Nebula Pictures

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In Latin the word nebula means 'a cloud, or a mist, or a vapor'; today the word nebula has a different astronomical meaning: 'a diffuse mass of interstellar dust or gas or both, visible as luminous patches or areas of darkness depending on the way the mass absorbs or reflects incident radiation.'  Yet, this definition is the product of the evolution of astronomy.  Before any advanced study of the sky was possible, people generally classified any cloud- or vapor-like form as nebulae.  However, in his time Galileo Galilei built a telescope that could magnify his vision into the heavens by eight times its former resolution.  While primitive in comparison to the telescopes of today, the observations that stemmed from his invention changed the world; most notably through his mapping of stars, planets, and his discovery of the four major moons of Jupiter.  Telescopes continued to evolve along with discoveries and knowledge of the universe, yet it was not until the year 1845 that scientists first applied the science of photography to astronomy.  After that, it was not until the 1870's that suitable film plates were developed for the photography of astronomical bodies such as nebulas.  These developments changed telescope design in the sense that lenses, mirrors and the optics of the instruments grew increasingly powerful and complex.  It was Edwin Hubble who, using his 100" telescope on Mt. Wilson in California, was finally able to provide enough data and observations to convince the world and astronomers that these objects were galaxies external to our own.

Using our present technology, we have learned to identify not just nebulae, but also the different types of nebulae that can occur.  The “dark nebula” is one example, also known as a diffuse nebula. This particular kind of nebula is a cloud of gas and dust that absorbs some light from behind it.  The absorbed light heats up the particles of dust, and because of this, those particles emit some of that absorbed energy as infrared light.  A beautiful example of this type of nebula in the Horsehead Nebula, found in Orion, named for the silhouette of the dark mass in front of a more luminous nebular region.  Furthermore, this type of nebula is a likely site of the formation of new stars, due to the processes of dust cloud condensation.

A supernova remnant is another type of nebula, which is just a fragment of a previous supernova explosion.  The Crab Nebula in Taurus, which is now fading at a rate of approximately .4% per year, is a supernova remnant.  Nebulas of this kind are powerful emitters of radio waves; this is because the explosions that formed the nebula had the probable pulsar remnants of the original star.

Very different from the supernova remnant is the reflection nebula, which only reflects and does not emit light.  The Merope Nebula is a perfect example of a reflection nebula; for it does not emit its own light, it simply reflects the light of the star Merope and other nearby stars.  Reflection nebulae are quite common finds in the night sky, usually found in close proximity to emission nebulae.

Emission nebulae emit their own light, largely due to the presence of ultraviolet radiation from the very young 'hot' stars that form the cluster.  The radiation emitted is powerful enough to 'excite' the atoms in the interstellar dust and gas so that they 'leap' to different levels of energy, and thus emit their own radiation form.  This radiation can be seen in the light spectrum of the nebula.  The Rosette Nebula very clearly demonstrates what an emission nebula looks like.  Found in the constellation monoceros, it's brilliant appearance can be attributed to the emission of its own light.

Finally, planetary nebulae, like the Eskimo nebula, are formed when very old stars that are around the same size as the Earth's sun have consumed most of their hydrogen fuel; this usually occurs only after billions of years have passed.  At the time of formation most of the hydrogen has been converted to helium, and the star has been expanded into what then becomes a 'red giant'.  The star then ejects gases at different times and at much lower speeds than before, and as the star evolves the core of it eventually becomes a white dwarf, which is an extremely hot star.  Thus, the very high temperature of the radiation makes the gases that have been ejected ionized and thus they begin to glow.

Margaret McGuire, 2004