# Green star (astronomy)

In astronomy, a green star is a white or blue star that appears green due to an optical illusion. There are no truly green stars, because the color of a star is more or less given by a black-body spectrum and this never looks green. However there are a few stars that appear green to some observers. This is usually because of the optical illusion that a red object can make nearby objects look greenish. There are some multiple star systems, such as Antares, with a bright red star where this illusion makes other stars in the system look green.

Finally I come to a book that says, "Mathematics is used in science in many ways. We will give you an example from astronomy, which is the science of stars." I turn the page, and it says, "Red stars have a temperature of four thousand degrees, yellow stars have a temperature of five thousand degrees . . ." -- so far, so good. It continues: "Green stars have a temperature of seven thousand degrees, blue stars have a temperature of ten thousand degrees, and violet stars have a temperature of . . . (some big number)." There are no green or violet stars, but the figures for the others are roughly correct. It's vaguely right -- but already, trouble! That's the way everything was: Everything was written by somebody who didn't know what the hell he was talking about, so it was a little bit wrong, always! And how we are going to teach well by using books written by people who don't quite understand what they're talking about, I cannot understand. I don't know why, but the books are lousy; UNIVERSALLY LOUSY!

Richard Feynman (1997, p.293), annoyed by a textbook discussing green stars.

## Why stars are not green

The colors of black-body radiation and most stars lie on the Planckian locus (the curved black line near the center of the diagram), with the corresponding temperature given in kelvins (in CIE 1931 x,y space). The spectral (rainbow) colors lie on the outer curved part of the diagram, with their wavelength given in nanometers.

A star is usually close to being a black body, give or take a few spectral lines, so its color is usually more or less the color of a black body. The color of a blackbody lies on the Planckian locus in the middle of the diagram on the right. As can be seen, this locus happens to pass through red, orange, yellow, white, and light blue areas, and one can indeed see many stars of these colors. On the other hand it does not pass through green, indigo (dark blue) or violet areas, so stars that appear to have these colors are rare and depend on some additional optical effect.

The (blackbody) colors of stars are sometimes confused with the colors of the spectrum, as in the textbook mentioned by Feynman in the quote above. The spectral (rainbow) colors are those on the curved part of the boundary of the diagram on the right. As can be seen, the red, orange, yellow and blue rainbow colors happen to be much the same as blackbody colors. However, stars whose peak emission is green light also emit much red and blue light, and the human visual system happens to interpret this mixture of colors as whitish rather than green. So the fact that some spectral colors appear as star colors is more a quirk of human color vision than a property of stars: if one uses an instrument such as a spectroscope that is better at distinguishing wavelengths of light, then all spectral colors look completely different from star colors. All sufficiently hot stars look about the same shade of blue (and not violet as claimed in some popular accounts). The reason for this is that at sufficiently large temperatures (above about 20000 degrees) all blackbody spectra look about the same in visible light, though they can differ lot at shorter wavelengths. Although their maximum output at visible wavelengths is at violet, they put out enough light at other wavelengths to look light blue: the color at the end of the Planckian locus rather than the color at the end of the spectrum.

Human color vision is in fact more complicated than suggested by the explanation above, and in particular the perceived color of an object depends not only on the light it emits, but also on the colors of nearby objects. For example, a blue object close to a red object may appear somewhat greenish; this effect accounts for many apparently green stars.

Strictly speaking, green stars are very unlikely rather than completely impossible. The spectrum of a star is not quite a black body spectrum but is altered by absorption and emission lines of the elements and compounds in it. It is conceivable that among the 1022 stars in the visible universe there are a few with such unusual structures and chemical compositions that their color is visibly altered from a black body color, but no examples of this seem to be known.

## Objects that resemble green stars

Although there are no truly green stars, there are many astronomical objects that can sometimes appear to be green stars. This section lists some of them.

### Multiple stars

Antares (red) and its companion star, Antares B

...there is no instance of an isolated deep blue or green star; these colors are apparently confined to the compound stars.

Elias Loomis, A Treatise on Astronomy, 1877, page 299

There a few stars in double or multiple star systems that appear greenish, even though they are really blue or white. This can happen if the star system contains a large red or orange star. An optical illusion causes things close to the red star to appear slightly greenish. The classical example is Antares B, the blue companion of the red supergiant Antares, and another example is the star system Almach.

### Nebulae

The green planetary nebula NGC 6826

Some planetary nebulae glow green, especially if they contain oxygen. These nebulae, or stars within them, may appear to be green stars. Some examples are the planetary nebulae NGC 6572, NGC 6826 and NGC 7009.

### Zubeneschamali

Another mystery concerns the fact that this white star has so often been described as "greenish" or "pale emerald". Olcott refers to it as "the only naked-eye star that is green in color" while T.W.Webb refers to its "beautiful pale green hue". Star colors are strangely elusive, of course, and there are many such discrepancies in the guidebooks, but modern observers generally agree that the only stars which definitely appear green are the close companions to red stars such as Antares itself.

Burnham (1978, p.1105) discussing Zubeneschamali.

The star Zubeneschamali is usually reported to be white by modern observers. However its color is controversial, and many earlier observers state that it is green. There seems to be no consensus about what its color really is, and no generally accepted explanation for why some observers see it as green.

### Uranus

Uranus

The planet Uranus, occasionally mistaken for a star, can appear greenish as it has a lot of methane that absorbs red light.

### The sun

Green flash

The sun can sometimes appear as a green spot for a second or two as it is rising or setting: this is known as green flash. Roughly speaking, the red light from the sun is blocked by the earth, the blue light is scattered by the atmosphere, and the green light is refracted by the atmosphere to the observer. A similar effect can occasionally be seen with other astronomical objects such as the moon and bright planets.

### False color images

The green blobs are false-color images of (non-green) stars.

Astronomical images are sometimes printed in false colors, which can make stars look green.

### Flying objects

Aircraft have a green navigation light on their starboard wing.