June 2013 Constellation of the Month – Corona Borealis

Almost directly overhead an hour after sunset, is a compact,  U-shaped constellation, Corona Borealis --  the Northern Crown.  (There is also a Southern Crown, Corona Australis, which is visible to people living in the southern hemisphere.)

  Coronal Borealis is just east of kite-shaped Bootes (our June 2012 constellation of the month).  It is about halfway between two very bright stars – Arcturus, which marks the bottom of the kite, and Vega, which is, the westernmost star in the Summer Triangle.

Its brightest star, Alpha Coronae Borealis, is 2^nd magnitude -- as bright as most of the stars in the Big Dipper.  The other 6 stars are considerably fainter, but still visible with the naked eye.  Alpha, like so many stars, is actually a double star, in which 2 stars orbit each other.  Our vantage point on earth is lined up with the plane of the orbit, so that one star will regularly pass in from of the other.  When the fainter star is in front of the brighter star, the pair will appear slightly fainter,  This happens every 17 days. This type of star is called an "eclipsing binary."

The constellation also contains 2 rarer types of variable stars.  T Coronae Borealis is an exploding variable star also known as the Blaze Star.  It is usually a very faint star, but once every 80 years or so it explodes, like a nova  In a matter of hours, it can increase its brightness 1,500 times – from magnitude 10 to magnitude 2.  

Another variable star is R Coronae Borealis.  It  periodically experiences drops in brightness, which can sometimes be very dramatic.  Scientists speculate that a buildup of carbon particles is the cause.

Corona Borealis has no easily seen nebulae or galaxies.  But it does have some attractive binary stars, which can be separated by small telescopes like those used at Discovery Center astronomy events.