the argument may not
be as clear-cut as the case for white being a color.

-- Jeff, in Minneapolis

eburacum45
2011-Apr-04, 10:17 AM
One star which is sometimes described as green is Beta Librae, Zubeneschamali, apparently:
http://stars.astro.illinois.edu/sow/zubenes.html
to be honest I've never looked at it to find out. It tends to be a bit low in the sky from here.

Hornblower
2011-Apr-04, 12:20 PM
One star which is sometimes described as green is Beta Librae, Zubeneschamali, apparently:
http://stars.astro.illinois.edu/sow/zubenes.html
to be honest I've never looked at it to find out. It tends to be a bit low in the sky from here.

Beta Librae is close to the threshold of color vision, according to a Sky and Telescope article on the perception of star colors some years ago. If the blue and red cones give up before the green ones, this might explain the greenish tint some observers have reported. With a brighter star with the same tint, all three sets of cones are working well and our visual cortex is adapted to see this tint as white.

It would be interesting to look at such luminaries as Sirius, Vega or Rigel, which have a similar tint, through suitable neutral density filters to bring them down to 3rd magnitude.

chornedsnorkack
2011-Apr-04, 12:27 PM
Beta Librae is close to the threshold of color vision, according to a Sky and Telescope article on the perception of star colors some years ago. If the blue and red cones give up before the green ones, this might explain the greenish tint some observers have reported. With a brighter star with the same tint, all three sets of cones are working well and our visual cortex is adapted to see this tint as white.

It would be interesting to look at such luminaries as Sirius, Vega or Rigel, which have a similar tint, through suitable neutral density filters to bring them down to 3rd magnitude.

Wouldn´t simplest "neutral density filter" be optical smallification? Such as employing inverted binoculars?

Hornblower
2011-Apr-04, 12:53 PM
Wouldn´t simplest "neutral density filter" be optical smallification? Such as employing inverted binoculars?Yes, that would work, but it might be excessive for the stars in question. Pinholes of various sizes held close to the eye might also be useful.

George
2011-Apr-04, 02:10 PM
The following should demonstrate why you won't see green stars, unless a companion star of another color causes the effect (eg Zubeneschamali).

Ignoring the really low temperature stars (T-Class), stars are blackbody radiators, but not quite. So, as has been mentioned, blackbody (Planck) radiators produce light throughout the visible spectrum; there are no large gaps in the spectrum that would allow green to shine (choosing the pun approach, of course :) ).

If they could, based on a Planck distribution model, the Sun might appear green because it has a surface temperature of 5777K. At this temperature it has a peak wavelength of 501.6 nm, which is on the boundary between green and cyan (blue green) within the spectrum. [The Sun's Planck curve, based on a juxtaposition of actual solar irradiance, is 5850K (495.4nm). Solar physicists have 5 different important temperatures they use for the Sun, ignoring internal temperatures.]

Here is a spectral irradiance (from space) plot that has been "colorfied":

http://img51.imageshack.us/img51/5703/solarirrcomp.th.jpg (http://img51.imageshack.us/i/solarirrcomp.jpg/)

Notice that the actual measured peak wavelength is in the blue portion of the spectrum. The reason for this, no doubt, is due to the fact that we can see deeper into the central portion of the Sun's disk. The central region of the Sun has a surface temperature of about 6390K, the limb is about 5000K, so we see the integrated result in a spectral irradiance measurement.

As for our vision of star color, especially the Sun, the better way to represent what we see, I think, is to convert the above plot to a photon flux, since photons better represent what happens within our eyes. Here 'tis...

http://img859.imageshack.us/img859/5168/solarflux.th.jpg (http://img859.imageshack.us/i/solarflux.jpg/)


Since E = Hf, then the blue end of the spectrum gets cut almost in half relative to the red end of the spectrum. The result is a surprisingly flat photon distribution of the colors. [I think the actual peak in this is.... yellow! Of all the bad luck since a yellow star it definietly is not!]

It would take a very strong green peak to allow a green color seen, but it can't happen in stars. Also, a combination of peaks in a color spectrum that has little or even no green can produce a green color effect. This would be a metamer. However, from the above information, there is no way that will happen for stars.

Red or orange stars are more prodigious in the red end of the spectrum, and very hot blue stars are also strong in the blue, no surprise. [You won't ever see, however, a saturated blue star because of the E= hf issue, which favors the red end of the spectrum, desaturating the blues. However, the center of star would look blue since around 15 million Kelvin produces a very steep slope such that the blues far out number the reds.]

No single green stars. Green eggs and ham, perhaps. :)

DonM435
2011-Apr-04, 08:15 PM
I was told that Antares B looks green because of the contrast with the orange-red primary star, but that it's really white or blue.