July 2014 Constellation of the Month – The Summer Triangle

Our July 2014 Constellation of the Month is not one of the official 88 constellations, but it is nonetheless a prominent and memorable shape made by stars.  The Summer Triangle is comprised of the brightest stars of 3 constellations.  You can't miss them; they are the brightest stars in the East an hour or two after sunset, about midway between the eastern horizon and overhead.

The brightest of the 3 stars – Vega, in the constellation of Lyra (the Harp) – is the easternmost one.   To the west and north is Deneb, in the constellation Cygnus (the Swan, also known as the Northern Cross).  To the west and south of Vega is Altair in the constellation Aquila, the Eagle. 

The Summer Triangle is a good non-constellation star pattern ("asterism") to get to know.  (Asterisms can also be well-known parts of constellations, e.g.,  the Big Dipper is a part of the constellation Ursa Major).   The Summer Triangle is visible about half the year, and it can be useful as a starting point for finding other constellations -- as we'll describe in coming months.

Vega is an important star, because it was used to define the zero point in the scale used to specify the apparent brightness of stars – the magnitude.  I say "apparent" brightness, because it describes how bright a star appears to us on earth.  A very bright star (one with a large "absolute" brightness) which is very far away can appear to us as less bright than a star of average brightness which is much closer to earth.  "Close" in astronomical terms, could be 50  trillion miles away.

So, Vega was defined as having a magnitude of 0.  And the brightness of other stars was measured in relation to that.  In order to avoid very large numbers, the magnitude scale is logarithmic, like the Richter scale for earthquake intensity.  A magnitude 1 star is 2.5 times as bright as a magnitude 0 star.  And a magnitude 2 star is 2.5 times fainter than that.  So a magnitude 2 star is 2.5 x 2.5 (6.25) times fainter than a magnitude 0 star. 

Stars brighter than 1.5 magnitude are called 1^st magnitude, from 1.5 – 2.5 are 2^nd magnitude and so on.  The naked eye in Ridgefield can probably see no fainter  than 4^th magnitude, due to light pollution. The other two stars in the Summer Triangle – Altair and Deneb – are magnitudes 0.77 and 1.2, respectively.  So they are also 1^st magnitude stars.  By way of comparison, the stars in the Big Dipper are magnitude 2 stars.