Summer Triangle Asterism
The Summer Triangle asterism still shine brightly in the skies. It pops out at nightfall and climbs up at the late evening. The Summer Triangle includes three bright stars in three different constellations.
The first stars to appear in late August evenings are the bright, white stars of the Summer Triangle asterism — Vega, Deneb, and Altair. At dusk, they are high in the eastern sky and pass the zenith at about 11 pm local time. This annual feature of the summer sky will remain visible until the end of December! At magnitude 0.03, Vega is the brightest star in the summer sky, mainly due to its relative proximity to the sun of only 25 light-years. Altair is only 17 light-years from the sun, but Deneb is a staggering 2,600 light-years away; so bright because of its far greater inherent luminosity.
Stars shine with a colouration that is produced by their surface temperatures, and this is captured in their spectral classification. Our sun is a yellowish G-class star with a surface temperature of 5,800 K. The three bright stars of the Summer Triangle are A-class stars that appear blue-white to the eye and have high surface temperatures in the range of 7,500 to 10,000 K. Look in the western sky for orange Arcturus, a K-class giant star with a temperature of only 4,300 K. Sitting low in the southwest, reddish Antares, the heart of Scorpius, is an old M-class star with a surface temperature of 3,500 K. By comparing these stars colours’ to other stars, you can estimate those stars’ temperatures.