The Summer Triangle

By John Hlynialuk

Summer is not officially over until September 21, so let’s have another look at the famous asterism called the Summer Triangle while it is still high in the sky. Most asterisms (an asterism is an easily recognized pattern of stars) are smaller than their parent constellations. But some include stars from two or more nearby constellations. The record holder is the Winter Hexagon, with a grand total of six bright stars from six adjacent constellations.

A reclining lawn chair is the best aid for finding the Summer Triangle. Face the chair to the south, sit down, and look straight up to see two bright stars. Deneb shines in the Milky Way, and a brighter star, bluish Vega, is about two outstretched fist-widths to its right. Now drop your gaze straight toward the south horizon (or follow the Milky Way), and you will see the third star of the triangle, Altair.

Deneb, Vega and Altair are the brightest stars of the constellations of Cygnus the swan, Lyra the harp and Aquila the eagle, respectively. (Cygnus is also known as the Northern Cross to modern stargazers.)

The stars Vega and Altair have more in common with each other than they do with Deneb. In fact, Deneb, all on its own, has some features that make it unique among the stars in our entire galaxy.

Vega and Altair are relatively close to Earth, at 25 light-years and 17 light-years, respectively, while Deneb is much farther, at 1,500 light-years. Out of the 314 stars listed in the Table of Brightest Stars in The Royal Astronomical Society of Canada’s Observer’s Handbook, only seven are farther than Deneb, and none of those is brighter. At such a distance, it makes Deneb an intrinsically very bright star.

Vega and Altair are slightly larger than our Sun (2.5 Sun diameters and 1.5 Sun diameters, respectively) and are roughly twice the mass of the Sun. By contrast, Deneb has recently been pegged at about 120 solar diameters—enough to fill the Earth’s entire orbit! Furthermore, Deneb’s mass of 20 times our Sun puts it in the group of exceptionally rare, massive supergiant stars that shine with the equivalent brightness of 60 000 Suns! These stars use their nuclear fuel at such a prodigious rate they live for only a few million years, compared with the several billion years that we expect for solar-type stars like Vega and Altair. When our Sun finally dies, it will do so relatively quietly, but Deneb, like other stars in its class, is destined to finish its life cycle in a titanic explosion—a supernova.

If you are ever lost in space, even on the other side of the galaxy, you could look for the blue-white supergiant star Deneb. If it hasn’t gone up in a colossal supernova explosion yet, it should still be shining away as a beacon to point the way home. I hope you have a safe return trip.