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Dust Cloud Menagerie
Interstellar dust clouds come in all shapes and sizes. One of the most recognizable dust clouds is the Horsehead nebula, located within the constellation of Orion in a vast nebula known as the Orion nebula. (Also known as Barnard 33 in the nebula IC434.) The dust clouds do not emit light as they are typically made from cold dust that does not radiate much energy, but they tend to be visible by the light they block, or reflect. As you can see in the image, the horse’s head is back-lit by the stars forming elsewhere in the cloud, and the solar winds from these stars have helped to sculpt the image we see today. It will not always resemble a horse, as in time, the stellar winds will dissipate the cloud.
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What Are They Made Of?
These dust clouds are vast collections of particles and molecules made up of carbon, silicates, ice (which could be water, carbon dioxide or ammonia ices), and even iron. They may even contain organic compounds such as formaldehyde, acetylene, acetaldehyde and methanol. The content of these clouds has been determined through the use of radio telescopes, which can detect the emission spectra of certain molecules and atoms. Reflected light in the visible and UV can also be used to identify elements in the clouds.
The source of the material that makes up these clouds comes in part from nova and supernova that occurred eons ago. These factories generate heavier elements as part of the fusion process as the star ages, and depending on the star’s mass, it can push off its outer layers in an elegant planetary nebula or blast the star’s contents into the heavens in a supernova. Over time (and the universe has been around a long time) these particles can move about, forming clouds that can ultimately be the birthplace of new stars. Even a star as modest as our Sun contributes particles to the interstellar medium through the solar wind and coronal mass ejections it generates.
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If you look at images of galaxies you often can see dark lanes of interstellar dust and gas that often have stars forming near them. This is beautifully illustrated in the image of the galaxy NGC-4921 shown below on the left. The enlarged image on the right clearly shows the dark lanes of dust as they meander through a fog of stars. Along the edge of the dust cloud you can see very hot young stars that formed.
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Images Beyond The Imagination
One last image of an interstellar dust cloud that tickles the imagination and is perfect for Halloween is the Witch Head nebula (IC2118), which is also in the constellation of Orion. We see this cloud of dust by the reflected light coming from the hot, blue super giant star, Rigel. The nebula has a strong bluish color because of the fact that the particles that make up the cloud are on the order of fractions of a micron in size and they selectively reflect the short wavelengths of light – blue light, better than the longer wavelengths. It also helps that the light coming from Rigel has a strong blue component because of its high temperature.
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All images courtesy of NASA