18 Scorpii AB?
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18 Scorpii is located about 45.7 light-years (ly) from Sol. It lies at the northern edge of (16:15:37.3-8:22:10.0, ICRS 2000.0) of Constellation Scorpius, the Scorpion -- just off its left claw. The star can be found northwest of Zeta Ophiuchi, south of Yed Posterior (Epsilon Ophiuchi) and Yed Prior (Delta Ophiuchi), north of Graffias or Acrab (Beta1,2 Scorpii), and east of Zubeneschamali (Beta Librae). The star may have a stellar companion in a wide orbit (as noted below). As 18 Scorpii has become one of the top 100 target stars for NASA's planned Terrestrial Planet Finder (TPF), images of this star and its position relative to the Milky Way in Earth's night sky are now available from the TPF-C team.
In late September 2003, astrobiologist Maggie Turnbull identified 18 Scorpii as one of the best candidates for hosting Earth-type life. The star was chosen from a shortlist of 30 stars (screened from the 5,000 or so stars that are estimated to be located within 100 ly of Earth) that were presented to a group of scientists from NASA's TPF and the ESA's Darwin planned groups of observatories (Astrobiology Magazine). The stars examined were selected from a larger list of 17,129 (of which 75 percent are located within around 450 ly, or 140 parsecs, of Sol) that were assembled into a Catalog of Nearby Habitable Stellar Systems (HabCat) by Turnbull and Jill Tarter of the SETI Institute (see: Margaret C. Turnbull, 2002, in pdf). Selection criteria for the 30-star shortlist included: X-ray luminosity, rotation, spectral types or color, kinematics, metallicity, and Strömgren photometry. On February 19, 2006, Turnbull named 18 Scorpii as a Sun-like star that is old enough to qualify as a top-five candidate for those listening for radio signals from intelligent civilizations (e.g., SETI Institute).
© ESA 2001
To find life around nearby stars,
the ESA's Darwin mission will look
for traces of water, oxygen, and
carbon dioxide in the atmospheres
of Earth-type planets found in
stellar habitable zones (more).
18 Scorpii is a yellow-orange main sequence dwarf star of spectral and luminosity type G1-5 V-Va. A little bigger and brighter than Sol, the star may have a mass similar to Sol's -- an inferred mass of 1.01 +/- 0.03 Solar-mass (Ryan et al, 2004; Guinan et al, 1999; and Porto de Mello and da Silva, 1997), 1.02 to 1.03 times its diameter (Ryan et al, 2004; and Johnson and Wright, 1983, page 685), and 1.05 times its luminosity. It may be 105 to to 112 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Porto de Mello and da Silva, 1997; and Cayrel de Strobel et al, 1991, 300).
A team of astronomers studying 18 Scorpii announced on January 6, 2004 -- at the 203rd Meeting of the American Astronomical Society -- that the star's surface temperature is 5,789 degrees Kelvin, slightly hotter than Sol's 5,777 degrees. The star takes 23 days to rotate, slightly faster than Sol's rotational period of 25.4 days. A middle-aged star that is slightly younger than Sol's 4.56 billion years, 18 Scorpii is now thought to be around 4.2 billion years old (superceding the previous estimate of 4.7 +/- 0.8 billion years by Guinan et al, 1999).
Some astronomers have regard 18 Scorpii as the nearest "Solar twin" (space.com; Ryan et al, 2004; and Porto de Mello and da Silva, 1997). Its starspot cycle lasts nine to 13 years while Sol's waxes and wanes every 11 years (Ryan et al, 2004). Observations through 2000, however, indicate that 18 Scorpii has a well-defined activity cycle which reached an apparent minimum in 1998 then showed a rapid rise through 2000. A comparison with contemporaneous Solar data using the same instrument suggests that 18 Scorpii's activity cycle may be of greater amplitude than Sol's and that its overall chromospheric activity level is noticeably greater than Sol's. Hence, this otherwise, "excellent solar photometric twin therefore may be a less perfect spectroscopic twin" (Hall and Lockwood, 2000). 18 Scorpii has been given the variable star designations: CSV 101566, NSV 7577, and SV ZI 1223. Other useful catalogue numbers and designations for the star include: 18 Sco, HR 6060, Gl 616, Hip 79672, HD 146233, BD-07 4242, SAO 141066, LHS 3172, LTT 6482, LFT 1259, and LPM 594.
The star is part of a survey of Sol-type stars in the local galactic neighborhood, by which astronomers hope to understand better Sol's past and future evolution. Young stars were observed to be 30 percent dimmer in brightness. Since they spin much faster, however, young stars were also found to have strong magnetic fields which apparently led to huge stellar flares that erupted one or twice a day with abundant emission of x-rays and ultraviolet light. Older stars appeared to have become quieter as they age, as the oldest emit only about a fifth as much x-ray energy as Sol before exhausting core hydrogen fusion into expanding into red giants.
An Earth-type planet could have liquid water in a stable orbit centered around 1.02 AU from around 18 Scorpii -- around the orbital distance of Earth in the Solar System. Such a planet would have an orbital period of around one Earth year. Astronomers are hoping to use NASA's Terrestrial Planet Finder (TPF) and the ESA's Darwin planned groups of observatories to search for a rocky inner planet in the so-called "habitable zone" (HZ) around 18 Scorpii. As currently planned, the TPF will include two complementary observatory groups: a visible-light coronagraph to launch around 2014; and a "formation-flying" infrared interferometer to launch before 2020, while Darwin will launch a flotilla of three mid-infrared telescopes and a fourth communications hub beginning in 2015.
According to some catalogues, Star A has a stellar companion at a wide separation of 25.8 arcseconds, or around 361 AUs. (See brief notation in SIMBAD and in Hipparcos Input Catalogue, Version 2 (Turon+ 1993).)
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 18 Scorpii.
|Star System||Spectra &|
|BD-07 4156||M0 V||3.1|
|HC+03 1919||M7 V||6.1|
|BK-05 9201||M1 V||7.1|
|O'Neil 723||M0 V||7.5|
|L 841-9||K-M2 V||8.0|
|HC+25 1902||K2 V||8.8|
|LP 684-17||M4.5-5 V||9.0|
|LP 805-10||M V||9.4|
|BK-02 75||M2 V||9.5|
|* plus bright stars *||. . .|
|HC-24 1124||F2 V||11|
|Psi Serpentis 3?||G2.5-5 V |
|DK-26 2485||G6 V||14|
|BK+00 2334||G2 V||14|
|BD+00 3593||G8 V||14|
|L. Serpentis 2||G0 V |
|L 989-20 AB||G-M3.5 V |
|HR 6516 AaB||G8-9 V-IV |
|BK+16 9747||G1 V||19|
|Gamma Serpentis 2?||F6 V |
|SS-36 2376||G0 V||20|
A discussion of "Nearby Stars Similar to the Sun" with a comparison of the spectrum of 18 Scorpii and Sol is available from SETIweb.org (taken from: Porto de Mello and da Silva, 1997). In addition, try Professor Jim Kaler's Stars site for other information about 18 Scorpii at the University of Illinois' Department of Astronomy.
Up-to-date technical summaries on these stars can be found at: the Astronomiches Rechen-Institut at Heidelberg's ARICNS, and the Nearby Stars Database. 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 night 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: Thanks to Hiroshi Takada for notifying us of at least one past finding that 18 Scorpii may have a wide binary companion.
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