Explore a Celestial Triangle

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A General Look at the Stars of the Triangle

The Summer Triangle and the constellations that lend their stars to it. Carolyn Collins Petersen

There are three stars up in the sky for the next several months that you can see from nearly anywhere on Earth. They're the three brightest stars in three constellations (patterns of stars) close together in the sky: Vega — in the constellation of Lyra the Harp, Deneb — in the constellation of Cygnus the Swan, and Altair — in the constellation of Aquila, the Eagle. Together, they form a familiar shape in the sky — a giant triangle.

Because they're high in the sky throughout most of northern hemisphere summer, they're often called the Summer Triangle. However they can be seen by many people in the southern hemisphere, which is experiencing winter right about now. And, they're visible in the sky in the evenings until well into October. So, they're really trans-seasonal. Which also gives you a good long time to watch them over the next few months.

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Vega -- the Falling Eagle

Vega and its dust disk, as seen by Spitzer Space Telescope. The disk glows in infrared light because it is warmed by its star. NASA/Spitzer/CalTech

The first star in the Triangle is Vega, with a name that comes to us via ancient Indian, Egyptian, and Arabic star observations. At one time, about 12,000 years ago, it was our pole star, and our north pole will appear to point at it again in about the year 14,000. It is the brightest star in Lyra, and the fifth brightest star in the entire night sky.

Vega is a fairly young blue-white star, only about 455 million years old. That makes it much younger than the Sun. Vega is twice the mass of the Sun, and due to that, it will will burn through its nuclear fuel more rapidly. It will probably live for about a billion years before leaving the main sequence and evolving to become a red giant star. Eventually it will shrink down to form a white dwarf.

Astronomers have measured what looks like a disk of dusty debris around Vega, and there are observations that suggest Vega may have planets (also known as exoplanets; astronomers have discovered many of them using the Kepler planet-finding telescope). None have been observed directly yet, but it's possible that this star, which — at a neighborly distance of 25 light-years — could have worlds orbiting around it.

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Deneb -- the Tail of the Hen

The constellation Cygnus with Deneb at the tail of the swan (top) and Albireo (the double star) at the nose of the swan (bottom). Carolyn Collins Petersen

The second star of the great celestial triangle is called Deneb (pronounced "DEH-nebb"). Like many other stars, its name come to us from ancient Middle Eastern stargazers who charted and named the stars.

Vega is an O-type star that is about 23 times the mass of our Sun and is the brightest star in the constellation Cygnus. It has run out of hydrogen its core and will begin to fuse helium in its core when it gets hot enough to do so. Eventually it will expand to become a very bright red supergiant. It still looks blue-white to us, but over the next million years or so its color will change and it may end up exploding as a supernova of some kind.

As you gaze at Deneb, you're looking at one of the brightest stars known. It's about 200,000 times brighter than the Sun. It's somewhat close to us in galactic space — about 2,600 light-years away. However, astronomers are still figuring out its exact distance. It's also one of the largest known stars. If Earth orbited this star, we'd be swallowed up in its outer atmosphere.

Like Vega, Deneb will our pole star in the very distant future — in the year 9800 A.D.

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Altair -- the Flying Eagle

The constellation Aquila and its bright star Altair. Carolyn Collins Petersen

The constellation Aquila (the Eagle, and pronounced "ah-QUILL-uh", which lies somewhat close to the nose of Cygnus, has the bright star Altair ("al-TARE") at its heart. The name Altair comes to us from the Arabic, based on observations by skygazerswho saw a bird in that star pattern. Many other cultures did, too, including the ancient Babylonians and Sumerians, as well as the inhabitants of other continents around the world. 

Altair itself is a young star (about a billion years old) that is currently passing through an interstellar cloud of gas and dust called G2. It lies about 17 light-years away from us, and astronomers have observed it to be a flattened star. It's oblate (flat-looking) because the star is a fast rotator, meaning it spins very rapidly on its axis. It took quite a few observations with special instruments before astronomers could figure out its rotation and the effects it causes. This bright star, which is the first one for which observers have a clear, direct image, is about 11 times brighter than the Sun and nearly twice as massive as our star.