While not a constellation itself, the Summer Triangle dominates the overhead sky in the northern summer and autumn months and guides stargazers to other stars, constellations, and deep-sky sights. The vertices of the triangle are marked by three bright stars Vega, Deneb, and Altair, each of which belong to true constellations Lyra, Cygnus, and Aquila, respectively. The image below shows the Summer Triangle rising as seen from mid-northern latitudes at 10 p.m. in mid July. The triangle is big: it spans about two full hand widths held at arm’s length. The triangle can be seen well south of the equator, too, above the northern horizon. Southern stargazers call it the “Northern Triangle” or the “Winter Triangle”.
(Image at top by Tien-Chu Chang under Creative Commons License).
Within the area of the Summer Triangle lie dozens of splendid deep-sky objects along the path of the northern Milky Way, objects that are easily accessible with binoculars or a telescope. We’ll visit some of these sights in future articles. But today, let’s have a look at the three main stars that make up the Summer Triangle, plus one more star that may be the prettiest double star in the sky.
The brilliant blue-white star Vega is the westernmost star in the Summer Triangle. It’s by far the brightest star in the small constellation Lyra, which in Greek legend represents the lyre of Orpheus. Vega is the 5th-brightest star in the sky and the 2nd brightest star north of the celestial equator after Arcturus. Vega is main sequence star, which means it creates energy by burning hydrogen in its core and otherwise behaving like a stable middle-aged star. It’s about 26 light years away and has an age of about 500 million years, some 1/10 the age of the Sun, and 40x the Sun’s brightness. Astronomers have found a dusty disk of debris around Vega, with tantalizing hints that planets may be coalescing within the disk.
Altair, the brightest star in the constellation Aquila, the Eagle, is the southernmost star in the Summer Triangle. It lies just 16 light years from Earth. It burns with a pure white light, a little cooler and whiter than Vega. Altair has about 1.8x the mass of our Sun and 11x its brightness. But whereas our Sun rotates once every 25 days, Altair rotates once every 9 hours, so fast it’s actually squashed a little at the poles (so is Vega, for that matter). Altair is attended just to the north and south by the stars Tarazed and Alshain, γ Aquilae and β Aquilae, respectively.
Deneb, the easternmost star of the triangle, is a bruiser compared to Vega and Altair. It’s one of the most massive and intrinsically luminous of all nearby stars. It appears slightly fainter than the other two stars, but in real terms, Deneb is far brighter, between 50,000 and 200,000 times brighter than the Sun. At a distance of 1,500 to 2,500 light years, the star is also much further away. The star marks the tail of the constellation Cygnus, the Swan.
Within the Summer Triangle, almost exactly at its center, lies Albireo, a star at the other end of the constellation Cygnus. In nearly any telescope, even at low magnification, Albireo resolves into a absolutely beautiful double star. The brighter 3rd magnitude component shines a golden-yellow; the fainter 5th magnitude component is a sapphire-blue. The color contrast is striking. If you have trouble seeing the colors, that’s because your eye is less sensitive to color concentrated in tiny bright points of light. Try de-focusing your telescope just a little to smudge out the image. The color should become obvious. Pretty much any telescope, even at low magnification, will split this fine double star. Even a pair of high-power binoculars, say 10x to 15x, will do the job.