Zeta Herculis (Rutilicus / Ruticulus) 3?
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Zeta Herculis A is a yellowish star that is brighter,
more massive, and further evolved than our Sun, Sol,
while its stellar companion B is dimmer, cooler, and
more orange in color. (See Sloan Digital Sky Survey
field images of the Zeta Herculis system from
This star system is located about 35.2 light-years (ly) away from our Sun, Sol. It lies in the west central part (16:41:17.2+31:36:9.8, ICRS 2000.0) of Constellation Hercules, the legendary hero of ancient Greece -- west of Epsilon Herculis, south of Eta Herculis, northeast of Kornephoros (Beta Herculis). The primary star has also been called "Rutilicus" or "Ruticulus," whose origin and meaning appears to be in dispute among historical guides (Richard Hinckley Allen, 1889: pp. 243-244), but Rutilicus or Ruticulus is also an alternate name for Beta Herculis (which is more commonly known as Kornephoros).
The visual duplicity of the system was discovered by Sir William (Friedrich Wilhelm) Herschel (1738-1822) as early as July 18, 1782 (Thomas J.J. See, 1895). Over the next two centuries, however, astronomers measured unexpected perturbations in the orbits of the binary system indicative of an unseen body now believed to be orbiting Star A only -- more below. (See an animation of the binary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.) Once designated the title member of the Zeta Herculis stellar moving group, some astronomers have proposed that the system be removed from that group of stars (i.e., because it is too metal rich) and that its associated group of stars be re-designated as the Zeta Reticuli "kinematic" or stellar moving group (del Peloso et al, 2000; and Olin J. Eggen, 1971 and 1958).
Zeta Herculis A(a?)
Star A is a yellowish subgiant star of spectral and luminosity type F9 to G0 IV, but it has been classed as orange as G2. This star has around as much as 1.45 (+/- 0.01) times Sol's mass (Morel et al, 2001; Chmielewski et al, 1995; Peter van de Kamp, 1938, ; and Y.C. Chiang, 1928), 1.7 to 2.6 times Sol's diameter (Fracassini et al, 1994; and Johnson and Wright, 1983: page 687), and 6.6 times its visual luminosity and perhaps around 8.1 times its bolometric luminosity (Morel et al, 2001; and Chmielewski et al, 1995). The star is probably closer to 1.1 (rather than 1.8) times as enriched as Sol with elements heavier than hydrogen ("metallicity") based on its abundance of iron (Morel et al, 2001); and Cayrel de Strobel et al, 1991, page 300; and Helfer el al, 1963). According to Professor James Kaler's summary of Zeta Herculis, massive Star A probably has halted core hydrogen fusion, although it may only be around 3.4 to 4.4 billion years old (Morel et al, 2001; and Chmielewski et al, 1995). Given its high estimated mass, Star A may have spent much of its main sequence life as a late A- to early F-type dwarf before evolving into a yellowish subgiant. It has been designated NSV 7915 as a New Suspected Variable star. Useful star catalogue numbers for the star include: Zet Her A, 40 Her, HR 6212, Gl 635 A, HD 150680, HIP 81693, BD+31 2884, SAO 65485, LHS 3234, LTT 14952, LFT 1299, Struve 2084 A, and ADS 10157 A.
According to the Sixth Catalog of Orbits of Visual Binary Stars, Star A and its visual stellar companion B have an average separation of 14.4 AUs (a = 1.33") in a highly elliptical orbit (e~ 0.46) with a period of 34.45 years that had the widest separation in 1956 and 1990 and is inclined by 131 degrees from the perspective of Earth (Staffan Söderhjelm, 1999; Wulff-Dieter Heintz, 1994; Sarah L. Lippincott, 1981; and Paul Baize, 1949). In addition, based on observations of orbital pecularities made over two centuries, Star A has been long suspected of having a unseen companion (W. Doberck, 1897: "due to a most curious and persistent constancy of error"), which may be as small as a brown dwarf mass of as little as 0.05 Solar-mass (Morel et al, 2001; Scarfe et al, 1983; D.W. McCarthy, Jr., 1983; Paul Baize, 1976; Louis Berman, 1941; George C. Comstock, 1917; T. Lewis, 1900; and W. Doberck, 1897).
The orbital distance from Zeta Herculis A where an Earth-type planet currently would be "comfortable" with liquid water is centered near 2.85 AU -- in the Main Asteroid Belt between the orbital distances of Mars and Jupiter in the Solar System. At that distance from the star, such a planet would have an orbital period close to 4 Earth years. As the separation between Star A and B varies between 7.8 and 21 AUs, however, any planets in the current "habitable zone" with surface water may have had its orbit disrupted over the past three to four billion years since the system formed. In any case, life on an Earth-like planet, moreover, would likely have developed closer to the star before it evolved and heated up into a subgiant, thereby shifting its habitable zone outwards away from Star A.
Zeta Herculis B
Star B is a orange-red main sequence dwarf star of spectral and luminosity type G7-K0 V (Morel et al, 2001; and Struve and Radcliffe, 1954). This star has around 98 +/- 2 percent of Sol's mass (Morel et al, 2001), a diameter around 86 to 88 percent of Sol's (Fracassini et al, 1994; and Johnson and Wright, 1983: page 687), and 62 percent of its visual luminosity and 77 percent of its bolometric luminosity (Morel et al, 2001; and Chmielewski et al, 1995). Useful star catalogue numbers for the star include: Zet Her B, Gl 635 B, BD+31 2884 B, Struve 2084 B, and ADS 10157 B.
The orbital distance from Zeta Herculis B where an Earth-type planet would be "comfortable" with liquid water is centered around 0.88 AU -- between the orbital distances of Venus and Earth in the Solar System. It possible that such an orbit would be stable even with the closest approach of Star A at 7.8 AUs. At that distance from the star, such a planet would have an orbital period close to 303 days. (See an animation of the binary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)
The following star systems are located within 10 light-years, plus more bright stars within 10 to 20 ly, of Zeta Herculis.
|Star System||Spectra &|
|LP 275-68||M V||3.4|
|BD+33 2777||K7 V||3.5|
|BD+25 3173||M2 V||4.5|
|G 169-29||M V||6.0|
|G 180-60||M V||6.3|
|G 180-11||M V||6.5|
|Wolf 654||M V||7.2|
|Ross 868 AB||M4 Ve |
|L 1346-53||M3 V||8.8|
|AC+41 726-154||M3-4 V||9.7|
|* plus bright stars *||. . .|
|Mu Herculis 4?||G5 IV |
|Gamma Serpentis||F6 V||12|
|72 Herculis A?||G0 V||13|
|BD+39 2947 AB||G8 V |
|44 (i) Boötis ABC|| F5-G1 Vn |
|Lambda Serpentis||G0 V||18|
|BK+00 2334 (?)||G2 V||18|
|Xi Bootis AB||G8 Ve |
|Chi Herculis||F8-9 V||20|
|Rasalhague AB||A5 III |
Up-to-date technical summaries on these stars can be found at: the Astronomiches Rechen-Institut at Heidelberg's ARICNS for Star A and Star B; the NASA Stars and Exoplanet Database; and SIMBAD. Additional information may be available at Roger Wilcox's Internet Stellar Database, and papers may eventually become available at the SAO/NASA ADS.
In Greek mythology, Hercules was the son of the God Zeus and the Alkmene, the wife of Amphitryon who was fooled by Zeus into believing that he was Amphitryon. Hera, the wife of Zeus, somehow arranged that the first born son of Alkmene became Eurystheus, who under Hera's influence eventually gave his half brother twelve tasks to complete or perish. For more information about the stars and objects in this constellation and an illustration, go to Christine Kronberg's Hercules. For another illustration, see David Haworth's Hercules.
For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.
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