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The Summer Triangle — 3 stars, 1 seasonal mystery
The Summer Triangle dominates the night sky long after summer begins to fade. So what makes Vega, Altair and Deneb a distinctly summer sight?
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On clear September evenings, I often find myself looking up just after dusk, expecting the sky to feel a little less like summer. Yet there, still commanding the overhead darkness, are Vega, Altair, and Deneb — the three brilliant stars of the famous Summer Triangle.
Their familiar pattern can make the season feel as though it is lingering above us, even as cooler nights and earlier sunsets suggest otherwise. In fact, even as late as mid-November, the Triangle is still nearly overhead soon after sunset at dusk. That contrast raises a simple but intriguing question: if this bright celestial landmark remains so prominent well into autumn, why do we still call it the Summer Triangle?
The answer lies not in the calendar, but in how the Triangle moves across the sky over the course of a summer night. The chief reason is that, at this time of year, the Triangle does not remain visible all night. By dawn, it has begun to drop beyond the west-northwest horizon. During summer, by contrast, it remains with us from dusk until dawn: already evident in the east-northeast at nightfall, near overhead around the middle of the night, and descending in the west-northwest as daybreak approaches.
The stars that compose the Triangle — Vega, Altair, and Deneb — lie at very unequal distances from us, so their apparent brightness can give a misleading impression of their true luminosity.
The closest of the three is Altair, 16.7 light-years away, followed by Vega at 25 light-years. Deneb, however, is vastly farther off, at roughly 1,600 light-years, though various sources place it between 1,400 and 2,600 light-years. Even though we see Deneb by light that may have begun its journey during the last days of the Roman Empire, it still shines at first magnitude (+1.3, to be exact) because it is a brilliant blue supergiant, at least 55,000 times brighter than the sun.
But their apparent superiority is largely a consequence of distance: Vega and Altair are only a few dozen light-years away, while Deneb lies well over a thousand light-years from Earth. Compared with the stellar lighthouse of Deneb, Vega and Altair are, comparatively speaking, penlight flashlights.
Before continuing with this discussion about the Summer Triangle, it is worth pausing briefly on the pronunciation of Vega, a point that became especially noticeable after the 1997 science fiction drama Contact. Based on Carl Sagan's 1985 novel, the film follows astronomer Dr. Ellie Arroway, played by actress Jodie Foster, as she discovers a radio signal from Vega containing a sequence of prime numbers.
Throughout the film, the star is pronounced "Vey-gah," perhaps echoing Chevrolet's Vega, the subcompact automobile sold in the 1970s. Astronomers, however, generally use a different pronunciation.