The Milky Way, with its myriad of stars and nebulae, rules the summer night skies.
The core of the Milky Way lies in the constellation Sagittarius. For those of us observing from mid-latitudes in the northern hemisphere, Sagittarius never rises high in the sky. At its best, it is seen low in the south during summer months. Over the next several weeks, the characteristic teapot asterism that marks the constellation Sagittarius will be in the best position for viewing around midnight.
The Milky Way contains many interesting nebulae. One of my favorites is the Eagle Nebula. It lies a little bit to the north of the constellation Sagittarius in the neighboring constellation Serpens Cauda.
Shown above is a picture of the most interesting part of the Eagle Nebula. The unusual cloud formation is called the Pillars of Creation.
True confessions: even though I took the picture shown above, I have never seen the Pillars of Creation with my own eyes. I, like most people, live in a light-polluted suburb. Only a few of the very brightest nebulae can be seen visually in a telescope of modest size when viewing under the light polluted skies where I live. The Pillars are not one of them.
I could pack up my equipment and travel to a dark-sky location but fortunately there is a simpler alternative: replace visual views with photographs. Many people prefer the visceral experience of seeing things visually through the eyepiece of a telescope. But, if that visual experience is just seeing a faint and fuzzy smudge, it is often a compelling alternative to opt for producing photographs of these objects.
Carefully done, a photograph can be dramatically better than the visual experience. The reason is that a photograph can be, and generally is, a long exposure. With your eyes, the view gets refreshed several times per second—it’s not possible to build up a long exposure on your retina. But, with a camera attached to a telescope sitting on a mount that tracks objects as they move across the sky, it is possible to make multiple-minute exposures and, with modern computer software, it is easy to stack several such photographs into one very-long-exposure final product.
That is exactly how I took the picture of the Pillars Of Creation shown above. My individual exposures varied in length between four minutes and ten minutes. I gathered these exposures on three separate nights—one night each summer over the course of three summers. The total exposure time was just shy of six hours.
In addition to being a long exposure, my picture also benefited greatly from the fact that I used a Hydrogen-alpha and an Oxygen-III filter. Most nebulae that we see in the Milky Way are of a type called emission nebulae. Light from the nearby surrounding stars excites the electrons of the atoms in the interstellar gas to higher energy levels. As an excited electron returns to its ground state it makes quantized transitions to lower energy levels and, as it does so, gives off photons of very specific wavelengths. In visible light, the predominant transitions produce the Hydrogen-alpha wavelength (6563 Angstroms), the Oxygen-III wavelength (5007 Angstroms), and the Sulphur-II wavelength (6724 Angstroms). Introducing a filter that lets essentially all of the light from these specific wavelengths through while blocking almost all of the light of other wavelengths provides a very powerful method for combating light pollution. Shown below is my picture taken using only a Hydrogen-alpha filter.
The color picture shown earlier is a combination of this Hydrogen-alpha picture with an Oxygen-III picture.
On a computer, a color picture is constructed by assembling three images: a red one, a green one, and a blue one. All the colors we see on the computer screen are just various combinations of red, green, and blue. For example, yellow is red and green in equal parts whereas red and blue combine to make purple. Anyway, I used the Oxygen-III image for both the green and the blue channels and I used the Hydrogen-alpha image for the red channel. Such a combination produces a picture that is fairly close to how the nebula would look visually through a telescope if one could get a big enough telescope and dark enough skies to actually see the nebula.
One of the iconic pictures taken from the Hubble Space Telescope is the Pillars of Creation picture shown below. This is a false color picture constructed from images taken with the three narrowband filters Sulphur-II (shown as red), Hydrogen-alpha (shown as green), and Oxygen-III (shown as blue). In reality, both Sulphur-II and Hydrogen-alpha are red. Hence, mapping Hydrogen-alpha to green produces a false coloring. That is why the stars have an odd pinkish appearance.