Stars in Gemini
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Stars in Gemini |
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Deep Sky Object Chart
| Stars in Gemini
| M35
| Eskimo
Alpha Geminorum (Castor) is, at magnitude 1.59 the 23rd brightest star in the sky, and is properly referred to as being of the second magnitude as astronomers consider a first magnitude to be from 0.5 to +1.5 - Castor barely falls short. Castor is also not the brightest star in Gemini; that honor goes to neighboring Pollux. For the amateur astronomer, Castor is a close but noteworthy double star, composed of stars of magnitudes 2.0 and 2.8, separated by just two arc seconds. Moderate telescopes will resolve this pair, provided the air is steady and the instrument's optics are in good working order; in fact, the Castor pair is a popular standard test among amateurs wishing to evaluate their equipment or the atmospheric conditions. The true nature of Castor is remarkable; the two stars we see are revealed by spectroscopes to be each a closer pair yet, making this a quadruple star system. Further, a ninth magnitude star in Castor's telescopic field (72.5 arc seconds away) is also known to be a member of the system - and this again has proved to be an invisibly close double! Castor is therefore a sextuple star, an unusual but not unheard of occurrence. This stellar family is located some 51 light years from us, much closer than many bright stars we see. The actual brightness of the stars is 12 and 12 (compared to our sun) for one of the brighter pairs, 6 and 6 for the fainter - all these are pure white in color, hotter than our sun. Together, the faint red dwarves only manage to put out five percent the light of our sun. The separations involved may seem tiny in your scope, but the close pair of white stars we can see are three times farther apart than our sun and Pluto! The red dwarf pair is a staggering 30 times farther out than that. Even inside the invisibly close pairs, the separations are well over a million miles.
Gamma Geminorum (Alhena) is the third brightest star is Gemini, just a 0.3 magnitudes fainter than Alpha. Alhena marks the foot of the twin whose head is marked by Pollux, and marks the southwestern border of the constellation, near Orion's club. At magnitude 1.93, Alhena is only slightly inferior in brightness to the big dipper stars, and definitely superior to better known stars such as Algol. One reason that Alhena attracts little attention is the fact that the star is not a double star - it is a single white star, spectral type A0-IV, putting out 160 times the light of our sun across a distance of some 105 light years. Another factor in the relative obscurity of this bright star is its placement in Gemini, the twins; with Castor and Pollux each marking a twin, there is a little psychological expectation for a third star. If this were the Triplets constellation, perhaps Alhena would be more famous! Delta Geminorum (Wasat) is, with Zeta, a fourth magnitude marker of the body of Pollux, the more southerly of the heavenly twins. Wasat is worth a look because of the eight magnitude companion about six arc seconds away; these are a true orbital pair, with the smaller star taking some 1200 years to complete a circuit. The true separation of the stars is estimated at 95 AU, or three times the diameter of our own solar system. Delta's two stars are very dissimilar: a yellow-white subgiant about 8 times brighter than our sun, with the orangish companion about 8 times less brilliant than ours. Delta is a fairly near neighbor to the sun, and also to Pollux and Castor, residing about 53 light years from Earth. Near in the sky, but a thousand light years in the background, stars Epsilon and Zeta Geminorum appear with the twins by chance alignment. Wasat has also been the site of historical discovery - although the star was merely in the background, it was in this part of the sky that the planet Pluto was discovered in 1930. Spying the new solar planet, one can only wonder if astronomer Clyde Tombaugh said, "Wasat?".. Delta is a binary star, with an eight magnitude companion 6.3 arc seconds away - an easy target for moderate telescopes. Epsilon Geminorum (Mebsuta) is the only bright star in the "body" of the northern twin, whose head is marked by that twin's name star, Castor. A yellow giant star of the third magnitude which is actually much brighter than Castor or Pollux, Mebsuta resides at the much larger distance of 1,100 light years. Remote as it is, you can imagine that Mebsuta must be quite a star to be one of Gemini's bright stars despite the handicap, and it is - this sun puts out around 6,000 times the light ours does. Epsilon is also a double star, with a very wide separation of 110 arc seconds; the companion is a fairly faint magnitude 9. This pairing seems to be accidental only, with the stars lying at different distances from us. Zeta Geminorum (Mekbuta) is slightly fainter than it's constellation neighbor Wasat - but the amount varies, because Zeta is one of the brightest examples of the famous and important "Cepheid" variable stars. These suns swell and shrink in a regular pattern of pulsation, and astronomers have learned that the length of the cycle is directly related to the actual brightness of the star; thus, we need merely watch the light pulses from one of these stars, deduce the star's true brightness from this - and based on how faint the star seems to us, we can calculate the distance of the star. Cepheid stars are a vital measuring standard for the universe, because direct measurements of star distances are only possible for closer stars (within 1500 light years or so) - fortunately, several Cepheids are within that range, allowing us to learn that wonderful trick they let us perform on distances that span millions of light years. The Hubble Space Telescope's primary mission, in fact, was to spot Cepheids in distant galaxies and thus measure the size (and therefore age) of the universe. Zeta Geminorum is thus worth more than a casual glance! Zeta is fairly close to us, about 1500 light years, and at maximum puts out 5700 times the light of our sun. The star is yellowish, and in a period of 10.15172 days swings between magnitude 4.4 and 5.2; Delta, by comparison, is magnitude 3.51. An eighth magnitude star in the telescopic field with Zeta is merely aligned by accident; the stars are in fact nowhere near one another in distance. Eta Geminorum (Propus) is a magnitude 3.33 red star, one of three marking the feet of Castor. It would be easy to confuse Propus with it's close neighbor Mu Geminorum, also a red star, just two degrees to the east. Mu is magnitude 2.86, slightly brighter, but perhaps the best test (if you have a good telescope) will be locating the close and elusive companion star to Propus, as Mu travels alone. The search will not be easy for smaller instruments; Propus B is just 1.5 arc seconds away - it is fairly bright, however, shining at magnitude 6.5 with a yellow light. The bright primary star is a red giant, of spectral type M3, about 160 times brighter than our sun. Like many red giants, Propus is not quite steady in it's light output: the magnitude varies by about a magnitude, in a period that averages 233 days. The distance is estimated to be 351 light years. An interesting note is that Propus A itself is not a single star, but in fact two red giants, orbiting one another every 8 years. We can't see this closer pairing, but we can certainly imagine the scene in a star system with two swollen red giant suns circled by a third yellow giant! Our own sun seems tame in comparison - but tame is better for stable planetary orbits, and uniform solar heating - let's not complain. Xi Geminorum (Alzirr) is a fairly bright star, of magnitude 3.38, and is noteworthy primarily because you may notice it four degrees southwest of second magnitude Alhena. 57 light years from us, this star is a white star 20 times brighter than our own. Xi is closely attended by the unrelated 4th magnitude star 30 Geminorum a little less than a degree to the northwest - the two make a pretty pair in binoculars. 30 Geminorum is a yellowish star, and is located far in the background, 274 light years from Earth.
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