New school year rides in on Great Square of Pegasus
A small but perceptive number of nature-aware children follow the seasonal turnings of the stars. To all longtime starwatchers, the constellations become old, reliable friends. But to those of school age, the emergence of the Great Square of Pegasus low in the east brings a special poignancy and foreboding. In fact, the first sighting of it is almost a shock. Its arrival means the end of summer is looming into sight and school will soon begin.
That’s certainly what the Great Square meant for me years ago. I’ve heard from many amateur astronomers who still have an emotional reaction to its arrival.
Now that September is here and school actually opens in a couple of days, if it hasn’t already, the Great Square no longer lurks low. It displays itself high and blatant in the east every clear evening.
Tonight the full moon shines below it. Full moon isn’t the best time for stargazing, but the Great Square’s four corner stars are bright enough to show even through the moonlight if you have reasonably good vision. The Square is balancing on one corner, as it always does when showing itself in the east. It’s a little larger than your fist held at arm’s length.
The rest of the constellation Pegasus is less apparent. It’s made of dimmer stars, so don’t expect to see much of the flying horse in the moonlight. Later this week, however, the moon will be gone, and the picture here shows the constellation’s arrangement. Judging by ancient representations, Pegasus was seen as only the front half of a horse. The Great Square is his body, his neck stretches out from one corner, and a somewhat brighter orange star marks his nose. (It’s even named “Nose’’ in ancient Arabic.) The lunging forelegs of Pegasus are dimmer still.
The horse is upside down to us, but as seen from Greece and the Mediterranean some 3,000 years ago, he crossed the sky upright and headfirst while at his highest.
Even though moonlight wipes out faint stars, I find that it has one interesting benefit. It makes the colors of stars a little easier to detect, for stars that are bright enough to show their tints to the naked eye at all.
For instance, the top star of the Great Square shows a trace of orange, while the other three are icy white. The top one is named Scheat, from the Arabic for “shin.’’ It’s an orange-red giant 200 light-years away, about 95 times larger than our sun and shining 300 times as brightly.
From Scheat, look along the Great Square’s upper right side. About halfway down it lies a faint star, hidden by the moonlight and Greater Boston’s light pollution, that in 1995 was plucked from obscurity to make astronomical history. The star is 51 Pegasi, the first normal star beyond our solar system to show clear evidence of having a planet.
At that time, every astronomy book in print said planets of other stars were so far and faint that they would forever be out of sight for earthbound telescopes. Even now nobody has actually seen the planet, nor will. But by 1995 astronomers had developed technology that enabled them to detect tiny, periodic changes in a star’s velocity (changes as little as a few feet per second) that betray the gravitational influence of an orbiting planet.
The field quickly exploded. Gravitational-wobble measurements have been key to finding or confirming most of the 373 extrasolar planets known as of this week.
If you have a pair of binoculars, they’ll show 51 Pegasi even through the sky glow. From Scheat, scan just a bit more than halfway down the Great Square’s upper-right side. Now move slightly upper right toward the Square’s outside. There’s a tiny pinpoint. That’s it.
It looks a lot like our sun would if you were there, looking back in our direction from 50 light-years away.
Its planet is a roasting “hot Jupiter’’ that circles the star very closely, completing an orbit every 4.2 days. Does the star have smaller, more Earthlike planets farther out? For this, we still have no way to tell.
Alan M. MacRobert is a senior editor of Sky & Telescope magazine in Cambridge (www.SkyandTelescope.com). His Star Watch column appears the first Saturday of every month. 
