Stargazing For Dummies
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A nebula is a cloud of gas and dust in space ("dust" meaning microscopic solid particles, which may be made of silicate rock, carbon, ice, or various combinations of those substances; "gas" meaning hydrogen, helium, oxygen, nitrogen, and more, but mostly hydrogen). Some nebulae play an important role in star formation; others form from stars gasping on their deathbeds. Between the cradle and the grave, nebulae come in a number of varieties.

Here are a few of the most familiar nebulae:

  • H II regions are nebulae in which hydrogen is ionized, meaning that the hydrogen loses its electron. (A hydrogen atom has one proton and one electron.) The gas in an H II region is hot, ionized, and glowing, due to the effects of ultraviolet radiation from nearby O or B stars. All the large bright nebulae that you can see through binoculars are H II regions. (H II refers to the ionized state of the hydrogen in the nebula.)
  • Dark nebulae are the dust bunnies of the Milky Way, consisting of clouds of gas and dust that don't shine. Their hydrogen is neutral, meaning that the hydrogen atoms haven't lost their electrons. The term H I region refers to a nebula in which the hydrogen is neutral. It's another name for "dark nebula."
  • Reflection nebulae are composed of dust and cool, neutral hydrogen. They shine by the reflected light of nearby stars. Without the nearby stars, they would be dark nebulae.
    Sometimes a new reflection nebula appears suddenly, and you may discover it, as amateur astronomer Jay McNeil did. In January 2004, he found a new reflection nebula in the constellation Orion with a 3-inch refractor in his backyard, and professionals now call it McNeil's Nebula. But don't hold your breath; this type of discovery is very rare.
  • Giant molecular clouds are the largest objects in the Milky Way, but they're cold and dark and scientists would've gazed right by them if not for the data gathered by radio telescopes, which can detect emissions of faint radio waves from molecules such as carbon monoxide (CO). Like all other nebulae, giant molecular clouds are mostly made of hydrogen, but scientists often study them by means of their trace gases, such as CO. The hydrogen in giant clouds is molecular, with the designation H2, which means that each molecule consists of two neutral hydrogen atoms.
    One of the most exciting nebular discoveries in recent decades showed that bright H II regions, such as the Orion Nebula, are just hot spots on the peripheries of giant molecular clouds. For centuries, people could see the Orion Nebula but had no idea that it's no more than a bright pimple on a huge invisible object, the Orion Molecular Cloud. But now we know. New stars are born in molecular clouds, and when they get hot enough, they ionize their immediate surroundings, turning them into H II regions. The part of a molecular cloud where the dust is thick enough to cut off the light of many or most of the stars behind the cloud, as visible from Earth, is called a dark nebula.

H II regions, dark nebulae, giant molecular clouds, and many of the reflection nebulae are located in or near the Milky Way's galactic disk.

Picking out planetary nebulae

Planetary nebulae are the atmospheres of old stars that started out resembling the sun but then expelled their outer atmospheric layers, as the sun will do in the far future. The nebulae are ionized and made to glow by ultraviolet light from the hot little stars at their centers, which are all that remain of the former suns. Planetary nebulae expand into space and fade as they grow larger. They can be well off the galactic plane, unlike H II regions.

For decades, astronomers believed that many or most planetary nebulae were roughly spherical. But now astronomers know that most are bipolar, meaning that they consist of two round lobes projecting from opposite sides of the central star. The planetary nebulae that look spherical, such as the Ring Nebula in the constellation Lyra, are bipolar, too, but the axis down the center of the lobes happens to point toward Earth (and so, like a dumbbell viewed end-on, they look circular).

Curious point: Respectively related and unrelated to planetary nebulae are protoplanetary nebulae, which are much studied by astrophysicists. One type of protoplanetary nebula is the early stage of a planetary nebula — a phase in the death of a star (not to be confused with the Star Wars Death Star). The other type of protoplanetary nebula is the birth cloud of a solar system's star and its planets. Yes, astronomers use the same term to refer to two completely different kinds of objects, but nobody's perfect.

Breezing through supernova remnants

Supernova remnants begin as material ejected from massive stellar explosions. A young supernova remnant is composed almost exclusively of the shattered remains of the exploded star that expelled it. But as the gas moves outward through interstellar space, it resembles a rolling stone that does gather moss. The expanding remnant creates a snowplow effect as it pushes along and accumulates the thin gas of interstellar space. By the time it has aged — tens of thousands of years later — the remnant is overwhelmingly composed of this "plowed up" interstellar gas, and the remains of the exploded star are mere traces. Supernova remnants expand along or near the galactic plane of the Milky Way.

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