Castor 6

In 1998, Castor was confirmed to be the title member of the Castor Moving Group of 16 stars (which includes Vega and Fomalhaut) whose stellar characteristics suggest a young average age of around 200 (+/- 100) million years (D. Barrado y Navascues, 1998; and Anosova and Orlov, 1991). Since its birth, the Castor system has moved away from its original cluster stars and into into the vicinity of Pollux. Although is Pollux actually the brightest star of Constellation Gemini (color photo), the Twins, Johannes Bayer (1572-1625) gave the first-rank Greek letter designation of Alpha to Castor (7:34:35.9+31:53:17.8, ICRS 2000.0) around 1600. Hence, it has been suggested that one of these stars may have changed in luminosity during the past four centuries. Indeed, Castor (the "Horseman" of the two warriors) is not much like its "twin" star Pollux. (See Akira Fujii's color photo of Castor -- blue star at the top left of photo.)

While Pollux is a highly evolved and relatively cool orange-red giant, single star, Castor is actually composed of three sets of tight, spectroscopic binary stars with a combined luminosity of 52.4 times that of Sol's. The Castor system has as many as four bluish-white, main sequence stars and two known, fainter red dwarf companions. The outer pair (YY Gem) was separated from the inner quadruple of bright stars by over 1,100 AUs (73" at a HIPPARCOS distance estimate of 51.6), moving in an orbit that may take some 10,000 years to complete; with a 40 percent probability of being quasi-elliptical and a 60 percent probability of being hyperbolic (Anosova and Orlov, 1991). According to Robert Burnham, Jr. (1931-93), the star's multiplicity was probably first resolved in 1678 by Giovanni Domenico Cassini (1625-1712) as the first gravitationally bound objects observed beyond the Solar System. (See an animation of the planetary and potentially habitable zone orbits of the Castor AabBab system and of the Cab system, each with a table of basic orbital and physical characteristics.)

ASCA SIS, ISAS


Castor AabBab and Cab have been
observed to flare (larger x-ray images).

Broadband and frequent x-ray flares and quiescent x-ray flux in both the inner bright stars Aab and Bab pairs as well as the outer pair of flare stars Cab, or YY Geminorum AB, have been repeatedly observed (Güdel et al, 2001; Gotthelf et al, 1994; Schmitt et al, 1994; and Pallavicini et al, 1990). The flares are believed to evolve in minutes to hours from magnetic instabilities in hot outer stellar atmospheres. While this was theoretically and observational anticipated for young red dwarfs, it was not expected for A-type dwarfs. Hence, the recent detection presence of flares from the Aab and Bab systems has led to debates over the actual spectral type of their companions and the magnetic stability of young A dwarfs since ROSAT had unexpectedly observed X-ray emission from early A-type dwarf stars with post-T Tauri companions. The Castor system (especially YY Gem) was also found to be relatively bright in radio, microwave, and some infrared wavelengths detected by IRAS (Freidemann et al, 1996). Over the coming decades, the two optically brighter binary pairs Castor Aab and Bab will move further apart in angular distance which will make it easier to redetermine their spectral types and masses.

Castor Aa

Castor Aa is a main sequence dwarf bluish-white star of spectral and luminosity type A1 V. Given its shared spectral type with well known neighbor Sirius A, Star Aa probably has a similar mass around 2.15 Solar. It may also have around 2.3 times Sol's diameter and 17 to 34 times its visual luminosity or more (depending on whether its unseen companion is an A-type dwarf). All the stars in this system may be less enriched than Sol (Güdel et al, 2001) -- despite being more enriched than Vega -- with elements heavier than hydrogen ("metallicity") based on its abundance of iron (Cayrel de Strobel et al, 1991, pp. 286 and 303). The star has a single-lined, spectroscopic stellar companion Ab. Useful catalogue numbers and designations for the star include: Alp or Alf Gem A, 66 Gem A, HR 2891*, Gl 278 A, Hip 36850, HD 60179, BD+32 1581, LTT 12038, WDS 07346+3153 A, ADS 6175 A, and Struve 1110 A.

Stars Aa and Ab have a combined luminosity of over 34 times that of Sol's. They are separated on average by only 0.022 AUs (of a semi-major axis). Their highly eccentric orbit (e= 0.499) takes only 9.21 days to complete (Joel Stebbins, 1914, pp. 465-466; and Heber D. Curtis, 1906). Currently, the circumbinary orbit of an Earth-like planet (with liquid water) around Stars Aab may be centered just beyond 6.0 AU -- just outside the orbital distance of Jupiter in the Solar System. Astronomers would find it very difficult to detect an Earth-type planet in the water zone of any star in this system using present methods.

Stars Aab are separated from its nearest binary companions Bab by around 107 AUs (6.805" of a semi-major axis at a HIPPARCOS distance estimate of 51.6 ly) in an orbit that takes 467 years to complete. The two binary pairs move in an eccentric orbit (e= 0.343) that is inclined by 114.5° from the perspective of an observer on Earth (Wulff Dieter Heintz, 1988). (See an animation of the planetary and potentially habitable zone orbits of the Castor Aab system, with a table of basic orbital and physical characteristics.)

NASA Observatorium



Castor Aa and Ba have
colors ranging from those
of Sirius A to Altair.



See a discussion of
the "main sequence"
as part of stellar
evolution and death.

Castor Ab

Castor Ab is a main sequence dwarf of uncertain spectral type, probably ranging from M5 to brighter ("earlier") type of probable luminosity type V (Golub et al, 1983). Its mass probably ranges from 40 to 60 percent or more of Sol's, with a diameter and luminosity less than Sol's (if not an A type dwarf. Based on EXOSAT observations, this object is believed to be a relatively bright, quiescent x-ray source that emits broadband flares and frequent x-ray flares (Güdel et al, 2001; Gotthelf et al, 1994; Schmitt et al, 1994; and Pallavicini et al, 1990). Such unexpected, X-ray emission has been observed with early type and post-T Tauri A dwarf companions with ROSAT. Useful catalogue numbers and designations for the star include: Struve 1110 C and ADS 6175 C.

Castor Ba

Castor Ba is a bluish-white main sequence dwarf star of spectral and luminosity type A2-5 Vm. Other than the metallic lines in its spectra, the star appears similar to Fomalhaut. Star Ba has an estimated mass around 1.7 times Sol's, 1.6 times its diameter, and over 14 times its visual luminosity. The star has a single-lined, spectroscopic stellar companion Bb, and frequent x-ray flaring from the binary system has been observed (Güdel et al, 2001). Useful catalogue numbers and designations for the star include: Alp or Alf Gem B, 66 Gem B, HR 2890*, Gl 278 B, HD 60178, SAO 60198, FK5 287, WDS 07346+3153 B, ADS 6175 B, and Struve 1110 B.

Stars Ba and Bb have a combined visual luminosity of more than 14 times that of Sol's. They are separated on average by only 0.03 AUs (of a semi-major axis). Their highly circular orbit (e= 0.01) takes only 2.93 days to complete (Joel Stebbins, 1914, pp. 465-466; and Heber D. Curtis, 1906), and suggests synchronous rotation (Schmitt et al, 1994, page 850). Currently, the circumbinary orbit of an Earth-like planet (with liquid water) around Stars Aab may be centered around 4.0 AU -- on the outer edge of the Main Asteroid Belt in the Solar System. (See an animation of the planetary and potentially habitable zone orbits of the Castor Bab system, with a table of basic orbital and physical characteristics.)

Castor Bb

Castor Bb is a main sequence dwarf star of uncertain spectral type, probably ranging from M2 to a brighter type ("earlier") -- possibly with metallic lines as well -- and luminosity type V (Golub et al, 1983). Its mass probably ranges from 40 to 60 percent or more of Sol's, with a diameter and luminosity less than Sol's. Observations with the XMM-Newton detected flares coming from the Bab system, possibly from this unseen companion Bb.

NASA -- larger image


Castor Cab are dim red dwarf stars,
like Gliese 623 A (M2.5V) and B
(M5.8Ve) at lower right.

Castor Ca (YY Geminorum Aa)

Castor Ca is a red main sequence dwarf star of spectral and luminosity type M0.5 Ve. It has an estimated mass around 62 percent of Sol's, 76 percent of its diameter, (Schmitt et al, 1994; from Joy and Sanford, 1926) and 2.6 percent of its visual luminosity. The star has an eclipsing, spectroscopic stellar companion Cb. Both are considered to be flare stars with frequent emission of x-ray flares and so have been given the variable designation YY Geminorum. Useful catalogue numbers and designations for the star include: Alp or Alf Gem C, 66 Gem C, YY Gem, Gl 278 C, and BD+32 1582.

Stars Ca and Cb have a combined visual luminosity that is only 5.1 percent of Sol's. They are separated on average by only 0.018 AUs (of a semi-major axis). Their highly circular orbit (e~ 0) takes only about 19.5 hours (0.814 days) to complete (Schmitt et al, 1994; Struve and Ebbighausen, 1959; and Joy and Sanford, 1926) and lies very nearly in the line of sight of an observer on Earth and so eclipse each other as they revolve (like CM Draconis Aab). Due in part to their close proximity, tidally enforced rapid rotation is expected to induce high chromospheric activity in both stars. The light curve for the system has recently been observed to exhibit three deep eclipses, indicating that the coronae of both stars are similarly active and relatively compact (Güdel et al, 2001); Currently, the circumbinary orbit of an Earth-like planet (with liquid water) around Stars Cab may be centered around 0.23 AU -- well within the orbital distance of Mercury in the Solar System. (See an animation of the planetary and potentially habitable zone orbits of the Castor Cab system, with a table of basic orbital and physical characteristics.)

Arnold O. Benz, Institute of Astronomy, ETH Zurich
-- high resolution and jumbo images (Benz et al, 1998).

Castor Cab may both be M-type flare stars, like
young UV Ceti (Luyten 726-8 B) shown flaring at left.
UV Ceti is an extreme example of a flare star that can
boost its brightness by five times in less than a minute,
then fall somewhat slower back down to normal
luminosity within two or three minutes before flaring
suddenly again after several hours.

Castor Cb (YY Geminorum Ab)

Castor Cb is a red main sequence dwarf star of spectral and luminosity type M0.5 Ve. It has an estimated mass around 57 percent of Sol's, 68 percent of its diameter, (Schmitt et al, 1994; from Joy and Sanford, 1926) and 2.6 percent of its visual luminosity.

Life Around a Flare Star

Many dim, red (M) dwarf stars exhibit unusually violent flare activity for their size and brightness. These flare stars are actually common because red dwarfs make up more than half of all stars in our galaxy. Although flares do occur on our Sun every so often, the amount of energy released in a solar flare is small compared to the total amount of energy that Sol produces. However, a flare the size of a solar flare occurring on a red dwarf star (Castor Cab) that is more than ten thousand times dimmer than our Sun would emit about as much or more light as the red dwarf does normally.

Flare stars erupt sporadically, with successive flares spaced anywhere from an hour to a few days apart. A flare only takes a few minutes to reach peak brightness, and more than one flare can occur at a time. Moreover, in addition to bursts of light and radio waves, flares on dim red dwarfs may emit up to 10,000 times as many X-rays as a comparably-sized solar flare on our own Sun, and so flares would be lethal to Earth-type life on planets near the flare star. Hence, Earth-type life around flare stars may be unlikely because their planets must be located very close to dim red dwarfs to be warmed sufficiently by star light to have liquid water (around 0.23 AU for Castor Cab), which makes flares even more dangerous around such stars. In any case, the light emitted by red dwarfs may be too red in color for Earth-type plant life to perform photosynthesis efficiently.