Ursa Major

March 2013 Constellation of the Month – Ursa Major

The constellation Ursa Major, the Great Bear, includes the group of stars (an "asterism") called the "Big Dipper."   The Big Dipper comprises less than half the area covered by Ursa Major; the remainder consists of 3 appendages extending from the back and bottom of the bowl.  That would be to the South in the early night time sky tonight.

Ursa Major is a circumpolar constellation.  This is explained in the blog post for our December 2012 constellation of the month, Cassiopeia, the "W" .  Basically, circumpolar constellations never set below the horizon.  They are visible all night every night of the year.  The W and the Ursa Major are always on opposite sides of the North Star, around which they seem to revolve every 24 hours and every 12 months.  Soon after sunset, Ursa Major is visible to the North and slightly East, relatively high in the sky, while Cassiopeia is to lower in the sky, to the North and slightly West.  At midnight, Cassiopeia will be at its lowest point near the northern horizon, and Ursa Major will be at its highest point.  

As is well known, the last two stars in the bowl of the dipper point to the North Star.  The second star, in the middle of the handle of the dipper, is a double star, which can be made out with the naked eye – if you have good eyesight.  Otherwise, binoculars or a telescope will make it visible.  The brighter of the two stars, called Mizar, is itself a double star, which can be seen in a small telescope.  

Ursa Major has a number of prominent deep-sky objects -- objects outside our Solar System – which can be seen with small telescopes such as those used at Discovery Center astronomy events.  The Owl Nebula, M 97, is a planetary nebula (explained in last month's blog post) with two dark round patches that give the appearance of an Owl's eyes.  It is located below the middle of the bowl of the dipper.  The Owl Nebula is in our Milky Way galaxy, about 2,600 light years away.   M81, Bode's Galaxy and M82, the Cigar galaxy are about 5,000 times as far away.  Some tens of millions of years ago, these two galaxies nearly collided.  Each of their gravitational fields affected the other, causing some disruption in their shapes, and facilitating the formation of new stars.