Saturday, July 7, 2012

July 2012 Constellation of the Month -- the Summer Triangle

The Summer Triangle is not one of the official 88 constellations, but it is nonetheless a prominent and memorable shape made by stars (an "asterism"). It is comprised of the brightest stars of 3 constellations.  You can't miss them; they are the brightest stars east of Arcturus, which is the brightest star in last month's Constellation of the Month: Bootes

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 "asterism" to get to know.  It is visible about half the year soon after sunset, 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 scale.  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) can appear to us as less bright than a star of average brightness which is much closer to earth.

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 fainter than 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 1st magnitude, from 1.5 – 2.5 are 2nd magnitude and so on.  The naked eye in Ridgefield can probably see no farther than 4th 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 1st magnitude stars.  By way of comparison, the stars in the Big Dipper are magnitude 2 stars. 

South and west of Arcturus (0.13) in Bootes are Spica (1.0) in the constellation of Virgo and two planets – Mars (currently 1.09)  and Saturn (currently 0.93) – which have been  visiting in that area in recent weeks.  The apparent magnitude of planets varies – in some cases considerably – over the course of weeks and months, since their distances from earth can vary greatly as they make their ways around the Sun.