Misty, Mysterious Nebulae
A variety of nebulae can be found throughout the Milky Way, but I'll describe just two main kinds. The first, called an emission nebula, is an immense store of hydrogen gas that has been excited (that is, energized) to incandescence by hot, newborn stars within or near it. Some emission nebulae are the sites of active star formation.
In the middle of Orion's Sword (or scabbard) lies a nebulous jewel, Messier 42 (M42), whose glowing tendrils, dusty indentations, and bright baby stars can be inspected with even a small telescope and even through moderate light pollution. (In the wide-field photo at left, the diagonal row of three bright stars at top is the familiar Orion's Belt.)
Left: Akira Fujii; right: Rick Fienberg
(M42), the Great Orion Nebula, is a superb example of an emission nebula. This celestial nursery, some 1,400 light-years away, cradles a quartet of bright infant suns called the Trapezium. The fan-shaped nebulosity illuminated by the Trapezium is an absorbing subject to study through any telescope. M42 is plainly visible in binoculars and can even be discerned by sharp-eyed people from dark backyards. Moreover, because Orion straddles the celestial equator, the Great Nebula is accessible to amateur astronomers all over the world.
A second type of nebula, called a planetary nebula, represents a brief moment in the last stages of a star's life cycle. A planetary nebula is a shell of gas that has been ejected from a dying star one that has exhausted its ability to generate energy by nuclear fusion in its core and is destined to fade to near-invisibility. Viewed through a small telescope, a typical specimen offers little more than a blue-green, vaguely planetlike disk hence the name "planetary."
The castoff mantles of dying stars, planetary nebulae like the Ring Nebula in Lyra (left) and the Helix Nebula in Aquarius (right) can be highlighted slightly by using a so-called 'nebula filter' at your telescope's eyepiece. The Ring, tiny but relatively bright, hardly calls for a filter. But the Helix, large and very faint, shows up much better when one is used.
Fortunately, the best planetaries have more character. The Ring Nebula
, Messier 57 (M57) in the constellation Lyra, is famous for its sharply defined structure. Viewed at high power, the small but conspicuous Ring is a cosmic Cheerio adrift in space 1,400 light-years from Earth. No other deep-sky object visible through amateurs' telescopes looks quite like it.
If your skies are truly dark, you'll also want to chase down NGC 7293, the Helix Nebula in Aquarius. Only 400 light-years distant, the Helix appears about 10 times larger than M57 on the sky, but it is extremely dim. Even so, this ghostly cousin to the Ring will materialize as a misty oval in a good pair of binoculars or a wide-field telescope used at low magnification under a black sky.