January 21, 2008


Two more constellations remain in the Winter Circle to cover, and today I'll talk about the constellation Gemini, which is found high in the SW sky near the zenith this evening, between Canis Minor and Auriga, just above Orion. The birth story of these twins is a little complicated. There were two sets of twin brothers mothered by Leda, one immortal (from which Polydeuces, or Pollux, came) and one mortal (Castor). The two brothers were alike in many ways, but each had his own specialty. Pollux was a champion boxer while Castor was a horseman and swordsman.

Castor was later killed in a swordfight, and his grieving brother Pollux asked their father Zeus to immortalize the two of them together in the sky. Unlike the myth, these two stars are not physically connected in the sky. In some stories, they were cattle thieves, and in these stories, the Milky Way is a herd of cattle in the sky, so perhaps these two are rustling a few cattle from the main herd.

Alpha Gemini is Castor, though it is not the brighter of the pair. Castor is a triple-double system. The main two stars are resolvable by a small telescope, both as bright blue A stars only a couple of arcseconds apart (with an estimated orbital period of 400 years), and each of these two stars is itself a very close binary seen spectroscopically. In addition, there is a faint red dwarf star much further away that is also a spectroscopic binary and gravitationally bound to this system. Kaler speculates that Castor C (the faint, remote companions) was perhaps captured by the main pair since it appears much younger.

The brightest star in Gemini is Pollux, a single yellow giant star about half as far away as Castor (about 34 light years), about 10 times larger than our Sun. Pollux holds the distinction as the brightest star in the night sky around which has been discovered an orbiting planetary companion. The companion of Pollux is a minimum of 3 Jupiter masses orbiting at roughly the same distance from its parent star as Mars orbits from the Sun. From that distance, Pollux is 16 times brighter in its planet's sky than our Sun is in ours.

At the foot of the twin figure Pollux is Alhena, a star similar to the brighter of the Castor triple system, but it is twice as far away and so fainter to our eyes. Another notable bright star, about a third of the way from Alhena to Pollux is Mekbuda, a supergiant Cepheid variable star, one of the few Cepheids in the night sky easily visible to the naked eye. Mekbuda varies in brightness with a period of just over 10 days. Knowing the period tells us the absolute luminosity and so makes it easy to determine the distance to this star.

Now for the deep sky objects in Gemini. Over on the Western edge of Gemini, about halfway between Mekbuda and Procyon and right on the border with Canis Minor is the Medusa Nebula, a planetary nebula nearly four light years across and 1500 light years away. This in the final evolutionary stage of stars like our own Sun. Nearby you can use Castor and Procyon as guide stars (look about halfway in between) to find another planetary nebula, the Eskimo Nebula. This one is a bit younger and more complex. At low resolution, the center of the nebula looks a bit like a face surrounded by a hooded parka, hence the name.

On the Eastern edge of the constellation, on the border with Auriga is an open cluster, NGC 2266. This is a very old cluster compared to most open clusters, and so it has a variety of different colors of stars as many have evolved off the main sequence to become red giants. Below this, near the foot of the twin figure of Castor is a pair of clusters, M35 and NGC 2158. M35 is a young open cluster with many bright blue stars while NGC 2158 is a much older cluster sort of on the borderline between what we would consider an open vs a globular cluster.

Very close to this pair, just to the West and a bit closer to the feet of Castor, is the supernova remnant IC 433, also known as the Jellyfish Nebula. This is somewhat similar to the Crab Nebula, a shell of hot gas surrounding a rapidly spinning neutron star that is rocketing away from the initial location of its explosion (probably because it was an off-center blast, this sort of thing is common in supernova remnants). The red glow you will see if you follow the first link is due to excited Hydrogen.

When electrons in Hydrogen atoms are excited, they move up to higher energy levels within the atom. As they cascade back down to lower energy levels, they give off a variety of energies (or wavelengths) of light, and one of the most common is Balmer Alpha, which represents the transition from the 3rd to the 2nd energy level and has a wavelength of 656 nm, right in the middle of the red part of the visible spectrum.

Finally, near the feet of the twins, just to the right of Alhena if they are upright, is a neutron star known as Geminga, which I discuss more fully here

Posted by Observer at January 21, 2008 09:58 AM

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