Color in the night sky: Photographing nebulas |

Color in the night sky: Photographing nebulas

Rick Spitzer
Special to the Daily

One of the most fascinating and beautiful deep space objects an astrophotographer can image is a nebula.

The word nebula means cloud or fog. The reality is that it is a distinct body of interstellar clouds made of hydrogen, helium, molecular clouds, cosmic dust and ionized gases.

It is, however, something humans will never really see in the way that they are imaged from Earth. The reason for that is that these objects in space are huge — really huge! Some are hundreds of light years across. The Orion Nebula is the brightest nebula in the night sky and can be seen with the naked eye, in Orion’s sword, as a bright area.

The Orion Nebula is about 20 light years from earth and 12 light years (about 71 trillion miles) in diameter. When viewed with a telescope, it is about twice the diameter of the full moon. If you weighed an area of a nebular cloud the size of the Earth, it would only weigh a couple of pounds. If you were flying toward it, the nebula would never have any real detail because of the huge size and low density.

When an astrophotographer takes a photo of a nebula, it does not have much color. The colors that we see in processed images are due to the different chemistry in the nebula cloud.

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The dust in the cloud can be microscopic. Small particles can scatter blue light and transmits the red. Scattered light from the nebula is just like on Earth where smoke in the air produces red sunsets. The nebula may appear as a smudge in the night sky, but by using some photo processing software, different colors can be saturated and made to stand out. When taking pictures of the images, filters of different colors and types can produce different colors in the final image. The results can be beautiful.

Stars often form in nebulae as the gas, dust and other materials form denser regions. At some point the area will become dense enough to form stars, planets and other planetary objects. Reflections of the starlight, or starlight that is blocked by dust, forms the patterns we see in the processed images of the nebula.

One of the most famous nebulae is the Horsehead Nebula. This nebula is approximately 1,375 light years from Earth and is one of the most identifiable nebulae because it resembles a horse’s head. It is composed of a dark cloud of dust and gas with sections of nearly complete opacity and other areas of transparency.

These images require a good apochromatic scope and a star tracker to match the rotation of the Earth so that exposures of 3-5 minutes can produce the colors and details seen. The colors of nebula are not what can actually be seen, but with computer processing they tell us a lot about the composition of the nebula.

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