But let's take a moment to really explore the obvious. What exactly are those stars which make up the Summer Triangle? First off, what are their names? From north to south, they are Deneb (Arabic for "tail"), Vega (Arabic for "falling") and Altair (Arabic for "flying eagle"). They are, respectively, the 19th, the 5th, and the 12th brightest stars in the entire sky. That is, at least as seen from the Earth. In and of themselves, their order in descending brightness would be Deneb, Vega, and Altair. In fact, although Deneb appears to from our vantage point to be the dimmest of the three, it is one of the brightest stars in the entire Milky Way Galaxy, shining with a luminosity of 200,000 Suns (yes, you read that correctly). Only its enormous distance from us makes it appear less bright than otherwise. Vega, which boasts a luminosity of 40 Suns, is but 25 light years distant, making it appear to be the most brilliant member of the triangle. And bringing up the rear, Altair shines out with a radiance of a mere 10 Suns, but at 16.75 light years, it is the nearest of the three.
So let's start with Deneb. "Tail" is certainly an apt name for this star, as it marks the tail feathers of the celestial swan Cygnus (Albireo being the beak). In addition to its fantastic brilliance, this star is HUGE. Were it our own star, our poor Earth would be just grazing its surface in its present orbit. Ouch! Deneb weighs in at a little less than 20 solar masses, but is spewing out into interstellar space roughly the equivalent of one Earth mass every 500 years. Currently a blue-white supergiant, astronomers believe that Deneb with ultimately expand even further, evolving into a red supergiant before collapsing in on itself and going supernova. But that's a few million years into the future, so there's no immediate threat to our lovely summertime asterism.
Next on our list is Vega, the brightest (to us) of the three. Vega has quite a history to it. It used to be the Earth's North Star (14,000 years ago), and will be again in about another 13,000 years. (It will be so much easier then to do polar alignment, with such a bright North Star.) It was the first star (other than the Sun) to be photographed, and the very first to have its spectrograph taken. At about 500 million years old, Vega is quite the youngster. But due to its mass (about twice that of the Sun), it is racing through its time on the Main Sequence. In another half billion years, it will begin to swell into a red giant before losing much of its mass in the form of a lovely planetary nebula. (So stick around!)
Vega and the Sun to scale. Note the enormous equatorial bulge on Vega.
Vega rotates so quickly (once every 12.5 hours) that it is very near the point where it would tear itself apart from centrifugal force. This was hidden from astronomers until recently, since we are looking down directly onto one of Vega's poles, and cannot observe its squashed shape directly.
Vega is surrounded by what appear to be multiple discrete disks of dust and small bodies, possibly in the process of coalescing into planets. So we may actually be observing a solar system in the very act of creation!
Last but not least on our list is Altair. Those of you who are my age, or are fans of 1950s B-grade science fiction movies, will recall that Altair was the parent sun of the "Forbidden Planet" in the movie of that name - home to the mad scientist Morbius and his beautiful daughter Alta, Robby the Robot, and the Monsters from the Id.
A Monster from the Id
In our far more boring reality, Altair is a fairly typical main sequence star. It is somewhat less than twice the mass of our Sun and shines at 11 times its brightness. Like Vega, it has an extreme rotation rate, spinning on its axis once every 9 hours. This results in an equatorial diameter a full 20% greater than its polar measurement.
Only 17 stars other than our own Sun have ever been imaged as anything other than a point source of light. Below is a 2007 image of Altair taken in the infrared. Note the oblateness. And unlike Vega, we are not looking down on one of the poles.
So now summer's most prominent asterism ought to be more than just a triangle. Its component stars are fascinating individuals, which are worth getting acquainted with.