I am often asked two questions: Are the colors in astronomical pictures real? Would you see them looking through a large telescope?
Our retinas contain two types of light receptors called rods and cones. There are approximately 120 million rods compared to about 7 million cones. Rods are more sensitive to light but only our cones detect color- cones require more light than rods to become active. This is why objects appear colorless in dimly lit situations- without enough light, our cones cannot function. Interestingly, light is comprised of three primary colors, red, blue and green. Of these, the cones in our eyes are most sensitive to the later. This means that the first color we are able to detect is usually green (which makes some evolutionary sense if your far ancestor's survival was dependant upon discerning plants.)
Astronomical telescopes are essentially used for two purposes: 1) to help separate distant, closely spaced objects and 2) to collect a lot of light. For example, the world's largest telescope, the Grantecan, located off the northwest coast of Africa on the Canary Island of La Palma, has a light gathering mirror 34 feet(!) in diameter that would enable an observer to see each headlight of a car located half a world away in Australia!
However, were it possible to attach an eyepiece and look through this gigantic instrument, the observer would be disappointed to find that the color of galaxies and nebulas would appear greenish-gray. This is because large telescopes produce large magnification and as magnification increases, the apparent surface brightness of the object being viewed decreases. As a result, objects seen with very large telescopes permit the observer to view tiny structures in greater detail but not with enough brightness to show colors! I have experienced this phenomenon personally when viewing the Ring Nebula through the Great 36-inch Telescope at the Lick Observatory. My view of the nebula was highly magnified and a true visual treat but it had no color whatsoever.
The rods and cones in our eyes are like light collecting buckets that are capable of gathering light for about 1/16th of a second. After that period, they empty themselves and send a signal to the brain telling it how much light each one contained. The brain transforms this information into our perception of vision. 1/16 of a second is not a very long exposure length so it's no wonder that our cones need quite a bit of light to detect color.
Cameras, however, do not have this restriction because it's possible to hold their shutter open over a much, much longer period of time.
Yesterday's cameras relied on film coated with an emulsion that contained crystals sensitive to each of the three primary colors of light. The ubiquitous color digital cameras we enjoy today use millions of microscopic red, green or blue filters to cover each of their tiny light sensors, known as pixels. Although each manufacturer uses its own proprietary filter placement pattern, it should be noted that only a portion of the total pixels in modern digital cameras are dedicated to the detection of a specific color. Regardless, because all pixels are equally sensitive to light and color, this helps digital cameras create full color pictures even under low light conditions.
Solid state astronomical cameras go one-step further- they use every pixel for each color.
Electronic cameras specifically designed for taking deep space images are unsurpassed for detecting very faint light but they have one drawback- they can only produce black and white images. To create a full color picture, astronomers, both professional and private, place a red, green or blue filter in front of the camera so that every pixel is used to detect one specific color. This, by the way, is a very time consuming process. For example, to create a full color picture, the astronomer exposes multiple images taken through red, green and blues filters then digitally combines or stacks them using commercially available software.
As a result, digital astronomical cameras are capable of producing deep space pictures with high color fidelity. The images I share with you are exposed using this type of equipment, too.
So, yes, the colors in astronomical images are real but, unfortunately no, you will never see a rainbow of hues when using a telescope to see a galaxy or nebula.
This talk about color is particularly appropriate because image on this page contains a wide range of hues!
At the close of each year, particularly during the month of December, the constellation of Orion can be spotted throughout the night by simply facing the sky to the south. For those who live near or in the Southern Hemisphere, Orion is an obvious sight flying high over head. Off the left shoulder of Orion lies Monoceros, the constellation of the mythical Unicorn. Hidden within this somewhat unremarkable asterism of stars is a truly magical place- a stellar nursery where new stars are actively being formed. This region includes the Fox Fur Nebula, the Cone Nebula and the Christmas Tree Cluster.
The Fox Fur Nebula
The blue hues seen in this image originate from a cloud of microscopic dust particles, far smaller than the stuff in cigarette smoke, that surrounds the bright stars seen here. Generally speaking, cosmic dust is the debris of a stellar explosion. Over millions and billions of years, dust particles drift from the scene of their violent creation, mix with the hydrogen and helium gas that's abundant throughout the Universe and form an thin, optically transparent cosmic soup. The mutual gravitational attraction of one dust particle to another (or the shock-waves of a distant supernova explosion) can gently herd and thicken this thin celestial broth until it becomes an thick, opaque cloud spanning hundreds or thousands of light years across. As the vast cloud continues to contract, it can begin to incubate new stars and form planets.
Incredibly, microscopic, seemingly insignificant, free floating dust grains are the seeds from which much of the world that surrounds us originates. Dust particles are also the progenitors of planetary inhabitants (you and me), too!
Interstellar dust has several optical properties that are also of interest when it comes to viewing a deep space picture. For example, the blue tint in the dust cloud and the color of Earth's skies are created by the same process! When light from the sun strikes the Earth's atmosphere, some is reflected back to space but a portion bounces around within our atmosphere before it strikes a eyeball looking in its direction. This bouncing, known as Raleigh scattering, makes our skies blue.
Here's why. Light is arranged into waves of weightless particles called photons. The distance between the tops of wave crests is different for each color. For instance, blue light has a shorter distance (or wavelength) between the top of its waves than red light. When light strikes an object larger than its wavelength, the wave bounces off and the color is reflected. Since the molecules of oxygen and nitrogen in our atmosphere are larger than a wavelength of blue light (but smaller than a wavelength of red), the gas in our atmosphere reflects blue light while the other colors in sunlight, including red, simply pass through. A similar situation creates the blue hues seen in the Fox Fur Nebula. Instead of atmospheric gasses, however, the blue scattering comes from infinitesimally small interstellar dust particles that are large enough to reflect blue light but too small to bounce red.
The mysterious Cone Nebula
Sometimes the dust within a cloud becomes so thick it prevents light from passing through to the other side by absorbing it. Such is the case with the Cone Nebula, seen near the bottom. We see it back illuminated by stars hidden on its far side. Though you might never guess it, the scale of this image is mind-boggling! For example, the towering Cone Nebula stands about seven light years in length!
Although the scene in this picture appears like a war zone, it's actually a stellar nursery comprised mostly of hydrogen gas and dust- no one said giving birth to a star was a neat and tidy proposition! Most of bright the stars in this picture are its babies and like most celestial newborns, they release a tremendous amount of ultraviolet radiation. Interestingly, ultraviolet energy produces a strange effect when it comes into contact with (normally transparent) hydrogen gas. When ultraviolet light strikes a molecule of hydrogen, one of the molecule's electrons is knocked out of orbit around its nucleus. When that electron is recaptured by another molecule of hydrogen, a photon of red light is released. Thus, the bright blue stars in this image are causing the invisible gas that surrounds them to glow red!
The Christmas Tree Cluster
The Christmas Tree Cluster takes its name, not from its shape but, from the brightness of its individual members- they illuminate the surrounding area with the sparkle of a lit Christmas tree display. This image was created by stitching six separate photographs, featuring over 52 total hours of exposure, into a single seamless mosaic picture so that the area around the Fox Fur and Cone Nebulae could be presented.