If you go to a very dark site on a moonless night—and I cannot recommend it highly enough that you do so—you’ll be faced with an awesome sight: a night sky seemingly brimming with millions of stars, all shining down on you.
In truth, only a few thousand of them are visible at once, but if you’re used to the light-polluted skies in and around cities and suburbs, it feels like you’ve been dropped into the middle of a deep-space scene from Star Trek.
Trying to find your way around that starry vault can be a little overwhelming. There’s a lot of sky up there, and stars are everywhere. How can you possibly figure out where you’re looking?
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Happily, nature presents us with some help: the brightest stars in the sky appear to form groups, patterns imagined by our brain interpreting what our eyes are seeing. Sometimes those patterns are simple geometric shapes, but others contain enough bright stars to trigger a sort of recognition in our mind, a passing resemblance to something with which we’re more familiar: a lion, a human figure or a leaping dolphin.
Astronomers call these collections constellations, meaning, literally, a group or set of stars. They’re pretty handy! Over millions of years, the brain has evolved a decently sophisticated pattern-recognition system, and for humans, this means we’re hardwired to spot constellations—and to use them as landmarks in the sky.
Cultures all over the world see these patterns in the stars, and nearly all assign them with stories and myths—not that all these stories are the same. For example, the ancient Greeks saw the constellation Pegasus as a flying horse, but in China it was part of a tortoise and in India a bed for the moon. Not bad for what’s essentially a big square of four stars with a few stellar lines arrayed off the corners.
Other patterns inspire more universal interpretations. Orion, for example, is generally seen as a larger-than-life human figure: a hunter, a king or even a god. It’s hard not to see it this way: three bright stars align to form Orion’s iconic belt, while some of the very brightest stars in the sky, including Betelgeuse and Rigel, give shape to this constellation’s arms and legs.
Over time, as the miasma of astrology coalesced into the science of astronomy, astronomers realized they needed a little more order to the system. Lines were drawn up (quite literally) to parcel up the entire sky into constellations; the International Astronomical Union (IAU), the official keepers and categorizers of all things cosmic, now recognizes 88 of them, each with borders meticulously defined. Every single object in the heavens—stars, galaxies, nebulae, black holes, everything—is “in” a constellation, making it easier for everyone to find them.
Mind you, though, just because stars are in the same constellation does not mean they’re physically associated with each other. Because stars are so far away—tens of trillions of kilometers at least, and sometimes much farther—we can’t perceive their true distance when we gaze upon the sky. As far as our eyes are concerned, they’re all at infinity, but that’s not really the case. Two stars can be right next to each other in the plane of the sky but very widely separated in three-dimensional space; even if they appear the same brightness, one might be intrinsically faint and relatively close to us, while the other is a luminous beast at much greater remove. Traveling even a few dozen light-years from Earth distorts the stars’ apparent position caused by perspective; one need not venture far, cosmically speaking, to render familiar stellar patterns nearly unrecognizable.
Constellations therefore exist almost entirely in our perception, not as a physical reality. But they’re tremendously useful. For one thing, they offer an immediate idea of where an astronomical object is in the sky, similar to someone saying, “I live in a town in Virginia.” It’s not specific, but it does narrow the field considerably. Plus, because of Earth’s motion around the sun, different constellations are visible at different times of the year, so just by knowing the date, you can figure out if an object can potentially be seen or not.
We can delineate them further by hemisphere, too. If you live in Earth’s Northern Hemisphere, for example, constellations too far south are always blocked by the bulk of the planet and are never visible (and vice versa for you austral hemispherians). Some, called circumpolar constellations, are so close to a celestial pole that they never dip below the horizon and instead just circle that fixed point on the sky every night. They’re always visible, all night long.
There are other groupings of stars besides constellations, too. Asterisms, for example, are like subconstellations; stars within them form a pattern that isn’t a full-fledged constellation. While there’s no sharp line differentiating them, one can take the somewhat arbitrary stance that constellations are officially defined by the IAU whereas asterisms are not. The most famous example is the Big Dipper, which is actually just one part of the constellation Ursa Major, the Big Bear. (The Dipper’s handle is the Bear’s tail—except bears don’t have long tails, so astronomers are better with stars than they are zoology.) It’s another example of a cultural interpretation; the Big Dipper is called the Plough in the U.K. and the Wagon in some Slavic countries. It’s also seen by other peoples as part of boat, a crustacean or a caribou.
And in this particular case, contrary to what I wrote earlier, most of the stars in the Big Dipper are in fact associated with one another; they are all at roughly the same distance and moving through space at the same speed and direction. All were likely born together from the same gas cloud long ago. Usually such groups are called clusters (such as the Pleiades, a group of hundreds of young stars about 450 light-years from Earth that coincidentally looks like a dipper and is often mistaken for the Little Dipper), but when they’re older and more dispersed, we call them associations.
Not that all constellations are instantly recognizable, though. Some, such as Aries, are little more than two or three middling bright stars in a row. Microscopium, a southern constellation representing (guess; go on, guess)—a microscope, has stars so faint that they’re invisible even in mild light pollution. Some really do look like their avatar; Scorpius is a long, sinuous collection of decently bright stars that strongly resembles a scorpion. My favorite is Delphinus, which consists of just four stars in a parallelogram with another star trailing behind: it’s a dolphin leaping from the water, and despite its minimalistic nature, that’s not too hard to picture. With a bit of cheek, I’ll note I’m also fond of Triangulum, the Triangle, because I appreciate truth in advertising.
To be honest, professional astronomers don’t really use constellations when talking about objects unless it’s in the name (such as, for example, the name of the star Alpha Centauri, which is in the constellation of Centaurus). Instead astronomers prefer referring to objects based on designated catalog names or even just coordinates on the sky. It’s prosaic, perhaps, but somewhat more practical when dealing with specific and usually extremely faint objects.
That’s not what constellations are for, anyway. They form an atlas to guide you whenever you look up into the overwhelming vastness of the night sky. When you see Corona Borealis (the northern crown) or Crux (the southern cross) or the brilliant stars of Orion, those patterns are telling you where you’re looking and welcoming you to look deeper to find further wonders within.