In the deep space there are not only stars, but galaxies like ours contain nebulae: diffuse matter made up of gas and dust. These nebulae display a variety of shapes and colors that reflect the diversity of processes that take place when nearby star light gets tangled among them.
Text: David Galadí-Enríquez
Image acquired with the 1.23 meter Zeiss telescope at Calar Alto Observatory.
Image Caption and Credits: Image of the NGC 6914 nebular complex in Cygnus, from the Documentary Photo Gallery of Calar Alto Observatory (Descubre/CAHA/OAUV/DSA). Vicent Peris (OAUV/DSA/PTeam), Jack Harvey (DSA/SSRO/PTeam), Juan Conejero (DSA/PTeam). Entirely processed with PixInsight 1.6. Click on the image to download a full-resolution version (image scale: 0.5 arcsec per pixel)
This photo of NGC 6914 released by Descubre Foundation, in collaboration with the Documentary School of Astrophotography (DSA) and the Astronomical Observatory of the University of Valencia (OAUV), captures this area of star formation in all its beauty. The collection of gas and dust in this image shows three main bright areas known as NGC 6914 (at the center), NGC 6914a (to the south, at left in the image) and NGC 6914b (to the north, at right in the image). Each bright nebula nucleus is centered on one or more massive, young hot stars. Some of these stars can be seen through small telescopes; but to discern details in the nebulae, a dark sky and a large aperture are really needed.
The field of view in this image is 27 arc-minutes wide by 15 arc-minutes high, with north towards the right and east is up. The image was acquired with the 1.23 meter Zeiss telescope at Calar Alto Observatory within the POP123 project of Descubre Foundation. It's made up of data through RGB broadband and H-α narrowband filters, with a total integration time of 23 hours. To make this landscape format image, a two-panel mosaic was assembled. H-α data were used to separate reflection from emission nebulas in the red channel, in order to raise the contrast of the faint hydrogen structures. We think this is perhaps the most detailed view of the object to date. Data acquisition and processing was carried out by Jack Harvey, Juan Conejero and Vicent Peris with all processing done in PixInsight.
Almost all the universe is made up of hydrogen and helium, thus composed of light and simple atoms; and interstellar nebulae aren't an exception. But the interstellar medium is made up in a small part of other materials like carbon, silicon and oxygen. These materials tend to aggregate, building tiny grains that form what we know as interstellar dust. Thus, a normal nebula is made up of a 99% of gas (hydrogen and helium) and a small proportion of material that composes the dust.
Interstellar dust is a small part of a nebula, but its interaction with light is so intense that it defines their shape. Light that reaches the dust is partially scattered and partially absorbed. This phenomenon is like what happens with the clouds of the Earth's atmosphere: the water droplets and the tiny ice crystals reflect and scatter part of the light hitting them. That's why, due to reflected light, the clouds have a bright appearance when looked from the same side as they are illuminated. However, when the same clouds are observed from the opposite side, light is absorbed proportionally to their density and thickness. That's why, when looking from below, we can see the Sun dimmed by the clouds. In the latter case, clouds appear to be deep gray.
The role of cosmic dust with stellar light is similar. A nebula containing a small quantity of dust will look darker, or even black, when illuminating stars are in the background. Then, we talk about dark nebulae. This same nebula, observed from the side where light is reflected, will appear bright. The interstellar dust is less reflective than the atmospheric clouds, but when the light from hot young stars reaches the dust, it shines with light blue tones. This is what we know as reflection nebulas. The blue cast of a reflection nebula is not only due to the color of the hot stars, but to the interaction of light with the dust grains that favors the reflection of blue light.
Nebular dust always comes with gas — mostly hydrogen gas. The same massive hot stars that illuminate the dust can cause the pure gas to shine. Ultraviolet light from the most massive stars pulls out electrons from the hydrogen atoms. When these atoms get back their electron, they emit a characteristic reddish or pinkish light: the H-α and H-β radiation. This excited gas forms the emission nebulae, which are a main feature of the star forming regions.
The nebular complex know as NGC 6914, in the Cygnus constellation, is a clear case of combination of the three types of nebulae described above. This region concentrates gaseous and dusty masses of the galactic plane, where stars are born. Fragments of nebula situated between the Earth and the young stars appear to be dark, with varying opacities and intricate shapes. In other areas, near the younger and hotter stars, dust shines with the bluish colors of the reflection nebulas. Lastly, all the image is covered by ionized hydrogen that gives to the scene a reddish and warm backdrop. Newly born stars, which are only a few million years old, mill around the scene. Their colors reveal their diversity of masses and temperatures. But in this small portion of the Galaxy, about 6,000 light-years away, massive and hot stars (O and B type stars) are so abundant that the area is classified as an OB association. Its name is Cygnus OB2.
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