Starwatch: the bright stars of the Summer Triangle

Starwatch chart 30 July 2018

One of the most obvious star patterns in the night sky at the moment is not a constellation at all. Instead, the Summer Triangle is made up of the three brightest stars from three constellations: Deneb from Cygnus, Altair from Aquila and Vega from Lyra. In the UK, British astronomer Patrick Moore popularised the term Summer Triangle in the latter half of the 20th century but its use dates back to at least 1913. Before that, the asterism was marked on 19th century star charts. All three stars give out blue-white light, indicating surface temperatures of roughly 2500-3500K hotter than the Sun’s 5800K. Whereas Altair and Vega are just a few times larger than the Sun, Deneb is fully 200 times the diameter of the Sun and pumping out around 200,000 times the Sun’s luminosity. Vega is interesting because it is surrounded by a disc of matter, which may indicate the formation of a planetary system.