Possibly found by G.D. Cassini in 1678, the brilliant white leader of Gemini was certainly known to be a double star in 1718 when Bradley and Pound noted the position angle by projecting the line between the two stars and referring it to lines drawn to the nearby bright stars. In 1722 they repeated the observation and a significant change had occurred. Sir John Herschel evaluated this and found that the PA had decreased by more than 7°.

Castor is the pair which Sir William Herschel first used to demonstrate his theory that the motion between the two stars is due to a physical attraction.

In the nineteenth century the large numbers of observations of Castor by double-star observers led to a plethora of orbits with periods ranging from 250 to more than 1000 years. As the pair had not then passed periastron, or even defined one end of the apparent ellipse, this was all preliminary. Even today, several orbits give similar residuals and the period would seem to be of order 450 years. A third star of magnitude 11, Castor C, located at 164° and 71'' (2000) and originally thought to be of use for measuring the parallax of AB is actually moving through space with Castor and is part of the system.

In 1896 Belopolsky showed that Castor B was a single-lined spectroscopic binary whilst Curtis at Lick Observatory9 showed that the same applied to the A component. In 1920, Adams and Joy10 announced that Castor C was also a short-period spectroscopic binary but in this case it was double-lined and it also turned out to be an eclipsing system and is now known as YY Gem.

Castor is a relatively nearby system and Hipparcos determined a parallax of 63.27 mas equivalent to a distance of 15.80 parsecs or 51.5 light years. From this and the semimajor axis of the orbit one can estimate the real size of the true orbit of Castor AB. The maximum separation of the stars is about 130 AU, some four times the distance of Pluto from the Sun.

Although the bright components A and B are single-lined spectroscopic systems, it was originally assumed that the stars in each system were similar in spectral type. Recent observations of X-ray emission from all three visible stars in the Castor system have proved that the companions to A and B are late-type stars, a conclusion borne out by the distribution of masses in the system. The total mass of the Castor AaBb quadruple is 5.6 M0. This is made up of Castor Aa (spectral types A1V and K7V and masses 2.6 and 0.7 M0) and Castor Bb (spectral types A1V and M0V and masses 1.7 and 0.6 M0). Star C, which is the eclipsing variable YY Gem, is also extremely active in X-ray and radio wavelengths and it is thought that the surfaces of both components are covered in star spots. Its two components are dwarf stars of spectral class M1. A recent paper by Qian11 speculates that a weak periodic variation in the period of YY Gem may be due to a perturbation by either a brown dwarf or giant planet or it may also be due to magnetic activity, so further research is needed.

Figure 9.2. The apparent orbit of Castor, period = 445 years. In this and subsequent figures the radius of the central circle represents the Dawes limit for a 20-cm aperture.

Figure 9.2. The apparent orbit of Castor, period = 445 years. In this and subsequent figures the radius of the central circle represents the Dawes limit for a 20-cm aperture.

Castor, like Mizar, is also part of a moving group that contains 16 other stars including the first magnitude objects Vega and Fomalhaut.

Two current orbits which give small residuals from recent observations show the pair widening for about 80-100 years before it reaches a maximum distance of about 8'' early in the twenty-second century. It will thus remain an easy and beautiful object in small telescopes for many years to come. Figure 9.2 shows the apparent orbit of AB.

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