Welcome to the last month in our 25th year! The evening sky welcomes the northern hemisphere to longer nights and views of the celestial vault, while in the south night is shrinking and Summer has arrived. The constellation of Gemini should be visible from a modestly dark site by 10 pm for citizens of Earth in the northern hemisphere. For many human civilizations, the constellation was associated with twins.

Two of the constellations bright stars, Castor and Pollux, are less than 5 degrees apart in the night sky from Earth although they are not gravitationally bound to each other. Pollux, the brighter of the two, shines at 1.14 and is easily the brightest star in the constellation. Despite its brightness, it has been designated as Beta Geminorum (Beta Gem). The star is much more yellow than its "twin" as it has a K0 spectral classification and is considered an orange giant. A mere 52 light years away, Castor (visual magnitude 1.93) was discovered to be a double star in 1678. As astronomical knowledge became more sophisticated, the system was eventually identified as a system of three components, now designated Castor A, B, and C. Castor A and B were studied by William Hershel in 1803, he identified that the stars were actually gravitational bound and thus, Castor A/B is the first true physical binary that was recognized. Castor A and B are separated by about 6 arc-seconds and their revolution period about 467 years. Castor B is a pair of stars, reported as type A5 in Burnham's Celestial Handbook. They are of a similar size and each is about 1-1/2 the size of our Sun. Castor C is a pair of red dwarf stars, each about half the mass of our own Sun. This component has been designated YY Geminorum due to its variable nature. Castor A and B have a faint companion (separated 72 arc-seconds) that have the same parallax and proper motion. With six individual stars gravitationally bound together, Castor is considered to be a sextuple star system.

Amateur astronomers will want to look for an object in the constellation of Gemini known as NGC 2392 or the Eskimo Nebula. This object is an excellent planetary nebula. Although it is not quite as large as the more popular "Ring Nebula", M57 in Lyra, NGC 2392 is brighter at 8th magnitude. The central star of this planetary is also much brighter. At 8th magnitude it is easily seen in amateur telescopes, whereas the central star of the Ring is too faint for the small telescope. Don't look at CCD images and expect your view to approach the color and detail of those. The visual observation has its own charm and wonder. On a good night the edges of the Eskimo's parka will thrill the eye.

No discussion of Gemini is complete without mention of the huge star cluster known as M35. This cluster spans an area greater than the full moon and is visible at dark sites with the naked eye. The cluster was known before the time of Messier, but was found in his catalog and so the M designation has remained. The cluster contains at least 120 stars covering an area of 30 light years. Viewing this object with an amateur instrument gives one the impression of many double stars. Also visible as a fuzzy patch of light in the same field of M35, is the rich cluster NGC 2158. This cluster is more distant than M35 and would take a large amateur instrument at high power to resolve into stars.

--See You Under the Stars!
Astra for Astra's Almanac

The star chart above was generated by Stellarium, a free open source planetarium program. The above image was created by Dawn Jenkins, using Stellarium and a graphic editing program to format the image for this web page. Editing was done for educational purposes only. Stellarium offers much more to amateur astronomers and is being used in planetariums and to guide telescopes in the field. Simple charts like the one above can be used on the internet for non-profit, illustration purposes. Thank you to the makers of this fine program from Astra's Star Gate.