Approximately 6,500 light-years away in the constellation Serpens lies an immense cloud of gas and dust that has become a symbol of space exploration – the Eagle Nebula. The image above, which shows a region spanning 25 light-years tall by 16 light-years wide, was taken on September 5, 2024, with Dyer Observatory’s 24″ Seyfert Telescope using a ZWO ASI6200MM Pro camera and hydrogen-alpha filter. The featured image is also a composite of 78 one-minute-exposure images stacked together. We could have taken a single long exposure image of 78 minutes to achieve pretty much the same result, however, if there were deviations in the telescope’s ability to accurately track the nebula as it moved across the sky, the entire exposure would have been ruined and 78 minutes of effort would have been for nothing. Thus, by taking multiple shorter exposures, we can select the best images to combine for a better final result. (And yes, we did have several exposures that had streaked stars that didn’t make the final cut.)
The featured image is greyscale because it was only taken with one filter that allowed through a single color. Colorful astronomical images are often the result of three separate images taken through different filters that each only let through a small portion of the rainbow (e.g., a red filter, green filter, and blue filter). Each of those three images, however, comes from the camera as greyscale images because a camera sensor can only detect the amount of light hitting each of its pixels, not the color. The various brightnesses of the image pixels corresponds to how much light was detected by the camera’s pixels while looking through a particular filter. If you then assign one image to be the red component, one to be green, and the third filter’s image to be blue and digitally combine them, you get a final color image. Single-shot color cameras, such as consumer digital cameras and smartphone cameras, operate the same way except that they possess a filter grid of alternating red, green, and blue filters over individual pixels. The onboard processor takes the intensity levels for the various pixels and converts them into colors for the final image. Some astronomical cameras are made this way as well; the drawback is that the final image has a reduced resolution and one doesn’t have the option of using different types of filters. These cameras are great for “pretty pictures” but monochrome cameras with individual filters, especially narrowband filters that isolate a single color emitted by a specific atom or molecule, are more suited to scientific work.
The darker regions with defined edges are not areas with much less gas – these are intervening regions of high dust concentrations that obscure the glow of the background gas. The three columns of dust in the center of the image, dubbed the “Pillars of Creation,” are cocoons of gas and dust that are still actively metamorphosing pockets of raw material into new stars. For scale, the leftmost pillar is about six light-years high. Within the clouds of gas and dust, where temperatures can drop to as low as a few tens of Kelvin, gravity has a chance to overcome pressure created by the motions of gas and dust, allowing the gas to slowly contract, pull in more gas, and eventually start forming stars. Stars, incredibly hot balls of gas, begin their lives in the extremely cold parts of nebulae such as this. The majority of the gas in the nebula is hydrogen, and when irradiated by nearby stars, hydrogen atoms produce a unique red hue (hydrogen-alpha) that the camera’s filter will let through, allowing the nebula show up especially well in the moderately light-polluted skies above Dyer Observatory.
Though many may not have heard of the Eagle Nebula, most have likely seen it thanks to the Hubble Space Telescope. In 1995, the orbiting telescope snapped an iconic image of the center of the nebula. Nearly two decades and several upgrades later, Hubble slewed once again to the center of the nebula and captured an even higher resolution view. It also imaged the region in the near-infrared (light just beyond the red portion of the rainbow that our eyes cannot see), allowing astronomers to peer through the obscuring dust to get a better look at what was going on inside.
Recently, the James Webb Space Telescope pointed its mammoth mirror toward the Eagle and returned some hauntingly beautiful starscapes including a mid-infrared view that really highlighted the dust composing the star-forming pillars.
Regions like the Eagle Nebula pervade spiral galaxies like our own Milky Way and show up in true-color images as reddish-pink splotches. As new stars are born within the clouds, their light excites the hydrogen gas, causing it to glow its distinctive red hue. Over time as stars continue to use up the raw materials of the cloud and the intense light and “stellar winds” take their toll, the clouds are gradually eroded away. On timescales of a few tens of millions of years, the cloud will gradually disappear. Left behind will be a glittering “open cluster” of stars, siblings born from the same mother cloud.
The Eagle Nebula, also known as M16 since it was the 16th object on French comet-hunter Charles Messier’s list of comet lookalikes, was first noted in 1745 by astronomer Jean-Phillippe Loys de Cheseaux who only described the cluster of stars residing within the region. Roughly two decades later, Messier made note of a faint glow around the stars, suggesting he did detect the nebula.
Want to see the Eagle Nebula yourself? The southern constellation Serpens is visible in the evening sky from early spring to late fall. A backyard telescope will easily pick up the nebula’s brighter stars, but a moderately large telescope and dark skies are required to pick up the dim glow of the surrounding gas. If you have trouble spotting it, late fall to early spring provides us with another example of a spectacular and easily visible star-forming region – the Great Orion Nebula.