Last year, a pair of Johns Hopkins astronomers goofed while tackling a simple question: What color is the night sky? Karl Glazebrook and Ivan Baldry studied the starlight from 200,000 galaxies, averaged the colors, and announced the surprise result: Taken as a whole, the universe is green. The world lived with a verdant cosmos for three months, until the researchers issued a red-faced retraction. The universe is actually a very pale beige, almost vanilla white.
This was not an isolated error. The heavens are flooded with color misconceptions, many of which are on view in the June sky. In this month of maximum daylight, let's start with the sun, a nice typical star. Ask people to describe the sun's color, and they will say it looks lemony. Ask the astronauts on the International Space Station, and they will insist it's white as snow. They are right. Earth's atmosphere scatters some of the blue component of sunshine, producing the azure color of the sky. The remaining mixture of light, white minus blue, yields a yellow-looking sun.
Even astronauts do not perceive the true solar colors, however. White is merely the retina's response to being bombarded by all the colors of the rainbow. Rather than debating the apparent color of the sun, we might be better off by asking: Of all the colors in sunlight, which one does the sun emit most intensely? The answer shows up clearly in the spectrum cast by a prism of cut glass and in the arc of a natural rainbow. The brightest color is green, because green is where the sun's energy output is strongest. If our sun peaks in the green, and galaxies are collections of suns, and the universe's light comes mostly from galaxies, no wonder those Johns Hopkins astronomers concluded the cosmos is green. Their error was neglecting to determine how human vision responds when a lot of green is blended with other colors.
Albireo, a bright double star in Cygnus, showcases the range of star colors. Seen through a telescope, one component is a cool yellow; the other is blue-white hot. The juxtaposition makes the contrast seem more intense, leading some observers to perceive topaz and sapphire.
At night, objects that emit evenly blended colors, such as the moon and the bright star Vega, look almost pure white. Everything in the night sky that is not white gives off just a single hue or radiates a lopsided mixture of light in which one color overwhelms the others. The most notable color is not green but red.
Mars appears a ruddy orange as it blazes low in the southeastern sky at midnight this month. Rustlike iron oxides on its surface absorb most of the sun's light but preferentially reflect the orange part of the spectrum. Antares, visible in the south most of the night, matches Mars's color—its name means "rival of Mars" in Greek—but the star's pumpkin hue has a different origin. Just like a piece of heated metal that glows red orange before it becomes white hot, Antares is a relatively cool star that emits far more red light than any other color. High overhead in the evening, brilliant Arcturus likewise shines with a tepid yellow-orange tinge.
In fact, green is a color that stars never assume. The hottest stars appear vaguely blue, and the coolest ones can be ruby red. For green to dominate the mix, both the hot blue and the cool red sections of a star's spectrum would have to be suppressed at the same time. That does not happen.
Nevertheless, we do sometimes see green in the sky. When charged particles from the sun excite oxygen atoms in Earth's atmosphere, the atoms can emit a green radiation: the glow of the aurora borealis, or northern lights. If we ever detect a similar glow around a distant planet, it will get our attention because green auroras signify free oxygen, something scientists expect to find only in the presence of plant life. When the era of interstellar exploration arrives, green will signal "go."
For a quick online demonstration of why hot stars look blue and cool ones look red, try this simple (but somewhat technical) interactive Web page: csep10.phys.utk.edu/guidry/java/wien/wien.html.
Why don't the real colors in the sky look like those spectacular images from the Hubble Space Telescope? Hubble researchers can explain: hubblesite.org/sci.d.tech/behind_the_pictures.
See www.pha.jhu.edu/~kgb/cosspec for Karl Glazebrook and Ivan Baldry's exploration of the color of the universe, based on the 2dF Galaxy Redshift Survey.
Learn about the color of the sun at casa.colorado.edu/~ajsh/colour/Tspectrum.html.
Colors of various stars: www.vendian.org/mncharity/dir3/starcolor.
A discussion of colors in the aurora—particularly green—from the University of Alaska at Fairbanks: www.gi.alaska.edu/ScienceForum/ASF14/1441.html.
