StarwatchAstronomyA look at the stars that make up the constellation of Gemini, including Castor, a tight binary, and Pollux, an orange giant
The evening star, Venus, reaches its peak brilliance at magnitude -4.6 this week. Obvious in Britain’s SW sky at nightfall, it sets in the W at 21:30. Mars, to its left and higher and now pulling away, is a good deal fainter at mag 1.2.
Our recent notes have featured Orion, which now strides westwards across the meridian at about 20:00 GMT. This time our chart looks above and to the left of Orion to the constellation of the twins of Greek mythology, Gemini.
Gemini’s two leading stars, Castor and Pollux, take the names of the twins but illustrations depict their bodies side by side with their feet towards Orion. The latter’s arm and club are represented on our chart by the line of dim stars reaching northwards from the red supergiant Betelgeuse at the Hunter’s shoulder.
Pollux, an orange giant star at a distance of 34 light years, shines at mag 1.1 and is Gemini’s brightest star. Even so, an apparent error by Johann Bayer in 1603 led him to designate it as Beta Geminorum rather than Alpha.
The Alpha star, Castor, lies 51 light years away and is perceptibly fainter at mag 1.6. Telescopes show it as double, with unequal component stars that lie 5 arcsec apart and take some 450 years to orbit each other. Spectroscopes reveal both to be tight binary stars, as is a third dim companion, making Castor a sextuple system.
Gemini’s star Zeta is much more remote at 1,200 light years. A pulsating Cepheid variable star, its brightness fluctuates between mag 3.6 and 4.2 every 10.2 days. Confirm its changes for yourself by comparing it to the steady mag 3.6 of Lambda and 4.1 of Nu. The red giant Eta shows a slow semi-regular variation between mag 3.2 and 3.9.
The foremost “deep sky object” in Gemini is the fifth magnitude star cluster Messier 35, or M35, 2° from Eta and some 2,800 light years from us. Binoculars show it easily and resolve some of its hundreds of component stars which formed together about 100 million years ago.
A more challenging target is the ninth magnitude Eskimo or Clownface Nebula, also known as NGC 2392. Named because it looks like a person’s head surrounded by a parka, this is a bluish-green planetary nebula that consists of multiple shells and filaments of gas which once formed the atmosphere of what is now a central white dwarf star. Perhaps slightly more distant than M35, it lies 2° ESE of Wasat but needs a better finding chart than we provide here and a large telescope to admire its detail.
ncG1vNJzZmivp6x7tbTEoKyaqpSerq96wqikaKuTnrKvr8RoaWlpZ2Szpq6Oamlon5Witq%2B1jJycpZ2jqbaiuIytrqKmo2KwsLrSrZylpJGptrC6jKyrmqqjYrumrtSlmGarpJa%2FuK3TnJ8%3D