The Summer Triangle still shining brightly overhead

Published 9:00 pm Friday, October 13, 2006

The first day of fall has come and gone, the nights are now longer than the days, the leaves are changing, and it’s gradually getting cooler. Summer is over, although we still can get those warm, sunny days – what’s often called an Indian Summer.

The evening sky still has a lot of summer in it. In fact, constellations are much like major league sports, as many seasons overlap. While autumn constellations such as Andromeda, Pegasus and Aries are lighting up our eastern evening skies, there still are many summer constellations stretched across the western half of the heavens. Eventually the Earth in its orbit around the sun will turn completely away from the stars of summer, but for now, they’re making their last celestial shine for this year.

After sunset this month, look overhead for the three brightest stars you can see. Those stars make up something called the “Summer Triangle”. Each of those bright shiners is the brightest star in three respective summer constellations and each is very special in its own way.

The brightest of them to us is Vega, the brightest star in a small constellation called Lyra, the harp. Vega is more than 25 light-years away (one light-year equals nearly six trillion miles). The light we see from Vega this week left that star when Ronald Reagan was just starting his presidency.

Vega also is bigger and hotter than our sun. It’s about three million miles in diameter, more than three times the girth of our sun, and 17,000 degrees Fahrenheit at its surface, or 7,000 degrees hotter than our sun. In fact, you can tell Vega’s one of the hotter stars in the sky by its faint blue tinge. The most significant fact about Vega, though, is that it marks the spot in space toward whcih the sun, the Earth and the rest of the solar system are racing at more than 12 miles a second.

The second-brightest star in the Summer Triangle is Altair, the brightest star in the constellation Aquila, the eagle. Altair is the nearest star in the triangle, a little more than 16 light-years away.

Even with the best of backyard telescopes, you can’t see anything really special. However, astronomers know from spectroscopic analysis that Altair really has a spin to it. It rotates on its axis every ten hours. It takes our sun nearly a month to make a complete spin. Altair spins on its axis so fast that it’s believed to be a lot fatter at its equator than it is at the poles. Because of centrifugal force, it’s actually an oval-shaped star.

My favorite star in the Summer Triangle is the faintest. Deneb, the brightest star in the constellation Cygnus, the swan (also known as the Northern Cross), is a large and luminous star. According to the latest data, this star at the tail of the heavenly swan is more than 3,200 light-years away. As well as we see this star, you have to figure that it’s one humongous star. In fact, it may be more than 250 million miles in diameter and kicking out more than 39,000 times more light than our sun. If you could magically pull Deneb in from 3,000 light-years away to the distance of Vega, about 25 light-years away, about the only thing brighter in the sky would be the moon. Deneb is the biggest single thing you can see with the naked eye in our October skies.

Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis and author of the new book “Washington Starwatch,” available at bookstores and on his Web site, www.lynchandthestars.com.