Notable Facts About Nebulae

Beautiful and colourful objects in the night sky, nebulae are sometimes called the flowers of cosmos. They come in a plethora of shapes, and they show us how stars are born and how they die. The word "nebula" comes from Latin for "little mists" or "cloud", and a nebula is a huge, diffuse cloud of gas and dust in intergalactic space. Intergalactic space is the space between galaxies, and it is generally free of dust and debris. Many nebulae are several light years across, and they are generally made up of hydrogen and helium.

1. Before the dawn of telescopes all deep-sky objects looked like small, light, foggy patches, and the term nebula was used to distinguish objects that looked non-stellar from the point-like stars. Once telescopes became more powerful it became possible to distinguish gas and dust clouds from objects made up from stars - between nebulae, and globular clusters, galaxies and supernova remnants.
2. There are several different classification schemes. One classification scheme is based on spectroscopy and the light, which is seen from the nebulae: emission, reflection and dark nebulae. Another class - planetary nebulae - form when dying Sun-like stars shed their outer layers. There are also supernova remnant nebulae, and they consist of the remains of a star that has exploded.
3. Emission nebulae emit their own light. They glow very brightly, like red or pink roses in the sky, and they are frequently the birthplace of stars. Close to an emission nebulae is a hot bright star, and the nebula glows because it is getting rid of extra energy given to it by nearby stars. Sometimes the star lies within the cloud itself, and in some cases, an entire star cluster provides the energy. The nebula is composed mainly of hydrogen, and stars inside the clouds charge hydrogen gas with extra energy. The hydrogen in these clouds is getting rid of the extra energy by emitting it in the form of red light. The cloud itself is actually glowing.
4. Gravity causes the emission nebula's dust and gas to pull together, and it contracts into a smaller body and after millions of years, the body is hot enough to shine. A new star is born.
5. The Orion Nebula is an emission nebula and it is the brightest nebula in the night sky. It can be seen with the naked eye. The astronomer William Herschel described the Orion Nebula as "an unformed fiery mist, chaotic material of future suns". The Orion Nebula is one of the richest star forming regions in the Galaxy. The Rosette Nebula's star-forming region is also famous and is carefully studied by astronomers.
6. The reflection nebulae scatter the blue light from a nearby cool star, in a similar way that our sky is blue because it reflects the light of the sun. The rest of the colours pass through the cloud undisturbed. Eventually the blue light escapes the cloud and travels to our eyes. A reflection nebula is visible because there are some nearby stars that light up the gas and dust. Blue light has the shortest wavelength, and it scatters light more than any other colour. The nearby stars ultraviolet light is not strong enough to interact with the nebula's hydrogen atoms.
7. The Pleidas and the Witch Head are beautiful blue reflection nebulae.
8. Many nebulae are combinations of the two types, for example the famous Trifid Nebula in Sagittarius.
9. Astronomers call non-glowing clouds of dust and cold gas dark nebulae. Dark nebulae, as seen from our perspective on Earth, block light from brighter objects behind them. A dark nebula can only been seen when they are placed in front of a bright nebula or field of stars. We see a silhouette of the cloud, but no details. Famous examples include the Coal Sack in the southern constellation Crux, and the Horsehead Nebula in Orion
10. A planetary nebula consists of a glowing shell of gas and plasma, and they have nothing to do with planets in the solar system. They look like giant planets and last only a few thousands of years. Most stars will end their lives as planetary nebulae; however, stars weighing more than our Sun will end in a supernova explosion. When our Sun begins to die it will expand, and become a giant star. After millions of years as a giant star, the sun will again shrink down to its normal size. As it shrinks, much of its surface layers will be shed leaving behind a ring. The beautiful ring is only visible for about 50,000 years.
11. The Dumbbell Nebula was the first planetary nebula discovered. Charles Messier observed it in 1764 and he listed it as M27 in his catalogue of nebulous objects. The Ring Nebula is another famous planetary nebula.
12. The central star in the Helix Nebula has gone through its chaotic evolution and is now a white Dwarf. There is evidence for a surviving planetary system.
13. A supernova remnant nebula looks chaotic, and it consists of the remains of a gigantic star that has exploded. A part of supernova remnants is in most cases a nebula. In the centre of the Gum Nebula is the smaller nebula called the Vela Supernova. The Vela Supernova is considered separate from the Gum Nebula.
14. A more modern scheme distinguishes star-forming or pre-stellar nebulae from post-stellar nebulae. Stars are born in giant molecular clouds, and the Pillars of Creation in the Eagle Nebula show how new stars emerge from pockets of interstellar dust.