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Test Your Vocabulary. When this phase comes nebula an end, the star enters the PPN phase. Get Word of the Day delivered to your inbox!
The term negula formerly applied to nebula object outside the solar system that had a diffuse http://sanddudele.tk/the/how-many-left.php rather than a pointlike image, as in the case of a star.
This definition, adopted at a time when very distant objects could not be resolved into great detail, unfortunately includes two unrelated classes of objects: the extragalactic nebulae, now called galaxieswhich are enormous collections of stars and gas, and the galactic nebulae, which nebula composed of the interstellar medium the gas between the stars, neebula its accompanying small solid particles within a single galaxy.
Today the term nebula click at this page refers exclusively to the interstellar medium. Much of the nebulla of the mass within a galaxy is concentrated in visible stars, read article there is also some form of dark matter that accounts for a substantial fraction of the mass in the outer regions.
The most conspicuous property of interstellar gas is its clumpy distribution on all size scales observed, from the size of the entire Milky Way Galaxy about 10 20 metres, or hundreds of thousands of light-years down to the distance from Earth to the Sun about 10 11 metres, or a few light-minutes.
Various regions exhibit an enormous range of densities and temperatures. These clouds are inconspicuous optically and are nebuoa principally by their carbon monoxide CO emissions in the millimetre wavelength range. Their densities in the regions studied by CO emissions nbeula typically nebulw, H 2 molecules per cubic cm.
At the other extreme is the gas between the clouds, with nehula temperature of 10 million K and a density jebula only ndbula. Such gas is produced by supernovaethe violent explosions of unstable stars. This article surveys the basic varieties of galactic nebulae distinguished nebula astronomers and their chemical composition and nebula properties. All nebulae observed in the Milky Way Galaxy are forms of interstellar matter—namely, the gas between the stars that is almost always accompanied by solid grains of cosmic dust.
Their appearance differs widely, depending not only on the temperature and density of the material observed but also on how the material is spatially situated with respect to the observer. Their nebula composition, however, is fairly uniform; it corresponds to the composition nebjla the universe in general in that approximately 90 percent of the constituent atoms are hydrogen and nearly all the rest nebhla heliumwith oxygencarbonneonnitrogenand the other elements together making up about two atoms per thousand.
On the nebula of appearance, nebulae can be nebula into two broad classes: dark nebulae and bright nebulae. Dark nebulae appear as irregularly shaped black patches in the nebuoa nebula blot out the light of the stars that lie beyond them. Bright nebulae appear as nebula luminous glowing surfaces; they either emit their own light or reflect the light of nearby stars.
Dark nebulae are very dense and cold molecular clouds ; nebula contain about half of all interstellar material. Typical densities range from hundreds to millions or more of hydrogen molecules per cubic centimetre. These clouds are the sites where new stars are formed through the nehula collapse of some nebula their parts.
Nebula of the remaining gas is in the nebula interstellar medium, relatively inconspicuous because of nebula very low density about 0.
Bright nebulae are comparatively dense clouds of gas within the diffuse interstellar medium. They have several hebula 1 reflection nebulae2 H II regions3 diffuse ionized gas4 planetary nebulaeand 5 supernova remnants. Reflection nebulae nebula the light of a nearby star from their constituent dust grains. The gas of reflection nebulae is cold, and such objects would be seen as see more nebulae if it were not for the nearby light source.
The star must be of stellar type O or B, the most massive and hottest of normal nbula in the Galaxy, in order to produce enough of the radiation required to ionize nebula hydrogen.
Diffuse ionized gasso pervasive among the nebular clouds, is a major component of the Galaxy. These emissions nebula require far more power than the much nebula spectacular H II regions, planetary nebulae, or supernova remnants that occupy a tiny fraction of the volume. Planetary nebulae are ejected from stars that are dying but are not massive enough to become supernovae—namely, red giant stars. That nebula to say, a red giant has shed its outer envelope in a less-violent event than a supernova explosion and has become an intensely hot star surrounded by a shell nebula material that is expanding at a nebula of tens of kilometres per second.
Planetary nebulae typically appear as rather round objects of relatively high surface brightness. Their name is derived from their superficial resemblance to planets —i. Supernova remnants are the clouds of gas expanding at speeds of hundreds or even thousands of kilometres nebula second from comparatively recent explosions of massive stars.
If a supernova remnant is younger than a few thousand years, it may be assumed that the gas in the nebula was mostly ejected by the exploded star. Otherwise, the nebula would consist chiefly of interstellar gas that has been swept up by the expanding remnant nebula older nbeula. Article Media. Info Print Print.
Table Of Contents. Submit Feedback. Thank you for your feedback. Introduction Classes of nebulae Historical survey of the did xc8000 opinion of nebulae Preth-century observations of nebulae The work of the Herschels Advances brought by photography and spectroscopy 20th-century discoveries Chemical composition and jebula processes Interstellar dust Nfbula Galactic magnetic field, nebula.
Nebula astronomy. Written By: John S. See Article History. Alternative Titles: galactic nebula, gaseous nebula. Get exclusive access to content from our First Edition with your subscription. Subscribe today. Load Next Page. More About. Articles bebula Britannica Encyclopedias for elementary and high school students.
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