Sigma Draconis (Alsafi)
|Home | Stars | Habitability | Life ||
Also known as Alsafi, Sigma Draconis is located about 18.8 light-years (ly) away from our Sun, Sol, in the northwestern part (19:32:21.59+69:39:40.32, ICRS 2000.0) of Constellation Draco, the Dragon -- south of Tyl (Epsilon Draconis) and north of Altais or Nodus Secundus (Delta Draconis). In the venerable Star Names: Their Lore and Meaning, Richard Hinckley Allen identified this star as Alsafi (derived from the Arabic "Athafiyy" for the "cooking tripod" of nomadic open-air kitchens). The star has a visual companion which is not gravitationally bound to it.
Sigma Draconis became one of the top 100 target stars for NASA's planned Terrestrial Planet Finder (TPF) before the project was indefinitely postponed. It was also selected as a "Tier 1" target star for NASA's optical Space Interferometry Mission (SIM) to detect a planet as small as three Earth-masses within two AUs of its host star (and so some summary system information and images of Sigma Draconis may still be available from the SIM Teams), but the SIM project manager announced on November 8, 2010 that the mission was indefinitely postponed due to withdrawal of NASA funding.
Astronomers had planned to use NASA's 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 Sigma Draconis. As originally planned, the TPF would include two complementary observatory groups: a visible-light coronagraph; and a "formation-flying" infrared interferometer, while Darwin would have launched a flotilla of three mid-infrared telescopes and a fourth communications hub. Both project, however, have been indefinitely postponed due to budget constraints. In November 2005, astronomer Margaret Turnbull, who has been working on identifying the best stars for the TPF to target its observatories, wrote to inform us that: "[S]igma Drac[onis] is one of our best TPF-C targets, and the 4th easiest star in the universe to detect terrestrial planets" -- more below).
Sigma Draconis is a main-sequence yellow-orange dwarf star of spectral and luminosity type of G9 V (RECONS; and NASA Star and Exoplanet Database, based on MK spectral classification system from Gray et al, 2003, see HIP 96100 in Table 1), which was previously classed as orange as K0 under the older HD system. The star has about 89 percent of Sol's mass (RECONS), 78 to 79 percent of its diameter (Boyajian et al, 2008, Table 2; and Pasinetti-Fracassini et al, 2001), and 39 percent of its visual luminosity and 41 percent of its bolometric luminosity (NASA Star and Exoplanet Database, derived from the exponential formula of Kenneth R. Lang, 1980). The star appears to be less enriched than Sol in elements heavier than hydrogen ("metals") with only about 56 to 72 percent of Sol's abundance of iron (Demory et al, 2009, Table B.1; Yoichi Takeda, 2007; Bryden et al, 2006; Valenti and Fisher, 2005; Nordström et al, 2004; Misha Haywood, 2001; and Cayrel de Strobel et al, 1991, page 33). Based on chromospheric activity and rotational period, it may be between 3.7 and 4.7 billion years old (Mamajek et al, 2008), but a range of 0.3 to 9.2 billion years has been estimated using different indicators and/or models in several other studies (Holmberg et al, 2009; and Lawler et al, 2009, see HD 185144 in Table 1; and Nordström et al, 2004). A variable star, it has the Catalog and New Suspected Variable designations of CSV 101868 and NSV 12176. Useful star designations and catalogue numbers for Sigma Draconis include: Sig Dra, 61 Dra, HR 7462, Gl 764, Hip 96100, HD 185144, BD+69 1053, SAO 18396, LHS 477, LTT 15713, and LFT 1486.
Estimates provided by the NASA Star and Exoplanet Database indicate that the inner edge of Sigma Draconis' habitable zone could be located around 0.557 AU from the star, while the outer edge edge lies around 1.016 AUs. According to the SIM project, the distance from Sigma Draconis where an Earth-type planet would be "comfortable" with liquid water is centered around only 0.65 AU -- between the orbital distances of Mercury and Venus in the Solar System. Assuming that Sigma Draconis has 89 percent of Sol's mass, such a planet would have an orbital period under 203 days -- just a bit over half of an Earth year -- at that distance from the star.
In November 2005, astronomer Margaret Turnbull, who worked on identifying the best stars for the TPF to target its observatories, wrote to inform us that: "[S]igma Drac[onis] is one of our best TPF-C targets, and the 4th easiest star in the universe to detect terrestrial planets. Due to its proximity, the angular habitable zone size is relatively large: from 68 to 150 milliarcseconds. And due to its low luminosity, an Earth-like planet would be about 2e-10 times as bright as the star at optical wavelengths. Believe it or not that is actually quite good for stars whose habitable zones are that large on the sky. Brighter stars, while their HZ's are large, swamp the light of their planets, which makes them difficult targets for TPF."
Hunt for Substellar Companions and Dust Disk
Using the Doppler radial-velocity technique pioneered by Geoffrey W. Marcy and R. Paul Butler, astronomers have failed to find a brown dwarf or large Jupiter- or Saturn-mass object in a "torch" orbit around Sigma Draconis (Wittenmyer et al, 2006; and Cumming et al, 1999). A subsequent radial-velocity search down of objects down to super-Earths was also negative thus far Howard et al, 2010). An infrared corongraphic search for brown dwarfs in wide orbits within 75 to 1,200 AUs of Sigma Draconsis and an older optical corongraphic search were also negative (McCarthy and Zuckerman, 2004; and Oppenheimer et al, 2001). No significant circumstellar debris disk has been detected (Lawler et al, 2009; Trilling et al, 2008; and Bryden et al, 2006).
The following star systems are located within 10 light-years of Sigma Draconis.
|Star System||Spectra &|
|BD+68 946 AB||M3.0 V |
|LP 71-165||M4.5 V||5.7|
|BD+61 2068 AB||M0 Ve |
|AC+65 6955||M3 V||7.6|
|Chi Draconis 2||F7 V |
|Struve 2398 AB||M3.0 V |
|Kruger 60 AB||M3 V |
|V1581 Cygni 2||M5.5 Ve |
|AC+79 3888||M3.5 V||8.6|
|G 227-29||M V||9.0|
|AC+54 1646-56||M1.5 V||9.3|
|BD+56 2966||K3 V||9.5|
Up-to-date technical summaries on these stars can be found at: the Astronomiches Rechen-Institut at Heidelberg's ARICNS, the NASA Star and Exoplanet Database, the Research Consortium on Nearby Stars (RECONS) list of the 100 Nearest Star Systems, and the SIMBAD Astronomical Database. Additional information may be available at Roger Wilcox's Internet Stellar Database.
Constellation Draco is associated with the dragon slain by Cadmus, the brother of Europa. It is a large and elongated constellation of the northern hemisphere and is one of the few constellations which really resemble the object they were named after. For more information about the stars and objects in this constellation and an illustration, go to Christine Kronberg's Draco. For another illustration, see David Haworth's Draco.
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 Mike Stevens for notifying us of a typo in the star's diameter, which also has been updated to a more recent estimate.
© 1998-2011 Sol Company. All Rights Reserved.