C J L MARR
Nebulae
an article written in 2009
The term “nebula” was originally used by astronomers to refer to any fuzzy patch in the sky that could be easily distinguished by a telescope, but was not sharp like stars or planets. “Nebulae” now refers to clouds of interstellar dust and gas made visible by their interaction with nearby stars or star remnants. In most cases, this material is about as dense as cigarette smoke, but the enormous quantity of it makes it visible thousands of light years away.
There are 5 types of nebula:
Emission - Hot young stars born within the nebula radiate their energy outward into the surrounding gas, resulting in electrons in the atoms of the nebula emitting their own light. It is this light we see as the eerie glow of an emission nebula, such as the Trifid Nebula, M20 (pictured top right, courtesy David Malin, Anglo-Australian Observatory).
Reflection – Reflection nebulae reflect light from a nearby star. Many small carbon grains in the nebula reflect the light. The blue color typical of a reflection nebula is caused by blue light being more efficiently scattered by the carbon dust than red light. The brightness of the nebula is determined by the size and density of the reflecting grains, and by the color and brightness of the neighboring star(s).
Absorption - Here, a high concentration of dust and molecular gas absorb practically all the visible light emitted from background stars. The eerily dark surroundings help make the interiors of molecular clouds some of the coldest and most isolated places in the universe. That no stars are visible in front of Barnard 68 indicates that it is relatively nearby, with measurements placing it about 500 light-years away and half a light-year across. It is not known exactly how molecular clouds like Barnard 68 form, but it is known that these clouds are themselves likely places for new stars to form.
Supernova remnant - When a high mass star (final mass greater than 1.4 solar masses) collapses at the end of its life, a supernova occurs. An enormous shock wave sweeps through the star at high speed, blasting away the various layers into space, leaving a neutron core and an expanding shell of matter known as a supernova remnant. The Crab Nebula, M1, the result of a supernova seen in 1054 AD, is filled with mysterious filaments, which are extraordinarily complex. The nebula spans about 10 light years, and in the very centre lies a pulsar — a neutron star as massive as the Sun but with only the size of a small town. The Crab Pulsar rotates about 30 times each second.
Planetary - A planetary nebula can result when a low or medium mass star (between say 1 and 8 times the mass of the Sun) ejects mass in the red giant stage, near the end of the its life. In this stage, the central part of the star is only about the size of the Earth. The rest of the star's envelope has expanded to about 70 times larger than it was during most of its lifetime (almost the size of the orbit of Mars). As more nuclear fuel is consumed, the inner core collapses to a final high density state that is rather unstable. The star pulsates, ejecting much of its outer envelope, leaving a small hot, dense core. This central star will eventually become a white dwarf. The ejected envelope becomes a spherical shell of cooler thinner matter spread over a volume about the size of our solar system - a planetary nebula.
They are called planetary because to early astronomers these fuzzy patches seemed like the disks of planets, but, of course, we now know they have no association with planets. The expansion speed of the material in a planetary nebula is about 20 - 30 km/s. After approximately 10,000 to 50,000 years (a short time, astronomically!) the density becomes too thin for the nebula to be seen. About 1,000 visual planetary nebulae have been catalogued.
Planetary nebulae are some of the most beautiful objects in the sky, and no two are exactly the same. However, in the main they all appear to have the same structure — a pair of expanding nodes ejecting from opposite sides of the original star. The Cats Eye Nebula, NGC6543 (shown above left, courtesy J.P. Harrington and K.J. Borkowski, University of Maryland, HST, NASA), is angled such that both nodes are visible to us — the higher node is the closer, and the material is so thin that we can see right inside it to the inner dwarf star, or stars (one theory is that it is a binary dwarf star system in there!). Both nodes look to be spherical and fairly uniform, but this need not be the case. Local conditions and the size and temperature of the original star most likely dictate the eventual shape of the nebula.
The material that makes up a nebula, is the stuff from which stars are made. Looking inside the great stellar nurseries like the Eagle Nebula (M16) and the Great Orion Nebula (M42) we can see massive young stars being formed. When you consider how much material is packed into each star, you begin to realise just how much of this gas and dust must have been lying around in the original nebula. Despite the huge number of stars that exist, there are still vast amounts of gas and dust floating around in nebulae. The next time you look up at the Large Magellanic Cloud (LMC) and see the glow of the Tarantula Nebula, 30 Doradus, remember that this particular cloud of material is more than 1,000 light years across — so big that even though it is attached to another galaxy 165,000 light years away, you can still see it with your naked eye!