In true colour, the Eagle nebula is a haze of glowing red gas, surrounding a bright open cluster of blue and white stars. The red glow is produced by molecular hydrogen gas, excited by ultraviolet radiation released by the stars within the nebula. As the molecules fall back down to their resting energy state, they release photons of red light, causing the nebula to glow.
Not all of the gas cloud is composed of hydrogen; other ions and molecules in the nebula also emit visible light when energised by UV radiation. Unfortunately, most of these emissions are towards the red end of the spectrum and although they are different wavelengths, it’s not possible for the eye to distinguish between them. In order to observe the distributions of the various components of the Eagle nebula, as well as to get a better understanding of its dynamics, instruments like the Hubble Space Telescope use filters to selectively image one wavelength of light at a time – essentially enabling astronomers to map the distribution of the gases in the molecular cloud.
The famous image of the Pillars of Creation, captured by the Hubble Space Telescope in 1995, is actually a composite of images taken through different filters, each focused on a separate wavelength. In combination, these separate images show the dark sulphur-containing dust columns glowing faintly red, with a surrounding green-blue mist of hydrogen and oxygen.
The glowing HII region of the nebula is easy to see, but the dark, cold molecular cloud obscures most of the visible light behind it. However, the longer wavelengths of infrared light are able to penetrate through the dust and can be used to generate a heat- map to pinpoint the locations of hidden stars.
The European Southern Observatory’s wide-field telescope, ANTU, has been used to probe the Eagle nebula for near-infrared emissions. These longer wavelengths render the Pillars of Creation almost transparent, enabling astronomers to look at the dense gaseous EGGs inside. These potential new stars are normally entirely obscured by the dust, but infrared images revealed over 70 warm EGGs nestled under the caps of the pillars. Several of these even have hot stars embedded in their tips, though whether the stars gave rise to the EGGs or the EGGs to the stars is not currently known.
Longer wavelengths of infrared light, as detected by the European Space Agency’s Infrared Space Observatory satellite and Herschel Space Observatory, reveal colder areas of the nebula. The majority of the molecular dust cloud is barely above absolute zero, but the faint infrared emissions from the cold clouds reveal the eerie outline of the frigid outer edges of the nebula. These cradle regions of relative warmth (-200 degrees Celsius/-328 degrees Fahrenheit).
The ESA’s XMM–Newton and NASA’s Chandra X-ray Observatory, have both scanned the Eagle nebula for the bright X-ray emissions given off by young stars. The images don’t reveal a lot about the structure of the dust cloud but when superimposed onto the visible light images captured by Hubble, or on infrared images of the nebula, it becomes clear that most of the new stars are in the central cavity. Few are forming near the remaining dense dust at the pillars, showing that the nebula is coming to the end of its active life.
Image Credit: NASA