Several bright objects pop out of the evening twilight this week.  In the southwest is Sirius, the brightest star in Earth's night sky, and just above it is Procyon.  Both are "dog stars" and both are fairly close by as stellar distances go.  We call Sirius The Dog Star because it is the brightest member of the constellation of Canis Major, the great hunting dog following Orion toward the western horizon.  Procyon is the brightest star — really the only bright star — in Canis Minor, Orion's other companion.  Sirius is about 8.5 light years away, Procyon about 11.5.  Contrast that with Betelgeuse, the reddish star in Orion's shoulder, at a distance of more than 400 light years, or Rigel at Orion's toe, over 700 light years distant.

Brighter than any of these is the planet Jupiter, blazing away in the west.  Jupiter is currently at magnitude -2.1 (that's minus 2.1), Rigel at 0.3, Betelgeuse at 0.6, Procyon at 0.4, and Sirius at -1.4.  That makes Sirius about five times as bright as Procyon and Jupiter about twice as bright as Sirius.

What do those numbers mean?

Well, for one thing, it means that astronomers are really fond of numbers.

It all started simply enough.  A few thousand years ago — specifically, the times of the Greek astronomers Hipparchus and Ptolmey — stars were divided into six classes with a first class star brighter than a second class star, a second class star brighter than a third class, and all the way down to a sixth class star, just barely bright enough to be seen in a dark sky.  In the 19th century a British astronomer worked out a mathematical formula to define these six magnitudes more precisely:  a first magnitude star would shine with exactly 100 times the light of a sixth magnitude star.  This means that a difference of one magnitude corresponds to a brightness of slightly more than two and a half times:  a first magnitude star will send 2.512 times as much light to our eyes as a second magnitude star will. 

With a mathematical formula, stars can be described with exact, down-to-the-decimal-point values and objects brighter than first magnitude can be given values of zero (the star Vega, for example) or even with negative numbers.  Venus reaches a magnitude brighter than -4 and a full moon will be nearly -13.

And our own sun?  Seen from the nearby star Alpha Centauri, our sun would shine at a respectable +0.5, brighter than Betelgeuse, not quite as bright as Rigel.  Seen from my porch on a warm spring day (should one ever arrive) it shines at -26.7.  Ahh!

Happy spring, by the way.  Dress warmly!


	 	 	 Keep looking up!
	 	 	 - Bob Hamlin
	 	 	 <rhamlinatdartmouth.edu>

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