Xi Scorpii 5

According to Robert Burnham, Jr. (1931-93), the binary nature of Star A was discovered in 1782 by Sir William Friedrich Wilhelm Herschel (1738-1822, portrait), who subsequently discovered the planet Uranus in 1781 -- which led to his appointment in 1782 as private astronomer to the King of England. The relative position of the AB pair to a third stellar companion "C" was first measured in 1825 by Friedrich Georg Wilhelm Struve (1793-1864), who also determined that the nearby wide binary (Stars D and E, or Struve 1999 AB) share a common proper motion with Stars ABC; Struve became director of Russia's Dorpat Observatory in 1817 and founded and directed the Pulkovo Observatory in 1837, surveyed 120,000 stars from 1819 to 1827, published an extensive monograph of Halley's Comet based on observations in 1835 and his findings on 2,640 double stars in 1837, and measured the parallax of Vega from 1835 to 1838. (See an animation of the orbits of Stars A and B and their potentially habitable zones, with a table of basic orbital and physical characteristics.)

Xi Scorpii A

Star "A" is a yellowish dwarf star of spectral and luminosity type F5-6 V-IV that appears to be evolving off the main sequence. It probably has more than one Solar mass, 1.5 times Sol's diameter (Pasinetti-Fracassini et al, 2001; and Johnson and Wright, 1983, page 685), and about 8.3 times its luminosity. Given its high luminosity, the star appears to be highly evolved as a near-subgiant star with a lot of helium ash at its core. Useful star catalogue numbers for the star include: Xi/Ksi Sco A, HR 5978, Wo 9540 A, Hip 78727, HD 144070, BD-10 4237 A, SAO 159665, WDS 16044-1122, ADS 9909 A, and Struve 1998 A.

Xi Scorpii A has a relatively close companion "B" of similar spectral and luminosity type, which also appears to be evolving off the main sequence. Their orbit takes about 45.648 years to complete and is inclined by i=42.3 +/- 1.1° from the perspective of an observer on Earth. Although the binary has an average separation of 19.0 AUs (semi-major axis a = 0.6712 +/- 0.0027"), the extremely high eccentricity (e= 0.7126 +/- 0.0079) of their orbit brings them as close together as 5.5 AUs and as far apart as 32.6 AUs (Mason et al, 1999; Robert Sutton Harrington, 1987; and Karl Schwarzschild, 1896). Because of its brightness, the orbital distance from Star A of an Earth-type with liquid water on its surface would likely be centered near 2.9 AUs. Unfortunately, the star's eccentric orbit with close binary companion B would likely disrupt the stability of an Earth-type planet in the liquid water zone.

USNO (updated to 2001),
Mason et al, 1999




Larger chart.




Star A has a relatively
close companion B in
a highly elliptical orbit
(more at USNO and
Mason et al, 1999).

The bright AB pair has a third companion "C" in a relatively wide orbit. However, Star C's orbit around the AB pair is probably elliptical and has a strong probability of being unstable at an observed separation of 210 AUs -- based on an angular distance of 7.4" (Anosova and Orlov, 1991). In turn, Stars A, B, and C share the same common proper motion with another binary pair (Stars "D" and "E") in a relatively wide orbit. (See an animation of the orbits of Stars A and B and their potentially habitable zones, with a table of basic orbital and physical characteristics.)

Xi Scorpii B

If this companion star is also a yellowish near-subgiant of spectral and luminosity type F5-6 V-IV like Star A, then it should be more massive than Sol with a larger diameter. According to one source, it has around 6.3 times its luminosity. Because of its brightness, the orbital distance from Star B of an Earth-type with liquid water on its surface would likely be centered near 2.5 AUs. Useful star catalogue numbers for the star include: Xi/Ksi Sco B, HR 5977, Wo 9540 B, HD 144069, BD-10 4237 B, ADS 9909 B, and Struve 1998 B.

Xi Scorpii C

This star is a yellow-orange main sequence dwarf of spectral and luminosity type G8 V. It may have a mass similar to Sol's, around 90 percent of its diameter (Pasinetti-Fracassini et al, 2001; and Johnson and Wright, 1983, page 685), and 80 percent of its luminosity. The orbit of an Earth-like planet around Star C in the liquid water zone would have to be centered around 0.80 AU -- between the orbital distances of Venus and Earth in the Solar System -- with an orbital period under one Earth year. Useful catalogue numbers for this star include: BD-10 4237 C SAO 159666, and Wo 9540 C.

Xi Scorpii D / Struve 1999 A

Star D appears to be a yellow-orange main sequence dwarf of spectral and luminosity type G8 V+. It probably has a mass similar to Sol's, 86 percent of its diameter (Pasinetti-Fracassini et al, 2001; and Johnson and Wright, 1983, page 685), and about 71 percent of its luminosity. The orbit of an Earth-like planet around Star D in the liquid water zone would have to be centered around 0.84 AU -- between the orbital distances of Venus and Earth around Sol -- with a probable orbital period of less than an Earth year. According to Robert Burnham, Jr. (1931-93), the star has a stellar companion "E" located around 329 AUs away in 2000 (11.6", according to the Astronomical League). The DE pair is separated from the ABC triple by around 8,000 AUs -- sep(DE)= 282.66" or 4.7' -- (van der Kamp and Harrington, 1964). Useful star catalogue numbers for the star include: Xi/Ksi Sco D, HIP 78738, HD 144087, BD-11 4057, SAO 159668, Wo/GJ 9541 A, ADS 9910 A, UGP 391, and STF 1999 A.

© Torben Krogh & Mogens Winther,




(Amtsgymnasiet and EUC Syd Gallery,
student photo used with permission)




Xi Scorpii E (or Struve 1999 B) is
an orange-red dwarf star, like Epsilon
Eridani at left center of meteor.

Xi Scorpii E / Struve 1999 B

Star E appears to be a orange-red main sequence dwarf of spectral and luminosity type K2 V. It probably has a mass less than Sol's, 81 percent of its diameter (Pasinetti-Fracassini et al, 2001; and Johnson and Wright, 1983, page 685), and about 43 percent of its luminosity. The orbit of an Earth-like planet around Star D in the liquid water zone would have to be centered around 0.66 AU -- a little closer than Venus's orbital distance around Sol -- with a probable orbital period of less than an Earth year. Useful star catalogue numbers for the star include: Xi/Ksi Sco E, HIP 78739, HD 144088, BD-11 4058, Wo/GJ 9541 B, SAO 159670, ADS 9910 B, UGP 392, and STF 1999 B.