![]() Here the dust particles are sufficiently numerous to (almost) completely block light from behind them. But please note that Sigma Scorpii is also lighting up a blue reflection nebula, because there is a significant amount of dust particles in the gas cloud near Sigma Scorpii, and the blue reflection nebulosity further attenuates the red light of hydrogen alpha.Ĥ) The very dark brown dust tendrils. The pink color is caused by the attenuation of the very red 656 nm light by the simultaneous emission of bluish cyan hydrogen beta 486 nm photons, emitted by electrons that were knocked "two levels up" by extra energetic photons. So why does the emission nebula look pink when all those "kicked-upstairs-and-falling-down" electrons emit all that red light? This is the wavelength of hydrogen alpha, which is very red. ![]() As the electron "falls down" again, it rids itself of the extra energy it gained when it was kicked into another electron shell, and it does so by emitting a photon of 656.281 nm. This is to say that a sufficiently large number of energetic photons from Sigma Scorpii knocks electrons in hydrogen atoms in the surrounding gas cloud into a "higher orbit" around their protons, or rather, the energetic photons knock electrons into a higher electron shell. Well, whichever combination is the correct one, Sigma Scorpii definitely produces more ultraviolet photons than Rho Ophiuchi does, and therefore Sigma Scorpii ionizes a pink emission nebula. Sigma Scorpii is another binary star, and I have seen many different suggestions as to what their spectral types are: B1III, B2III+O9.5V, and O9.5V+B7V. The Rho Ophiuchi nebula is one of the largest blue reflection nebulas in the sky.ģ) The pink Sigma Scorpii nebula. In the case of Rho Ophiuchi, there is just the right amount of dust behind the stellar pair to scatter their blue light back at us. Such stars pump out huge amounts of blue and violet photons, while at the same time just falling short of emitting enough ultraviolet photons to ionize an emission nebula. The star Rho Ophiuchi itself is a binary star made up of two stars of spectral class B2, which is ideal for creating a blue reflection nebula. The Antares reflection nebula is by far the largest of the yellow reflection nebulas visible in the sky.Ģ) The blue Rho Ophiuchi reflection nebula. Its reflection nebula is even less orange and more yellow than the star itself, because the dust particles in reflection nebulas preferentially scatter shorter wavelengths. So Antares isn't actually a red star, but yellow-orange. (If you want a redder star, check out Mu Cephei, with a B-V index of cirka +2.2, or, better yet, carbon star T Lyrae, with a variable B-V index which is almost always larger than +5.) Its B-V index is around +1.9, which makes it a yellow-orange star. Antares is, despite being called a red supergiant, not actually a red star. The fact that the heads of the cometary clouds of IC2118 point northeast towards the association is strong support of that relationship.Why are the colors the way they are in the APOD?ġ) The yellow Antares reflection nebula. ![]() The wind blown appearance and cometary shape of the bright reflection nebula is highly suggestive of a strong association with the high mass luminous stars of Orion OB1. As the supershell expands into the interstellar medium, favorable circumstances for star formation occur. ![]() The molecular clouds of IC 2118 are probably juxtaposed to the outer boundaries of the vast Orion-Eridanus bubble, a giant supershell of molecular hydrogen blown by the high mass stars of the Orion OB1 association. In fact candidates for pre-main sequence stars and some classic T-Tauri stars have been found deep within the nebula. Radio observations show substantial carbon monoxide emission throughout parts of IC 2118, an indicator of the presence of molecular clouds and star formation in the nebula. The nature of the dust particles, reflecting blue light better than red, is a factor in giving the Witch Head its blue color. The nebula lies in the Eridanus Constellation, about 900 light-years from Earth. IC 2118 (also known as Witch Head Nebula due to its shape) is an extremely faint reflection nebula believed to be an ancient supernova remnant or gas cloud illuminated by nearby supergiant star Rigel in the constellation of Orion. ![]()
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