Colors of the Cosmos
© Neil deGrasse Tyson
Adapted from Natural History magazine March 2002
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.
Or is it?
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 degrees Kelvin
and are generally described as red. Yet the filament of
a high-wattage incandescent light bulb rarely exceeds
3,000 degrees Kelvin (tungsten melts at 3,680 degrees)
and looks very white. Below about 1,000 degrees, 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
diaplays a dull red on its surface with regions og 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.
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Astrophysicist Neil deGrasse Tyson is the Frederick P.
Rose Director of New York City’s Hayden Planetarium
and is a visiting research scientist at Princeton University
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