By Dan Brady and Ryan Wyatt
Colors of the Cosmos
Red, green, and blue may mean one thing to a scientist and something different to everybody else.
Only a few objects in Earth’s nighttime sky are bright enough to trigger our retina’s color-sensitive cones. The red planet Mars can do it. As does the blue supergiant star Rigel (Orion’s right kneecap) and the red supergiant Betelgeuse (Orion’s left armpit). But aside from these stand-outs, the pickings are slim. To the unaided eye, space is a dark and colorless place. Not until you aim large telescopes does the universe show its true colors.
Glowing objects, like stars, come in three basic colors: red, white, and blue—a cosmic fact that would have pleased the founding fathers. Interstellar gas clouds can take on practically any color at all, depending on which chemical elements are present, and depending on how you photograph them, whereas a star’s color follows directly from its surface temperature: Cool stars are red. Tepid stars are white. Hot stars are blue. Very hot stars are still blue. How about very, very, hot places, like the fifteen million degree center of the Sun? Blue. To an astrophysicist, red-hot foods and red-hot lovers both leave room for improvement. It’s just that simple.
A conspiracy of astrophysical law and human physiology bars the existence of green stars. How about yellow stars? Some astronomy textbooks, many science fiction stories, and nearly every person on the street, comprise the Sun-Is-Yellow movement. Professional photographers, however, would swear the Sun is blue; “daylight” film is color-balanced on the expectation that the light source (presumably the Sun) is strong in the blue. The old blue-dot flash cubes were just one example of the attempt to simulate the Sun’s blue light for indoor shots when using daylight film. Loft-artists would argue, however, that the Sun is pure white, offering them the most accurate view of their selected paint pigments.
No doubt the Sun acquires a yellow-orange patina near the dusty horizon during sunrise and sunset. But at twelve-noon, when atmospheric scattering is at a minimum, the color yellow does not spring to mind. Indeed, light sources that are truly yellow make white things look yellow. So if the Sun were pure yellow then snow would look yellow—whether or not it fell near fire hydrants.
To an astrophysicist, “cool” objects have surface temperatures between 1,000 and 4,000 Kelvin and are generally described as red. Yet the filament of a high-wattage incandescent light bulb rarely exceeds 3,000 Kelvin (tungsten melts at 3,680 Kelvin) and looks very white. Below about 1,000 Kelvin, objects become dramatically less luminous in the visible part of the spectrum. Cosmic orbs with these temperatures are failed stars. We call them brown dwarfs even though they are not brown and emit hardly any visible light at all.
While we are on the subject, black holes aren’t really black. They actually evaporate, very slowly, by emitting small quantities of light from the edge of their event horizon in a process first described by the physicist Stephen Hawking. Depending on a black hole’s mass, it can emit any form of light. The smaller they are, the faster they evaporate, ending their lives in a runaway flash of energy rich in gamma rays, as well as visible light.
Modern scientific images shown on television, in magazines and in books often use a false color palette. TV weather forecasters have gone all the way, denoting things like heavy rainfall with one color and lighter rainfall with another. When astrophysicists create images of cosmic objects, they typically assign an arbitrary sequence of colors to an image’s range of brightness. The brightest part might be red and the dimmest parts blue. So the colors you see bear no relation at all to the actual colors of the object. As in meteorology, some of these images have color sequences that relate to other attributes, such as the object’s chemical composition or temperature. And it’s not uncommon to see an image of a spiral galaxy that has been color-coded for its rotation: the parts coming toward you are shades of blue while the parts moving away are shades of red. In this case, the assigned colors evoke the widely recognized blue and red Doppler shifts that reveal an object’s motion.
For the map of the famous cosmic microwave background, some areas are hotter than average. And, as must me the case, some areas are cooler than average. The range spans about one one-hundred-thousandth of a degree. How do you display this fact? Make the hot spots blue, and the cold spots red, or vice versa. In either case, a very small fluctuation in temperature shows up as an obvious difference on the picture.
Sometimes the public sees a full-color image of a cosmic object that was photographed using invisible light such as infrared, or radio waves. In most of these cases, we have assigned three colors, usually red, green, and blue (or “RGB” for short) to three different regions within the band. From this exercise, a full-color image can be constructed as though we were born with the capacity to see colors in these otherwise invisible parts of the spectrum.
The lesson is that common colors in common parlance can mean very different things to scientists than they do to everybody else. For the occasions when astrophysicists choose to speak unambiguously, we do have tools and methods that quantify the exact color emitted or reflected by an object, avoiding the tastes of the image-maker or the messy business of human color perception. But these methods are not public-friendly—they involve the logarithmic ratio of the flux emitted by an object as measured through multiple filters in a well-defined system corrected for the detector’s sensitivity profile. When that ratio decreases, for example, the object is technically turning blue no matter what color it appears to be.
The vagaries of human color perception took their toll on the wealthy American astronomer and Mars fanatic Percival Lowell. During the late 1800s and early 1900s, he made quite detailed drawings of the Martian surface. To make such observations, you need steady dry air, which reduces smearing of the planet’s light en route to your eyeball. In the arid air of Arizona, atop Mars Hill, Lowell founded the Lowell observatory in 1894. The iron-rich, rusty surface of Mars looks red at any magnification, but Lowell also recorded many patches of green at the intersections of what he described and illustrated as canals—artificial waterways, presumably made by real live Martians who were eager to distribute precious water form the polar icecaps to their cities, hamlets, and surrounding farmlands.
Let’s not worry here about Lowell’s alien voyeurism. Instead, let’s just focus on his canals and green patches of vegetation. Percival was the unwitting victim of two well-known optical illusions. First, in almost all circumstances, the brain attempts to create visual order where there is no order at all. The constellations in the sky are prime examples—the result of imaginative, sleepy people asserting order on a random assortment of stars. Likewise, Lowell’s brain interpreted uncorrelated surface and atmospheric features on Mars as large-scale patterns.
The second illusion is that gray, when viewed next to yellow-red, appears green-blue, an effect first pointed out by the French chemist M. E. Chevreul in 1839. Mars displays a dull red on its surface with regions of gray-brown. The green-blue arises from a physiological effect in which a color-neutral area surrounded by a yellow-orange appears bluish green to the eye.
In another peculiar but less embarrassing physiological effect, your brain tends to color-balance the lighting environment in which you are immersed. Under the canopy of a rain forest, for example, where nearly all of the light that reaches the jungle floor has been filtered green (for having passed through leaves), a milk- white sheet of paper ought to look green. But it doesn’t. Your brain makes it white in spite of the lighting conditions.
In a more common example, walk past a window at night while the people inside are watching television. If the TV is the only light in the room, the walls will glow a soft blue. But the brains of the people immersed in the light of the television actively color-balance their walls and see no such discoloration around them. This bit of physiological compensation may prevent residents of our first Martian colony from taking notice of the prevailing red of their landscape. Indeed, the first images sent back to Earth in 1976 from the Viking lander, though pale, were purposefully color-tinted to a deep red so that they would fulfill the visual expectations of the press.
At mid-twentieth century, the night sky was systematically photographed from a location just outside of San Diego, California. This seminal database, known as the Palomar Observatory Sky Survey, served as the foundation for targeted, follow-up observations of the cosmos for an entire generation. The cosmic surveyors photographed the sky twice, using identical exposures in two different kinds of black-and-white Kodak film—one ultrasensitive to blue light, the other ultrasensitive to red. (Indeed the Kodak corporation had an entire division whose job it was to serve the photographic frontier of astronomers, whose collective needs helped to push Kodak’s R&D to its limits.) If a celestial object piqued your interest, you’d be sure to look at both the red- and blue-sensitive images as a first indication of the quality of light it emits. For example, extremely red objects are bright on the red image but barely visible on the blue. This kind of information informed subsequent observing programs for the targeted object.
Although modestly sized compared with the largest ground-based telescopes, the 94-inch Hubble Space Telescope can take spectacular color images of the cosmos. The most memorable of these photographs are part of the Hubble Heritage series that will secure the telescope’s legacy in the hearts and minds of the public. What astrophysicists do to make color images will surprise most people. First, we use the same digital “ CCD ” technology found in household camcorders, except that we used it a decade before you did and our detectors are much, much higher quality. Second, we filter the light in any one of several dozen ways before it hits the CCD. For an ordinary color photo, we obtain three successive images of the object, seen through broad-band red, green, and blue filters. In spite of their names, taken together these filters span the entire visible spectrum. Next, we combine the three images in software the way the wetware of your brain combines the signals from the red, green, and blue-sensitive cones in your retina. This generates a color picture that greatly resembles what you would see if the iris in your eyeball were 94 inches in diameter.
Suppose, however, that the object were emitting light strongly at specific wavelengths due to the quantum properties of its atoms and molecules. If we know this in advance, and use filters tuned to these emissions, we can narrow our image sensitivity to just these wavelengths, instead of using broad-band RGB. The result? Sharp features pop out of the picture, revealing structure and texture that would otherwise go unnoticed. A good example lives in our cosmic back yard. I confess to having never actually seen Jupiter’s red spot though a telescope. While sometimes it’s paler than at other times, the best way to see it is through a filter that isolates the red wavelengths of light coming from the molecules in the gas clouds.
In the galaxy, oxygen emits a pure green color when found near regions of star formation, amid the rarefied gas of the interstellar medium. Filter for it and oxygen’s signature arrives at the detector unpolluted by any ambient green light that may also occupy the scene. The vivid greens that jump out of many Hubble images come directly from oxygen’s nighttime emissions. Filter for other atomic or molecular species and the color images become chemical probe of the cosmos. Hubble can do this so well that it’s gallery of famous color images bear little resemblance to classical RGB images of the same objects taken by others who have tried to simulate the color response of the human eye.
The debate rages over whether or not these Hubble images contain “true” colors. One thing is certain, they do not contain “false” colors. They are the actual colors emitted by actual astrophysical objects and phenomena. Purists insist that we are doing a disservice to the public by not showing cosmic colors as the human eye would perceive them. I maintain, however, that if your retina were tunable to narrow-band light, then you would see just what the Hubble sees. I further maintain that my “if” in the previous sentence is no more contrived than the “if” in, “If your eyes were the size of large telescopes.”
The questions remains, if you added together the visible light of all light-emitting objects in the universe, what color would you get? In simpler phrasing, What color is the universe? Fortunately, some people with nothing better to do have actually calculated the answer to this question. After an erroneous report that the universe is a cross between medium aquamarine and pale turquoise, Karl Glazebrook and Ivan Baldry of Johns Hopkins University corrected their calculations and determined that the universe is really a light shade of beige, or perhaps, cosmic latte. Glazebrook and Baldry’s chromatic revelations came from a survey of the visible light from more than 200,000 galaxies, occupying a large and representative volume of the universe.
The nineteenth-century English astronomer Sir John Herschel invented color photography. To the frequent confusion but occasional delight of the public, astrophysicists have been messing with the process ever since—and will continue forever to do so.
Colors of the Cosmos
By Dan Brady and Ryan Wyatt
This past Thursday at Nightlife, Dan Brady presented Colors of the Cosmos in the Morrison Planetarium. We thought we’d also share it with our Science Today readers. Click through the links to see the colorful images and enjoy!
The phrase “Colors of the Cosmos” makes for a catchy title, but real scientific value lies behind those alliterative words. Generally, we can divide our exploration of space pantones into two groups: the kind we see directly with our eyes, and the kind we use special instruments to reveal. This article is brimming with visual aids, so be sure to click on all the links below to appreciate the full beauty of this topic.
Just looking at the night sky reveals some subtle variations that are often obscured by San Francisco’s city lights and persistent fog. Orion occasionally shines through these obstacles, revealing some noticable color differences. Contrast the red of Betelgeuse with the blue of Rigel down below. Or on either side of Orion, the white Sirius with Taurus’s bloodshot eye, Aldebaran. These are subjective experiences: visible for all of human history, they’ve inspired myths and curiosity, but in the last few centuries, we’ve been able to focus some questions into answers.
In fact, most of what we know about the Universe comes from studying light from far away destinations. Not all of it is directly visible, however: either because our eyes are not sensitive to it (think radio waves, x-rays, and infrared), or because our atmosphere has shielded us from detecting the subtleties. Color, or the colors we see with our eyes, offer important clues to understanding the Universe, but from now on we may also use some representative colors to help us visualize the stuff beyond our normal perception.
The way we observe star clusters is a great example of how our digital telescopes can objectively quantify light in brand new ways. NGC2547 lies in the constellation Vela (not visible to us in the Northern Hemisphere) and while this photo is full of color, it is actually a composite of several black and white images. Just like the pixels in your phone or TV, the light we see is a combination of red, green, and blue wavelengths. Telescopes don’t work this way: they have to take photos of red, green, and blue light one at a time, with filters covering their digital sensors. These give us raw, black and white images—but the information is separated into the three primary colors of light. Look closely, and you’ll notice significant differences between each of these three filtered images. Then we add the appropriate colors, merge the images, and, voila: a complete, color image of a nearby group of stars.
Astronomers are interested in the individual wavelengths as well as the combined color image: the various stars in NGC2547 can emit very different wavelengths of light from even their closest neighbors. By measuring and comparing the difference in light output in different colors, astronomers can quantify the stars’ colors and even determine the age of a star cluster. (NGC2547 turns out to be between 20 and 35 million years old, BTW.)
At this point, it’s fair to ask yourself, “So what?” Colors sure are pretty, and astronomers can turn colors into numbers, but how important are they? Well, are you in your work place? Look up. What kind of lights do you see? Fluorescent? Incandescent? Halogen? We all know these lights shine with different colors. Neon signs are the clearest example—so, okay, maybe I should have had you imagine Las Vegas. Apologies. We can compare this everyday light sources to more distant examples…
Take a look at these different images from the Orion Nebula (pictured, above right). Light bulbs shine in different colors because there are different elemental gases inside them. And the same thing goes on with these formative stars: the colors (or wavelengths of light) that we don’t see tell us what gases and elements are inside, because their atoms absorb specific wavelengths as they pass through. The red stuff is hydrogen, seventy five percent of our visible universe by mass. The rest are elements that make up you and me: carbon, oxygen, nitrogen, and all the others in the periodic table.
In stars, colors reveal something even more meaningful: temperature. We know that some things burn red or white hot, but more specifically, we know that anything in the Universe of a certain temperature glows most brightly at a specific color (or wavelength of light). Knowing how color relates to temperature allowed us to calculate the ages of stars in NGC2547, but we use this relationship to pin down temperatures of much cooler objects as well. Information like this has been hidden to us until very recently—and one sky survey, called 2MASS, has revealed that there are many more light sources than we imagined hanging out in the infrared spectrum. And stars aren’t the only bright things out there: even the dust in our galaxy glows dimly in the spaces between the stars.
Asking “why is the sky blue?” even fits in here: when the white light of our sun hits our atmosphere, shorter-wavelength (blue) light scatters more than longer-wavelength (red or green) light. This basic childhood mystery turns out to have serious implications when we look at exoplanets orbiting other stars. Just in the past few months, astronomers have announced the colors of some of these distant planets—and they provide clues as to what’s on the surface. We’re not quite at the stage where we can see what’s fashionable in alien clothing trends or whether the leaves on their trees are green or purple, but we’re getting closer with each new generation of telescope.
Okay, the briefest of reviews: colors are everywhere in the cosmos; we quantify them using scientific instruments; different colors mean different temperatures or different energetic molecules emitting light; and a lot of the colors (or wavelengths of light) in the Universe are invisible to us humans.
Now it’s time to look at the rest of the electromagnetic spectrum… The wavelengths of light we cannot see!
Gamma rays are the most energetic waves in physics—carrying around a billion times more energy than visible light—so they usually come from the most powerful things in the Universe. In this map from the FERMI sky survey, the brightest sources lie along the galactic plane, where there’s the most stuff close to our telescopes. But other sources include pulsars and supernovae: collapsed or exploded stars that shoot x- and gamma-rays deep into the night sky. These sources show up as bright spots away from the streak of our Milky Way.
But the biggest questions in cosmology today come from radiation that is all but invisible to most of us, although it permeates every inch of the Universe. And to see it, we zoom way out: past our local group of galaxies, through the brightly colored galaxies of the Sloan Digital Sky Survey, the Two Degree Field Survey, and even past the oldest primordial stars known as quasars. It’s here that we can finally see the oldest image of the entire Universe, the Cosmic Microwave Background (CMB).
In the image of the CMB taken by the Planck space telescope, the colors represent variations in the temperature and density of the early Universe: blue-black corresponds to the coolest, densest parts of the image, and red marks the hottest, least dense regions. The midpoint of this color representation, the bright green, has a temperature associated with it: 2.7 Kelvins above absolute zero, the current average temperature of the cosmos. Amazingly, the difference between the hottest part of the image and the coolest works out to only one part in a hundred thousand! The darkest blue is only one-one-hundred-thousandth cooler than the reddest red.
This remarkable cosmic observation also has some fans on the Internet, most famously Randall Munroe of the webcomic xkcd—you can even buy t-shirts on his website to express how warmly you feel about this discovery. The overall shape of the curve in the xkcd comic tells us the temperature (the aforementioned 2.7 Kelvins), and the tiny variations indicate the temperature above and below that average.
This temperature map is the oldest image of the Universe because it’s the farthest back we can look back in time. Before this time, three hundred and eighty thousand years after the Big Bang, the Universe was so hot and dense that light couldn’t even travel through: this image dates from the precise moment when the Universe was cool enough to let electrons and protons come together to form the first hydrogen atoms.
The Planck CMB is as far back and as far out as we can go, so let’s head back home, diving in through the quasars, distant galaxies, towards our local group and the Milky Way. The colors we’ve explored have ranged from the extremely hot, where energetic gases form new stars or signal the death of the old; to the frigid, blackness of space, where temperatures hover near absolute zero and shelter secrets from the origins of the Universe. We’ve seen the visible colors of nebulae and distant planets, and the false colors of the Milky Way using x-rays, infrared, and microwaves. All these colors are visible only with the collaboration of astronomers all around the world. Their hard work has revealed a Universe that has been hidden until very recently—but here all along.
Dan Brady is a planetarium presenter at the California Academy of Sciences. He earned his BS in Physics from UCLA and has taught science since 2008.
Ryan Wyatt is the director of the Morrison Planetarium and Science Visualization at the California Academy of Sciences.
