No, these are not the cirrus clouds that you would find in ths sky on a clear day. These clouds are what you would call interstellar clouds of gas and dust because these unusual clouds are found in space. Just as clouds appear in the daytime sky, they also appear at night. These unusual hazy patches are called nebula (plural-nebulae), which is latin for the word "cloud". Charles Messier (1730-1811),the renowned comet hunter credited for cataloging the the sky, is recognized as the first person to acknowledge the existence of nebulae. Nebulae are the starting place and ending place of stars.
An important aspect of the nebulae exist in the heart of them. This "aspect" of nebulae are called Bok globules. They aid in the formation of stars. These globules are composed mainly of hydrogen. They are formed when a supernova explodes (the ending of the star's life cycle) causing shock waves to ripple through a gas cloud. The shock waves cause the gas to condense into these Bok globules which become protostars. The new stars ignite causing furthur shock waves. Younger stars form on subsequent shock waves.
Nebula are clouds of interstellar gas and dust. The gases that these clouds are composed of are hydrogen, oxygen, nitrogen, and sulfer. The majority of the cloud is comprised of hydrogen, and then there are traces of the other gases.The combinations of these gases and the dust, and the light from nearby stars, give nebula their amazing , spectacular shapes and colors that you see in photographs.
The spectacular colors that you see in these pictures are not the colors that you would see looking up at the night sky. Your eyes do not see the same colors because of the sensitive rods and cones in your eyes Only a photograph or electronic detector, such as a CCD (charge coupled device that absorbs and builds up light over over a period of time picking up color) picks up these brilliant colors.
The gas and dust of these clouds are at extremely low densities. It is the gas of these nebula, that of which bright stars shine on, that the cloud gets its color. The gas is about 100 times denser than the dust, but still has an extremely low density. Interstellar dust comes from stars. It can be blown off of them as they shine or in a supernova explosion. The dust is made of cabon or iron with a coating of ice or frozen gas. Dust particles are approxiametly the size of particles that you would find in wood smoke. The dust appears blueish when light shines on it. The dust blots light from more distant stars where nebulae clouds appear dark. Although only a few particles are dispearsed through a cubic mile of space, the distances between the stars are so great that the dust can block the light fron distant stars.
There are three types of nebluae. They are categorized by the density of
the gas and dust that they are composed of, their chemical compositions, and
the presence or abscence of nearby stars. The three types of are EMISSION
NEBULAE, DARK NEBULAE, AND REFLECTION NEBULAE.
All three of types of nebulae are found in the picture above.
These "glowing" nebulae consist of 75% hydrogen and nearly all the rest is
helium. It glows because it is heated to temperatures close to 10,000 K by
ultraviolet radiation from other nearby or embedded hot stars. the UV
radiation ionizes the interstellar gas atoms and light energy is emitted
by the atoms as they interact with the free electrons in the nebula. The
energy is then redistributed in the form of visible light. Hydrogen gas
glows red, oxygen and helium gas glows green. Emission nebula can be in
the form of H-11(formed by shock wave or UV), planetary nebula( formed
when red giant or supergiant shed outer layer), or supernova remmnants
(formed after a supernova explosion).
Dark nebulae, or absorption nebula, are not luminous. Their
interstellar gas and dust absorbs visible and UV light from background stars
stars producing dark patches in the sky. They are detected by what they
obscure. There are no nearby stars to illuminate them because their dust
clouds have a higher than average density that is thick enough to block light
in and beyond them.
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This page is written by Karen Brown in the Astronomy class of BCC/Broward County in July of 1998. |