From the June 2004 issue

Am I seeing the real thing?

Astrophotography shows the true colors of celestial objects.
By | Published: June 28, 2004
June 28, 2004
Lots of people wonder if the colors they see in images of celestial objects (like the ones from the Canada-France-Hawaii Telescope atop Mauna Kea) are real or the result of some sort of imaginative image processing. Are all those reds, blues, and greens really there? Can we see them through a telescope?
The Trifid Nebula (M20)
This nebula in the constellation Sagittarius the Archer is one of the most beautiful of all nebulae and an example of a combination reflection/emission nebula. The blue color comes from the light of very bright, young stars that is being scattered by dust in the outer regions of this nebula. The red color is created by gas excited by the energy given off by the stars.
Jean-Charles Cuillandre (CFHT)
Without question, the colors are real, but we usually don’t see them when we look through a telescope. Different kinds of objects in space show us their true colors in several ways. The planets in our solar system shine by reflected sunlight. The creamy yellows and browns of Saturn, the blue of Neptune’s atmosphere, and the red rocks and soil on Mars appear that way because those are the colors each reflects best.

Nebulae come in a variety of beautiful colors that are captured by astroimagers and reproduced in Astronomy. But why don’t we see these vivid colors through telescopes? The answer, quite simply, is that most objects are not bright enough to trigger the color receptors in our eyes.

Human vision is divided into two types. “Photopic” vision is the type we employ mainly in the daytime, when we’re looking at objects flooded with light. It’s sometimes just called “color vision” and involves the receptors in the eye known as cones. “Scotopic” vision is used under low levels of illumination and involves the receptors called rods. For our photopic vision to be triggered, the object we’re looking at must be reasonably bright, or our telescope must be large enough to make it appear bright. Take the colors of nebulae, for example.

NGCs 6823 and 6820
Two celestial objects can be seen in this image: star cluster NGC 6823 (upper left) and emission nebula NGC 6820. Stars form from nebulae in clusters rather than individually, and all the stars don’t form at once, so light from the first-formed stars illuminates the remaining nebula.
Jean-Charles Cuillandre (CFHT)
Ninety-nine percent of all nebulae appear white or whitish when we look at them through telescopes. Why? It’s not because all those nebulae are white. It’s because they’re not bright enough for their light to trigger the cones in our eyes. Several nebulae do appear colored, however, when viewed through a large telescope — the Orion Nebula (M42), for example, shows subtle colors in telescopes with apertures of 12 inches or larger.

You’re more likely to see color in deep-sky objects of a different type — planetary nebulae. Planetary nebulae are generally not large, so their light is concentrated in a relatively small area of an eyepiece’s field of view. The Blue Snowball (NGC 7662) in Andromeda is a great example. We see objects like this as colored because telescopes collect much more light than our eyes. The bigger the scope, the more colored nebulae it will show you.

The detectors used by amateur astrophotographers also have a big advantage over our eyes. A film or CCD camera can collect light from a celestial object over seconds, minutes — even hours. The human eye is limited to a 1/30-second exposure. That is, our brain refreshes our view 30 times a second. This limitation (similar to a camera shutter) allows only so much light into the eye at any one time, and it guarantees cameras will record much greater detail and color. So enjoy the pictures — they’re real.

Star-forming region NGC 281
Stars are born in nebulae – vast clouds of interstellar gas and dust. Before the first stars within a nebula begin to shine, the cloud is dark. After star formation begins, however, gas within the nebula is excited by the new stars’ energy and the nebula becomes a colorful celestial object. NGC 281 lies within the boundaries of the constellation Cassiopeia the Queen.
Jean-Charles Cuillandre (CFHT)
Deeper in space, the most colorful objects are nebulae. A small percentage of these are “reflection” nebulae. As the name implies, we see these objects because they reflect light from nearby stars. Because nebulae are made of gas and dust, these object don’t just reflect light, they also scatter it, causing reflection nebulae to appear bluish. The same thing happens on Earth: Sunlight is scattered by our atmosphere, coloring the clear sky blue.

Most visible nebulae are “emission” nebulae (and, yes, there are combination emission/reflection nebulae). An emission nebula shines because new, hot stars within the cloud are exciting the atoms of gas (raising them to a higher energy level). Individual atoms of gas in the nebula absorb the energy from starlight. Because the atoms can’t hold this extra energy for long, they re-emit it as light with a distinctive red color.