α Ursae Majoris

Alpha Ursae Majoris is “Dubhe”, shortened from the Arabic Thahr al Dubb al Akbar: The Back of the Great Bear. It's an orange giant with a close companion.

Dubhe forms the ‘lip’ of the dipper and in binoculars is just out of view from beta, about 5.5º due south.

The asterism of the Big Dipper (or Plough, in the UK) is so large that binoculars can only get at most two stars at a time in the same field of view: dipper.
     The classic means of finding the North Pole is to draw a line from beta through alpha and continue it in the same direction.

Dubhe is a binary (BU 1077): 2.0, 4.9; 10º, 0.6 with an orbit every 44.4 years. Presently the companion is drawing away from the primary, and will achieve its maximum distance (0.84") in the years 2022-2025.

In the same field with Dubhe are two other binaries—one a binocular binary, the other with a very close companion:
      Otto Struve 235: 5.7, 7.6; 32º, 0.8".
      Struve 1495: 7.3, 8.8; 36º,34.1".


Beta UMa is called Merak, ‘the Loin of the Bear’; it's one field south of alpha: beta,

Beta UMa is a main-sequence white star with a visual magnitude of 2.3, meaning four stars are brighter than beta in Ursa Major. It has a radius of about 2.8 Suns and is 80 light years away.

Two difficult Messier objects are in the same field of view:
      M108 wasn't originally added by Messier himself, and it was long after his death that the object found its way into his catalogue. Very dim at an apparent visual magnitude of 10 (some references give 13m), M108 is a very small galaxy that requires a large telescope to get any detail.
      The Owl Nebula (M97), one of the largest known planetary nebulae, also requires a large telescope as it is only a tenth-magnitude object with a fourteen-magnitude central star.

Easier to resolve is Struve 1520, at the southeastern edge of your field of view, with a nice colour contrast of a yellow primary and a blue companion: 6.5, 7.8; 343º, 12.1".


The other corner of the dipper is gamma, eight degrees southeast of beta, with another Messier galaxy as well: binoculars.

Gamma UMa (Phad or Phecda, ‘Thigh of the Bear’) is a white sub-giant with a radius of three times the Sun. The Messier object M109 is about a half-degree southeast of gamma.

M109 is an extremely faint barred galaxy (apparent magnitude 10.6) brought to life with very large telescopes, which show a soft orange central glow to contrast the blueish tint of the spirals. M109 is the central galaxy of a group of 50 or so galaxies in Ursa Major which go under the name of the M109 Group.

Click on theta Ursae Majoris to continue.

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