Originally, the word "nebula" advert to nearly any extended astronomical object (otherthan planets and also comets). The etymological source of "nebula" means "cloud". Together isusual in astronomy, the old ax survives in modern usage in occasionally confusingways. We periodically use the word "nebula" to refer to galaxies,various types of star clustersand assorted kinds that interstellar dust/gas clouds.More strictly speaking, words "nebula" need to be booked for gas and dust clouds andnot for groups of stars.

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Galaxies

Early in this century, there to be a good debate regarding the nature the the nebulae likethis one which at the time can not be resolved into individual stars. Many thanks inlarge part to the work-related of Edwin Hubble who famous record "The kingdom of the Nebulae"finally placed the worry to rest, we now understand that these are really vastconglomerates billions that stars i beg your pardon are an extremely much much more distant from theEarththan various other nebulae. Our very own Milky means galaxy is just oneof the billions the galaxies now recognized to exist.A usual galaxy is 100,000 light-years in diameter.(M 51 shown; watch theSEDS Messier catalogfor much more examples.)

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Globular Clusters

Globular clusters space gravitationally bound teams of many thousands(sometimes as numerous as a million) the stars. They consist mainly ofvery old stars.Globular clusters space not concentrated in the plane of the galaxy however rather arerandomly dispersed throughout the halo.There are numerous hundred globular clusters linked with our galaxy.A typical globular cluster is a few hundred light-years across.(M 13 shown; watch theSEDS Messier catalogfor more examples.)

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open Clusters

Open swarm are loosened aggregations of dozens or hundreds ofyoung stars.They are normally not gravitationally bound and will disperse in a reasonably shortperiod that time, astronomically speaking.They space often associated with much more diffuse nebulosity, as well.Also called "galactic clusters" since they are usually uncovered in the aircraft of thegalaxy.A common open swarm is less than 50 light-years across.(M 44 shown; view theSEDS Messier catalogfor more examples.)

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emissions Nebulae

Emission nebulae are clouds the high temperature gas.The atoms in the cloud are energized by ultravioletlight native a adjacent star and also emit radiation together theyfall ago into lower power states (in lot the same means as a neon light).These nebulae are usually red because the predominant emission line of hydrogenhappens to it is in red (other colors are produced by other atoms, but hydrogen isby far the most abundant).Emission nebulae room usually the sites of recent and also ongoing star formation.(M 42 shown)

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reflection Nebulae

Reflection nebulae room clouds the dustwhich are merely reflecting the irradiate of a nearbystar or stars.Reflection nebulae are likewise usually sites of star formation.They room usually blue since the scattering is more efficient because that blue light.Reflection nebulae and emission nebulae are often seen together and also are sometimesboth referred to as diffuse nebulae.(NGC 7023 shown)

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Dark Nebulae

Dark nebulae room clouds the dustwhich are simply blocking the light from every little thing is behind.They space physically very comparable to reflection nebulae; they look various onlybecause that the geometry the the irradiate source, the cloud and also the Earth.Dark nebulae are also often checked out in conjunction with reflection and also emission nebulae.A usual diffuse nebula is a couple of hundred light-years across.(NGC 2264 shown; see also the Horsehead Nebula)

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Planetary Nebulae

Planetary nebulae room shells the gas thrown the end by part stars close to the finish of theirlives.


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Ours Sunwill probably develop a planetary nebula in about 5 billion years.They have actually nothing at all to carry out withplanets;the terminologywas invented because they regularly look a small like planets in tiny telescopes.A common planetary nebula is less than one light-year across.(M 57 shown)

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Supernova Remnants

Supernovae take place when a huge star ends its life in an impressive blaze of glory.For a couple of days a supernova emits together much energy as a totality galaxy. Once it"s every over,a large fraction of the star is puffy into an are as a supernova remnant.A usual supernova remnant is in ~ most few light-years across.(M 1 shown)
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© bill Arnett 1995 might 17