When you leave the planets, moons, and comets of our solar system behind, you enter the realm of the deep sky, a place of subtle glows and faint lights. This is where you find clusters of stars and wispy nebulae scattered along the Milky Way.
Beyond our galaxy the deep sky expands to include countless galaxies each containing their own wonders. To start you on your journey into the deep sky, here are two clouds of interstellar gas that are easy targets in binoculars from suburban locations.
The Orion Nebula
There's no brighter nebula visible from midnorthern (or southern) latitudes than the Orion Nebula (M42), located within
Orion, the Hunter (You can find Orion the Hunter and other constellations using SkyandTelescope.com's Interactive Sky Chart).
It lies below that constellation's three belt stars and makes a great target for any brand new Christmas telescope. Through binoculars the nebula looks like a fuzzy star with a bright center.
If you zero in with a telescope, you should see a tiny crooked square of blue-white stars called the Trapezium. They are surrounded by a glowing cloud with a mottled texture. If the sky is relatively dark (no moonlight and your backyard is in the suburbs) you should see two wings of light arcing from either side of the nebula's bright core.
If it looks like the Trapezium stars are lighting up the nebula, you're right. These are hot stars recently formed in collapsing clouds of dust and hydrogen gas. Ultraviolet light from these and other stars embedded in this emission nebula causes the gas to glow.
More on Orion
You probably won't see any color. The pinks and blues that show up so vividly in long-exposure photographs are too subtle to trigger the color receptors in your eyes, especially when viewed through small telescopes. If anything, the Orion Nebula might have a greenish tinge to it, from the glow of oxygen atoms excited by radiation.
When you look at the Orion Nebula you are looking across 1,500 light-years of space. That may seem vast, but on the cosmic scale it's in our galactic neighborhood. The nebula sits in an outlying spur of the same Milky Way spiral arm that contains our Sun.
The Orion Nebula is also known as M42 — the 42nd entry in Charles Messier's catalog of objects. While you're in the area, check out M43, part of the Orion Nebula just to the north of the Trapezium. Also, if you're scanning this area with binoculars or a telescope, you've probably already seen M45 — the Pleiades, or Seven Sisters. This is the sky's brightest star cluster. It's an easy naked-eye object about 35 degrees above and to the right of Orion. (See "A Sampling of Star Clusters".)
The Dumbbell Nebula
For our other target, we go after another type of nebula. Like Orion, this object is a glowing cloud of gas but with a different origin. The Dumbbell Nebula (M27) named for its resemblance to a weight-lifter's equipment, is a planetary nebula. It and all its kind are the cast-off remnants of dying stars. Most stars like the Sun — average well-behaved stars — bloat up into red giants at the end of their lives. They lose weight by blowing off their outer layers of gas. As these layers expand into space they form temporary shrouds around the central star.
At a distance of 3,500 light-years, the Dumbbell is one of the closest planetaries and appears larger than most. It's worth looking for in binoculars just off the summer Milky Way. To find M27, first locate the three bright stars of the Summer Triangle. They're overhead in the evenings from July through September. They're Altair (in the south), Vega (to the west), and Deneb (to the east).
Halfway between Altair and Vega, track down Albireo, the 3rd-magnitude star that marks the head of Cygnus, the Swan. Now look halfway between Albireo and Altair for a faint but distinctive pattern that resembles a little arrow. This pattern of four 4th-magnitude stars is the constellation Sagitta. Gamma (γ) Sagittae lies at the tip of the "arrowhead."
M27 lies about half a binocular field above (due north of) this star. Look for a small puffy cloud sitting amid a rich field of stars. If skies are dark, it should be visible in most telescope finders.
A telescope of any aperture will show M27's dumbbell shape, or perhaps you might think of it as a cosmic apple core. The nebula looks brighter in larger telescopes but doesn't really show a lot more structure until you bring 12- to 20-inch scopes to bear on it.
Here's where one observing aid comes in handy — the nebula, or light-pollution filter. While not particularly useful for star clusters and galaxies, these filters really help bring out planetary nebulae. They block most of the colors of the spectrum except the prominent green light emitted by glowing oxygen atoms. Even in a dark sky, a nebula filter will boost the contrast between the nebula and its background.