Eye Astronomy #4: The Winged Horse
Here’s a curious thing about the night sky. In the summer, you’ll see a good number of bright stars. In the winter, you’ll see a lotof bright stars. In the spring and autumn, though, not so much. There’s a reason for that. Our galaxy, the Milky Way, is shaped like a huge pancake with a lump in the center. The Earth’s orbit around the sun is aligned in such a way that we end up looking right through that pancake of stars in the summer and winter, while in spring and autumn we’re looking out of it.
Well, it’s autumn now. If you go outside on a November evening, most of the brightest stars will be close to the horizon if not below it. But there are still sights to see. A great place to start is the constellation Pegasus, the winged horse, which right now is flying high over your head, as shown in this chart:
Pegasus is a big constellation, but it’s not incredibly bright. Its most prominent feature is the body of the horse, the “Great Square of Pegasus.” And it is exactly that: a great big square of reasonably bright stars with almost nothing in the middle. The critter’s head and front legs extend eastward from the square. Curiously, the poor beast has no back legs. What you might take for one is actually the constellation Andromeda, the chained maiden. Go figure.
Of course, that big, empty square isn’t truly empty. There are innumerable stars within it, but most are too faint to see with just your eyes, while those that could be seen are pretty faint, about magnitude 4 and dimmer, making them hard to glimpse in suburban skies. (In case you’re wondering, the four stars of the square are all roughly magnitude 2, more than six times brighter than the brightest stars in the interior.)
Andromeda, as I mentioned, sprouts from the back of the square, in fact from the square’s brightest star, Alpheratz. If you follow the line of stars forming Andromeda, the second one out from Alpheratz is a star called Mirach. That’s your jumping-off point to the Andromeda galaxy. From Mirach, go up two stars, if you can. The second one is as faint as the brightest stars inside the Great Square, so you may not see it without at least binoculars. A bit beyond that star lies the Andromeda galaxy. The Andromeda galaxy is the most distant object visible to the naked eye, over 2.5 million light years away. (But you’d need rather dark skies to see it. Otherwise, binoculars or a small telescope will reveal it.) It’s also remarkably big. If we could see the whole thing with our eyes, it would be six times as wide as the full moon!
Okay, back to Pegasus. Pegasus sits just north of the zodiac, the band of sky through which the sun, moon, and planets move. On the chart, you’ll see some of those constellations marching by:Aquarius, Pisces, Aries, and Taurus. Right now, Saturn is in Aquarius and Jupiter is blazing forth from Taurus. Aquarius and Pices are relatively faint constellations. Aries contains two reasonably bright stars. But Taurus lies close to the plane of the Milky Way, so it is brighter. Indeed, its brightest star, the red giant Aldebaran, is the fourteenth brightest star in the sky.
Somewhat west of Aldebaran (above it, right now), you might notice a curiously bright haze. If you focus on it, you’ll discover it to be a small collection of bright stars. That’s the famous Pleiades, also known as the Seven Sisters, the brightest open star cluster in the sky. An open cluster is a group of stars that were born together from the same cloud of gas and dust. The Pleiades are so bright because they are so young—only 100 million years old—and hot.
Just as the zodiac arcs south of Pegasus, the Milky Way arcs over its northern flank, passing through Cygnus, Cassiopeia, and Perseus (the hero who rescued Andromeda). The winged horse therefore has a rather special place in the sky, carrying us to a great many other constellations. Because of this, once you find the Great Square, you’re well on your way to exploring the rest of the autumn sky.
