Alpha Canis Majoris (Sirius)

α Canis Majoris

Alpha Canis Majoris is better known as Sirius, which some authorities believe to be a distortion of a Greek word meaning "scorching". It is the brightest star in the heavens at -1.44 visual magnitude. The star is 8.6 light years away.

The star is due south in early January: binoculars.

The field is interesting not only because M41 is found near the southern edge, but for two pleasant binaries on the southeastern edge.

Pi CMa, on the southeastern edge, is a telescopic binary: 4.6, 9.6; 13 and separation 11.6"

17 CMa, just southwest of the brighter pi CMa, is a multiple star, and is a rarety in that its present label, H V 65, is that given to it by its discoverer, William Hershel, who catalogued the star 3 March 1782.

Hershel was the pioneer of binary stars (a term he coined in 1802); it was he who proved the then theory that two or more stars orbited about a common gravitational field. His was the first double star catalogue, published in 1782, with subsequent revisions.

In his catalogue of 1785, he described his observations of 17 CMa, calling it a 'Treble' system: "The three stars [i.e. 17ABC] form a rectangle, the hypotenuse of which contains the largest and smallest."

A third companion was later found. All three companions are binocular objects:
      17AB: 5.8, 8.7; 147, 43"
      AC: 5.8, 9.2; 187, 49"
      AD: 5.8, 9.7; 187, 128"

Hershel's original 1785 catalogue is reproduced online, while a recently restored version by Bruce MacEvoy is also available online (a search will quickly find these items). Many of Hershel's discovered doubles were later incorporated into others' catalgues, and Hershel's contributions would nearly become forgotten.

On to M41. This open cluster, four degrees south of Sirius, has naked-eye visibility, with an apparent visible magnitude of 4.5. It's nearly as large as the full moon and contains at least 100 stars, perhaps more. Its age is thought to be over 200 million years. While easily found with the naked-eye, the larger the viewing instrument, the more pleasure in this splendid object.

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1999-2014 by Richard Dibon-Smith.