D. Indumathi, The Institute of Mathematical Sciences, Chennai
"Nebula" means "cloud". The word nebulous means that which is in the form of a cloud or haze; hazy, so that it is unclear, vague, or ill-defined.
Nebulae are interstellar clouds of dust that are visible either to the naked eye, or with a telescope. Interstellar means "between the stars". Nebulae were originally the clouds of dust (seen as smudges) around distant stars. Later, these clouds were also seen in regions free of stars. So sometimes nebula means the smugdes in galaxies or star clusters, sometimes it refers to interstellar dust clouds. However, in modern times nebula is used for gas and dust clouds only.
Most nebulae are extended, diffuse objects without any well-defined edges or boundaries. They mostly consist of hydrogen gas, with some helium, apart from cosmic dust. They are loosely held together by gravity.
Objects in the Sky
Galaxies are vast conglomerates billions of stars which are very much more distant from the Earth. 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. Shown is the galaxy NGC1300.
Globular clusters are gravitationally bound groups of many thousands (sometimes as many as a million) of stars. They consist primarily of very old stars, distributed throughout the galaxy.
A globular cluster is spherical in shape; hence the name. It orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centres.
There are several hundred globular clusters associated with our galaxy. Shown is the cluster M80 (NGC 6093), one of the densest of the 147 known globular star clusters in the Milky Way galaxy. It is located about 28,000 light-years from Earth and contains hundreds of thousands of stars. A typical globular cluster is a few hundred light-years across.
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, for a few hundred million years. In contrast, globular survive several billions of years since they attract their neighbours more strongly. A typical open cluster is less than 50 light-years across. Shown is the cluster NGC265 in the Small Magellanic Cloud galaxy, close to us.
Nebulae are diffuse structures of different types. A typical diffuse nebula is a few hundred light-years across.
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Kinds of nebulae
These are found near stars and are clouds of high temperature gas. The ultraviolet light from the star falls on the gas and heats it. In turn, the hot gas emits radiation. These nebulae are usually red because the predominant emission line of hydrogen happens to be red. Hence these reddish nebulae are seen in regions where star formation is occurring. Shown is the Flame Nebula in the Orion Constellation.
Reflection nebulae are clouds of dust (what else?!) which simply reflect the light of a nearby star(s). Reflection nebulae are also usually sites of star formation. However, they do not themselves produce light, but reflect all the light that falls on them. So they are blue in colour since scattering is more efficient for blue light (the same reason as whythe sky is blue). Reflection nebulae and emission nebulae are often seen together and are sometimes both referred to as diffuse nebulae.
Shown is the reflection nebula called the Witch Head nebula (IC 2118) about 1000 light years away. It glows primarily by light reflected from Rigel (located just outside the top right corner of the image). Rigel is the bright star in the Orion constellation. Fine dust in the nebula reflects the light.
Dark nebulae are clouds of dust (again!) that 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. Shown is the Horse-head nebula, a cold, dark cloud of gas and dust, silhouetted against the bright nebula, IC 434. A typical diffuse nebula is a few hundred light-years across.
A supernova occurs when a massive star (about 20 times more massive than our Sun) reaches the end of its life. This happens when nuclear fusion in the core of the star stops; the star collapses.
The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode. For a few days a supernova emits as much energy as a whole galaxy. A large part of the star is blown into space as an expanding shell of gas, which forms a supernova remnant, a special diffuse nebula. A typical supernova remnant is at most few light-years across.
Shown is the most famous remnant, the Crab nebula, in the constellation
Planetary nebulae are shells of gas thrown out by some stars near the end of their lives. The difference between a supernova remnant and this is that these stars are not massive enough to explode as a supernova, but will simply evolve into cold objects called white dwarfs at 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.
-Sources include <http://bill.nineplanets.org/arnett.html>, with images from NASA and Wikipedia.