Nebulae

Originally, the word "nebula" referred to almost any extended astronomical object (other than planets and comets). The etymological root of "nebula" means "cloud". As is usual in astronomy, the old terminology survives in modern usage in sometimes confusing ways. We sometimes use the word "nebula" to refer to galaxies, various types of star clusters and various kinds of interstellar dust/gas clouds. More strictly speaking, the word "nebula" should be reserved for gas and dust clouds and not for groups of stars.

	The Orion Nebula is a star-forming region in the constellation Orion. Astronomers have found at least 153 protoplanetary disks, which are believed to be embryonic solar systems where planets will form. Each vibrant color corresponds to a different gas within the nebula. Light emitted by oxygen is seen as blue, by hydrogen as green, and by nitrogen as red. (23K) Image: C. R. O'Dell (Rice University) and NASA
	The Cateye Nebula is a planetary nebula. This nebula formed about 1,000 years ago when a fast "stellar wind" of gas blown off the central star created the elongated shell of dense, glowing gas. This structure is embedded inside two larger lobes of gas blown off the star at an earlier phase. (44K) Image: J. P. Harrington and K. J. Borkowski (University of Maryland), and NASA
	It is easy to see why NGC 2024 is also called the Horsehead Nebula. NGC 2024 is about 1,100 light-years from the Sun, in the constellation Orion. (30K) Image: Jason Ware
	The Hourglass Nebula is a planetary nebula. When a star such as the Sun starts to die it becomes red and expands into a giant star. The old star will eventually eject its outer layers. The gaseous shell is illuminated by the dense stellar core, which is now exposed. We see the illuminated gas as a planetary nebula. (71K) Image: Raghvendra Sahai and John Trauger (JPL), the WFPC2 science team, and NASA
	This unusual nebula is the tip of a column of gas and dust in the star-forming nebula M16. The pillar is slowly being eroded by the ultraviolet light from nearby stars. As a result, globules of especially dense gas are exposed. These globules are called "EGGs" (Evaporating Gaseous Globules). Within each EGG is a forming star. When a star reaches maturity it evaporates the gas around it, revealing itself to the universe. In M16, some of these EGGs are being evaporated prematurely by massive neighboring stars, giving astronomers a rare glimpse of the different stages of star formation. (76K) Image: Jeff Hester and Paul Scowen (Arizona State University), and NASA
	These pillars are columns of cool interstellar gas and dust that serve as incubators for new stars. Ultraviolet light from massive newborn stars is slowly boiling off the gas from around the pillars. The pillars survive because they are more dense than the surrounding gas and dust. The pillars are not unaffected, as they too are being evaporated. (72K) Image: Jeff Hester and Paul Scowen (Arizona State University), and NASA

Galaxies

Early in this century, there was a great debate as to the nature of the nebulae like this one which at that time could not be resolved into individual stars. Thanks in large part to the work of Edwin Hubble whose famous paper "The Realm of the Nebulae" finally put the issue to rest, we now know that these are really vast conglomerates billions of stars which are very much more distant from the Earth than other nebulae. Our own Milky Way galaxy is just one of the billions of galaxies now known to exist. A typical galaxy is 100,000 light-years in diameter.

Globular Clusters

Globular clusters are gravitationally bound groups of many thousands (sometimes as many as a million) of stars. They consist primarily of very old stars. Globular clusters are not concentrated in the plane of the galaxy but rather are randomly distributed throughout the halo. There are several hundred globular clusters associated with our galaxy. A typical globular cluster is a few hundred light-years across.

Open Clusters

Open clusters are loose aggregations of dozens or hundreds of young stars. They are generally not gravitationally bound and will disperse in a relatively short period of time, astronomically speaking. They are often associated with more diffuse nebulosity, as well. Also called "galactic clusters" because they are usually found in the plane of the galaxy. A typical open cluster is less than 50 light-years across.

Emission Nebulae

Emission nebulae are clouds of high temperature gas. The atoms in the cloud are energized by ultraviolet light from a nearby star and emit radiation as they fall back into lower energy states (in much the same way as a neon light). These nebulae are usually red because the predominant emission line of hydrogen happens to be red (other colours are produced by other atoms, but hydrogen is by far the most abundant). Emission nebulae are usually the sites of recent and ongoing star formation.

Reflection Nebulae

Reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. Reflection nebulae are also usually sites of star formation. They are usually blue because the scattering is more efficient for blue light. Reflection nebulae and emission nebulae are often seen together and are sometimes both referred to as diffuse nebulae.

Dark Nebulae

Dark nebulae are clouds of dust which are simply blocking the light from whatever is behind. They are physically very similar to reflection nebulae; they look different only because of the geometry of the light source, the cloud and the Earth. Dark nebulae are also often seen in conjunction with reflection and emission nebulae. A typical diffuse nebula is a few hundred light-years across.

Planetary Nebulae

Planetary nebulae are shells of gas thrown out by some stars near the end of their lives. Our Sun will probably produce a planetary nebula in about 5 billion years. They have nothing at all to do with planets; the terminology was invented because they often look a little like planets in small telescopes. A typical planetary nebula is less than one light-year across.

Supernova Remnants

Supernovae occur when a massive star ends its life in an amazing blaze of glory. For a few days a supernova emits as much energy as a whole galaxy. When it's all over, a large fraction of the star is blown into space as a supernova remnant. A typical supernova remnant is at most few light-years across.

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Nebulae

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