In April 1957 Sir Patrick Moore began his monthly “The Sky at Night” which is still going strong today making it the longest-running program with the same presenter in television history. Sir Patrick, in his own unique eccentric and fast talking way often referred to “The Summer Triangle” which is an asterism or distinct pattern of stars located in the northern sky. This is a title which he said he coined but the triangle was recognised as a distinct pattern of bright stars as far back as 1816. The three stars making up this imaginary triangle are called Altair, Deneb, and Vega, each of which is the brightest star of its respective constellation – Aquila (The Eagle), Cygnus (The Swan), and Lyra (The Lyre). The sky is divided into 88 internationally agreed separate constellations which are based on star patterns considered to represent an object, person, or animal, often mythological, e.g. Orion the Hunter. They are regions of sky containing all the celestial objects within their boundaries. Asterisms do not have boundaries and can be made up of stars like The Summer Triangle from three different constellations.
Many of the first observational astronomers were Arabs so the Arabic word Deneb means “tail” as it occurs at the “tail” end of the flying swan. Altair is Arabic for “flying eagle” in Aquila “The Eagle”. Vega’s name comes from the Arabic word “waqi,” which means “swooping” and refers to the time when people saw the constellation Lyra as a swooping vulture rather than a musical instrument. The Arabs obviously saw these constellations as birds flying in the night sky.
Even though these three stars straddling the Milky Way are called The Summer Triangle as they are overhead in the summer, they can also be seen well into winter. The photo taken last September shows the star Vega in the Summer Triangle asterism as the brightest due to its closeness to us. The second-brightest star is Altair, and the third is Deneb. But looks aren’t everything and the stars can be deceptive and not what they seem. The relative brightness of a star is classified by it’s magnitude. The smaller or more negative value means it seems brighter. In the Summer Triangle, Vega has an apparent magnitude of 0.026 making it seem the brightest with Altair being dimmer at 0.76. Deneb seems the dimmest at 1.25 but is actually the farthest star of the Summer Triangle at 2,615 light-years away. It is the brightest star in the constellation Cygnus, a blue-white super giant star, being nearly 200,000 times brighter than our Sun and about 200 times wider! Even though it seems the brightest, Vega is only about 2.5 times wider and around 40 times brighter than our Sun being only 25 light-years away. Altair is only about 1.8 times wider and around 11 times brighter than our Sun and 16.73 light-years distant.
A deceptive Summer Triangle!
Glynn Bennallick