Xi Scorpius 5
|Home | Stars | Orbits | Habitability | Life ||
Xi Scorpii is a multiple system of five components located about 92.6 light-years (ly) from Sol. The system lies in the northern part (16:04:22.2-11:22:22.6, ICRS 2000.0) of Constellation Scorpius, the Scorpion. It can be found: southwest of Phi Scorpii Nu Ophiuchi, Yed Posterior (Epsilon Ophiuchi), and Yed Prior (Delta Ophiuchi); northeast of 48, Theta, and Gamma Librae; north of Graffias or Acrab (Beta1,2 Scorpii), and southeast of Mu Serpentis.
According to Robert Burnham, Jr. (1931-93), this multiple star system 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 became director of Russia's Dorpat Observatory in 1817 and founded and directed the Pulkovo Observatory in 1837. (Struve also 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 star groups ABC and DE and their potentially habitable zones, with a table of basic orbital and physical characteristics.)
This star is a yellow-orange main sequence dwarf star of spectral and luminosity type F6 V-IV, with about 10 percent of Sol's mass (Wulff Dieter Heintz, 1996), (Pasinetti-Fracassini et al, 2001; and Wesselink et al, 1972), , and about 8.13 times its luminosity. It appears to be almost 1. times as enriched than Sol in elements heavier than hydrogen ("metals") based on its abundance of iron (B.J. Taylor, 2003; and (Cayrel de Strobel et al, 1991, page ). Useful star catalogue numbers for the star include: Xi/Ksi Sco A, Wo 9540 A, Hip 78727, HD 144069 J, BD-10 4237, SAO 159665, ADS 9909 A, and Struve 1998 A.
Spectroscopic and astrometric analyses reveal a companion Ab. First detected as a periodic orbital perturbation in 1905 by N. E. Norlund, this companion may be separated "on average" from Star Aa by about 0.45 AU (of a semi-major axis of 0.054" at 27.3 ly) with a period of 1.83 years in a highly elliptical orbit (e=0.61) that swings between 0.2 and 0.7 AUs, at an inclination from the perspective of an observer on Earth of 91° (Heintz, 1996). However, recent radial velocity analysis suggests a slightly lower orbital eccentricity of 0.53 (Griffin, 1998). The orbit of an Earth-like planet (with liquid water) around Xi Scorpii A would have to be centered around 2.65 AUs -- within the Main Asteroid Belt in the Solar System -- with an orbital period between Earth years. Given that the widest separation of the stars in the binary pair Aab is around 0.7 AU, however, such a planetary orbit appears unlikely to be stable. Furthermore, the noncoplanarity of the orbits of star pairs Aab and Babc? reduces the likelihood of stable planetary orbits at increasing distances from each star or binary pair Alan Hale, 1994.
On the other hand, the wide binary pairs Aab and Bab are separated by an "average" distance of about 21.2 AUs (of a semi-major axis of 2.533" at 27.3 ly) in an elliptical orbit (e= 0.412) of 59.9 years, so that the two star pairs get as close as 12.5 AUs and as far away as 39.9 AUs (Wulff Dieter Heintz, 1996; revising earlier earlier estimates, including Mason et al, 1995). The orbital inclination of the two pairs from Earth fluctuates from 122.1° from 1935 to 1995 to 121.2 from 1995-2034. (See an animation of the orbits of star groups Aab and Babc? and their potentially habitable zones, with a table of basic orbital and physical characteristics.)
NASA -- larger image
Xi Ursae Majoris Ab may be a dim red dwarf star, like
Gliese 623 A (M2.5V) and B (M5.8Ve) at lower right.
Xi Ursae Majoris Ab
This companion object appears to have about four-tenths of Sol's mass according to Heintz (1996), which would correspond with a star around spectral type M3 (Mason et al, 1995). It may have around one percent of Sol's luminosity. Despite its faintness, its radius may be half as great as star Aa (Griffin, 1998, page 20).
This star is a yellow-orange main sequence dwarf star whose spectral and luminosity type has been estimated in the range from G0-5 Ve. The star may have about 90 percent of Sol's mass, 91 times of its diameter (Johnson and Wright, 1983, page 671), and 67 percent of its luminosity. It is about 300 °K cooler than Star Aa. Star Ba may be only 76 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Cayrel de Strobel et al, 1991, page 291). Moreover, the star seems to be depleted in lithium because it has maintained comparably dynamo-induced, chromospheric activity resulting from a relatively fast, synchronous (tidally-locked) rotation with its companion and so it may have lost about 10 percent more of its matter than would a single star of its mass and age (Strobel et al, 1994). Star Ba may have a brown dwarf companion (see Bb below) in a "torch orbit," with an average separation of 0.06 AU in a highly circular orbit (e=0.00) whose period is completed within four days.
The orbit of an Earth-like planet around the tight binary system that star Ba forms with its brown dwarf companion in the liquid water zone would have to be centered around 1.1 AU -- a little farther than Earth's orbital distance around Sol -- with an orbital period exceeding one Earth year. However, if the existence of a relatively close, second companion (see Star Bc below) around Bab -- with an orbital period of 2.2 to 2.9 years or less -- is confirmed, then a planetary orbit in Star Ba's water zone may not be stable over the long run. Useful catalogue numbers for this star include: HR 4374, Gl 423 B, HD 98230, and LHS 2391.
© John Whatmough (Artwork from Extrasolar Visions, used with permission)
Xi Ursae Majoris Bb (HD 98230 b) appears to be a brown dwarf -- like
Gliese 229 b with its own dark satellite, as imagined by Whatmough.
HD 98230 b or Xi Ursae Majoris Bb?
This probable brown dwarf companion was discovered using radial velocity measurements in 1996, possibly confirming Louis Berman's discovery of a spectroscopic companion to Star B in 1931. It has at least 37 times the mass of Jupiter and a circular orbit (e~ 0) with a period of 3.98 days (see Extrasolar Planets Encyclopaedia). A recent analysis of radial velocities, however, discusses the possibility of a Bb companion as a orange-red (late K-type) dwarf star based on suspected mass ratios among the binary pairs without any mention of the 1996 brown dwarf finding (Griffin, 1998, pp. 293-294). Past calculations of orbital elements and system mass ratios based on astrometry -- and other visual observations -- and the spectral type of Star Ba (G0-5) indicate that HD 98230 b is not massive enough to fully account for subsystem B (e.g., Wulff Dieter Heintz, 1996, page 411), and suggest the existence of a stellar companion (i.e., Bc).
© Torben Krogh & Mogens Winther,
(Amtsgymnasiet and EUC Syd Gallery,
student photo used with permission)
If it exists, Xi Ursae Majoris Bc may be an
orange-red dwarf star, like Epsilon Eridani
at left center of meteor.
Xi Ursae Majoris Bc?
Analysis of only one of 27 speckle interferometric observations (obtained with Kitt Peak and Canada-France-Hawaii telescopes) uncovered a fifth visual component to this multiple system (Mason et al, 1995). This object, however, apparently "never [has] had any effect whatsover upon the astrometric and radial-velocity behaviour of the observable components whose existence is reliably established" Griffin, 1998), pages 275-276). The star may be an orange-red, main sequence dwarf of spectral and luminosity type K2-3 V and have an orbital period with Star Ba or 2.2 to 2.9 years. Subsequently, Heintz (1996, page 411) suggested that such a companion to Star Ba would have to have a mass of at least half Sol's to reach detectable brightness, and that, among other orbital requirements, Bc's period would have to be less than an Earth year in order to account for the absence of effects on Ba's radial velocities and positions.
Brown Dwarfs or Planets?
When brown dwarfs were just a theoretical concern, astronomers differentiated those hypothetical objects from planets by how they were formed. If a substellar object was formed the way a star does, from a collapsing cloud of interstellar gas and dust, then it would be called a brown dwarf. If it was formed by gradually accumulating gas and dust inside a star's circumstellar disk, however, it was called a planet. Once the first brown dwarf candidates were actually found, however, astronomers realized that it was actually quite difficult to definitely rule on the validity of competing hypotheses about how a substellar object was actually formed without having been there. This problem is particularly difficult to resolve in the case of stellar companions, objects that orbit a star -- or two.
© American Scientist
(Artwork by Linda Huff for Martin et al, 1997; used with permission)
Although brown dwarfs lack sufficient mass (at least 75-80 Jupiters) to
ignite core hydrogen fusion, the smallest true stars (red dwarfs) can
have such cool atmospheric temperatures (below 4,000° K) that it is
difficult to distinguish them from brown dwarfs. While Jupiter-class planets
may be much less massive than brown dwarfs, they are about the same
diameter and may contain many of the same atmospheric molecules.
University of California at Berkeley astronomer Ben R. Oppenheimer, who helped to discover Gliese 229 b, is part of a growing group that would like to define a brown dwarf as an substellar object with the mass of 13 to 80 (or so) Jupiters. While these objects cannot fuse "ordinary" hydrogen (a single proton nucleus) like stars, they have enough mass to briefly fuse deuterium (hydrogen with a proton-neutron nucleus). Therefore, stellar companions with less than 13 Jupiter masses would be defined as planets.
Other prominent astronomers, such as San Francisco State University astronomer Geoffrey W. Marcy who also has helped to discover many extrasolar planets, note that there may in fact be many different physical processes that lead to the formation of planets. Similarly, there may also be many different processes that lead to the creation of brown dwarfs, and some of these may also lead to planets. Hence, more observational data may be needed before astronomers can determine how to make justifiable distinctions in the classification of such substellar objects.
The following table includes all star systems known to be located within 10 light-years (ly), plus more bright stars within 10 to 20 ly, of Xi Scorpii.
|Star System||Spectra &|
|* plus bright stars *||. . .|
Up-to-date technical summaries on these stars can be found at: the Astronomiches Rechen-Institut at Heidelberg's ARICNS for Star A, Star B, and Star C; the HIPPARCOS Catalogue using the VizieR Search Service mirrored from the Centre de Données astronomiques de Strasbourg (CDS); NASA's ADS Abstract Service for the Astrophysics Data System; the SIMBAD Astronomical Database mirrored from CDS, which may require an account to access; and the NSF-funded, arXiv.org Physics e-Print archive's search interface. Additional information may be available at Roger Wilcox's Internet Stellar Database.
In Ancient Greek legend, Orion (the Hunter) boasted that his might and skill were so great that he could kill all the animals on the face of the Earth. Gaea, Goddess of Earth, was alarmed and sent a giant scorpion to kill him. After a brief battle, the scorpion managed to sting Orion on the heel (at the star Rigel), but the Gods decided to give both Orion and the scorpion honored places at opposite ends of the noight sky so that they would never engage in battle again. For more information about the stars and objects in this constellation and an illustration, go to Christine Kronberg's Scorpius. For another illustration, see David Haworth's Scorpius.
For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.
Note: Nominated as a "notable nearby star" by Mike Stevens.
© 2003 Sol Company. All Rights Reserved.