Ursa Major: a brief guide

Ursa Major is one of the most familiar constellations visible in the Northern Hemisphere. Here is a brief guide to some of its notable features.

Ursa Major (the Great Bear) is known by several names, including the Plough and the Big Dipper. It is an easily recognised constellation consisting of a rough rectangle and three other stars that appear to attach to it in a handle shape. These are the ones that are easily seen with the naked eye, but there are many more in this region of the night sky that can be seen with the aid of a telescope or binoculars.

As with all constellations, one has to remember that they are optical illusions in that their members usually have no relationship with each other and are at many different distances from Earth.

The two “outer” stars of the rectangle are Dubhe and Merak. These are known as the Pointers because an imaginary line drawn through them leads the eye to Polaris, the North Celestial Pole and a guide to finding one’s direction at night. Dubhe is a yellow giant star with high luminosity, being the 35th brightest star visible from Earth. It has a smaller companion star. Merak is a main sequence star that is nearly three times larger than our Sun and 68 times more luminous.

The second star in the “handle” is Mizar, although most people can soon make out that it is in fact two stars, the fainter companion being Alcor. However, Alcor is in fact a binary star and Mizar is a quadruple, which means that one is actually looking at six stars and not two.

If one has a good pair of binoculars it is possible to see two galaxies to the north of Ursa Major. These are Bode’s Galaxy (M81) and the Cigar Galaxy (M82). M82 is almost edge-on to us but M81 is tilted at a lesser angle. These galaxies are around 10 million light years away but close enough to each other to interact gravitationally.

The spiral galaxy M101 can be seen on the other side of the constellation. Binoculars will reveal it as a circular smudge but better magnification will show that it is a well-formed spiral that is face-on to us.

A good telescope can reveal the Owl Nebula (M97), although the “owl eyes” will need a relatively high resolution telescope in order to see them.

© John Welford