Star Map 40
Rigel or grains, that are so small (in proportion to the wavelength of light) is their property of selectively scattering light of a particular wavelength, If a beam of white light shines upon a cloud containing the grains, the
Manorial chromony p raw em autorskim blue light is scattered in all directions, a phenomenon similar to that seen in the Earth's sky, hence its colour blue, This is one reason why reflection nebulae appear so blue on photographs, it is just the blue wavelengths of the light from (usually) hot blue stars nearby- To be scientifically correct, the nebulae should be called scattering nebulae instead of reflection nebulae> but the name has stuck. This property is often called interstellar reddening. An interesting property of the scattered light is that the scattering process itself polarises the light, useful in the studies of grain composition and structure-
Another phenomenon associated with dust grains that should be mentioned, as it affects ALL observations, is interstellar extinction. Astronomers noticed that the light from distant star clusters was fainter than expected, and this was due to dust lying between us and the cluster. This in fact makes all objects fainter than they actually are, and has to be taken into account when making detained measurements.
Several reflection nebulae reside within the same gas clouds as emission nebulae, the Triftd nebula is a perfect example- The inner parts of the nebula are glowing with the tell-tale pink colour, indicative of the ionization process responsible for the emission, whereas further out from the centre, the edge material is definitely blue, thus signposting the scattering nature of the nebula. Visually, reflection nebulae are very faint objects having a low surface brightness, thus they are not easy targets. Most require large aperture telescopes with moderate magnification in order to be seen, but there are a few visible in binoculars and small telescopes. Mote that excellent seeing conditions are necessary and very dark skies.
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