18 Scorpii

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18 Scorpii
Diagram showing star positions and boundaries of the Scorpius constellation and its surroundings
Cercle rouge 100%.svg

Location of 18 Scorpii (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Scorpius
Right ascension 16h 15m 37.26946s[1]
Declination –08° 22′ 09.9870″[1]
Apparent magnitude (V) 5.503[2]
Spectral type G2 Va
U−B color index +0.18[3]
B−V color index +0.64[3]
Variable type Sun-like[4]
Radial velocity (Rv) +11.6[2] km/s
Proper motion (μ) RA: 230.77[1] mas/yr
Dec.: -495.53[1] mas/yr
Parallax (π) 71.94 ± 0.37[1] mas
Distance 45.3 ± 0.2 ly
(13.90 ± 0.07 pc)
Absolute magnitude (MV) 4.77[2]
Mass 1.02 ± 0.03[5] M
Radius 1.010 ± 0.009[5] R
Luminosity 1.058 ± 0.028[6] L
Surface gravity (log g) 4.45[7] cgs
Temperature 5,433 ± 69[6] K
Metallicity [Fe/H] 0.04[7] dex
Rotation 22.7 ± 0.5[8]
Age 4.1–5.3[9] Gyr
Other designations
BD-07 4242, Gl 616, HD 146233, HIP 79672, HR 6060, SAO 141066, GC 21864, CCDM 16156-0822.[3]
Database references
Exoplanet Archive data

18 Scorpii is a solitary star located at a distance of some 45.3 light-years (13.9 parsecs) from Earth at the northern edge of the Scorpius constellation. It has an apparent visual magnitude of 5.5,[2] which is bright enough to be seen with the naked eye outside of urban areas.

18 Scorpii has many physical properties in common with the Sun. Cayrel de Strobel (1996) included it in her review of the stars most similar to the Sun,[10] and Porto de Mello & da Silva (1997) identified it as a solar twin.[11] Some scientists therefore believe the prospects for life in its vicinity are good. However, no planet has yet been discovered orbiting this star.


18 Scorpii is a main sequence star of spectral and luminosity type G2 Va,[11] with the luminosity class of 'V' indicating it is generating energy through the nuclear fusion of hydrogen in its core region. Sousa et al. (2008) found its metallicity to be about 1.1 times that of the Sun, which means the abundance of elements other than hydrogen or helium is 10% greater.[7][12] The radius of this star, as measured using interferometry by Bazot et al. (2011), is 101% the radius of the Sun. When combined with the results of asteroseismology measurements, this allows the mass of the star to be estimated as 102% of the Sun's mass.[5] This star is radiating 106% of the Sun's luminosity from its outer atmosphere at an effective temperature of 5,433 K.[6] It is this heat that gives the star the yellow-hued glow of a G-type star.[13]

According to Lockwood (2002), it has a temporal photometric behavior very similar to the Sun.[14] Its brightness variation over its entire activity cycle is 0.09%, about the same as the Sun's brightness variations during recent solar cycles.[15] Using the technique of Zeeman-Doppler imaging, Petit et al. (2008) have detected its surface magnetic field, showing that its intensity and geometry are very similar to the large-scale solar magnetic field.[16] The estimated period for the activity cycle of 18 Scorpii is about seven years,[4] which is significantly shorter than the Sun's, and its overall chromospheric activity level is noticeably higher.[15][17] Like the Sun, it has a hot corona with a temperature in the range of 1.5–2 MK and an X-ray luminosity of 8 ± 1.5 ergs s−1.[8]

Though 18 Scorpii is only slightly more metal-rich overall than the Sun, its lithium abundance is about three times as high; for this reason, Meléndez & Ramírez (2007) have suggested that 18 Scorpii be called a "quasi solar twin", reserving the term solar twin for stars (such as HIP 56948) that match the Sun, within the observational errors, for all parameters.[18]

18 Scorpii was identified in September 2003 by astrobiologist Margaret Turnbull from the University of Arizona in Tucson as one of the most promising nearby candidates for hosting life, based on her analysis of the HabCat list of stars. This is a solitary star and, as of 2005, radial velocity measurements have not yet revealed the presence of planets orbiting it.[19] Nor does this star display the level of excess infrared emission that would otherwise suggest the presence of unconsolidated circumstellar matter, such as a debris disk.[20]


The age of 18 Scorpii, shown relative to the Sun, the older solar twin HD 197027, here annotated as HIP 102152 and the formation of the Milky Way


  1. ^ a b c d e van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664, arXiv:0708.1752, Bibcode:2007A&A...474..653V, doi:10.1051/0004-6361:20078357 
  2. ^ a b c d Nordström, B. et al. (May 2004), "The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of ˜14 000 F and G dwarfs", Astronomy and Astrophysics 418: 989–1019, arXiv:astro-ph/0405198, Bibcode:2004A&A...418..989N, doi:10.1051/0004-6361:20035959 
  3. ^ a b c "18 Sco -- Variable Star", SIMBAD (Centre de Données astronomiques de Strasbourg), retrieved 2011-10-13  The ubv information is per Compilation of Eggen's UBV data, transformed to UBV (unpublished) 1986. See the Measurements section.
  4. ^ a b Hall, Jeffrey C. et al. (July 2009), "The Activity and Variability of the Sun and Sun-Like Stars. II. Contemporaneous Photometry and Spectroscopy of Bright Solar Analogs", The Astronomical Journal 138 (1): 312–322, Bibcode:2009AJ....138..312H, doi:10.1088/0004-6256/138/1/312 
  5. ^ a b c Bazot, M. et al. (February 2011), "The radius and mass of the close solar twin 18 Scorpii derived from asteroseismology and interferometry", Astronomy and Astrophysics 526, arXiv:1209.0217, Bibcode:2011A&A...526L...4B, doi:10.1051/0004-6361/201015679 
  6. ^ a b c Boyajian, Tabetha S. et al. (February 2012), "Stellar Diameters and Temperatures. I. Main-sequence A, F, and G Stars", The Astrophysical Journal 746 (1): 101, arXiv:1112.3316, Bibcode:2012ApJ...746..101B, doi:10.1088/0004-637X/746/1/101 . See Table 10.
  7. ^ a b c Sousa, S. G. et al. (August 2008), "Spectroscopic parameters for 451 stars in the HARPS GTO planet search program. Stellar [Fe/H] and the frequency of exo-Neptunes", Astronomy and Astrophysics 487 (1): 373–381, arXiv:0805.4826, Bibcode:2008A&A...487..373S, doi:10.1051/0004-6361:200809698 
  8. ^ a b Coughlin, Jared et al. (January 2010), "The Night Time Sun: X-Ray Observations of the Solar Twin 18 Scorpii", Bulletin of the American Astronomical Society 42: 333, Bibcode:2010AAS...21542417C 
  9. ^ Mamajek, Eric E.; Hillenbrand, Lynne A. (2008), "Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics", The Astrophysical Journal 687 (2): 1264–1293, arXiv:0807.1686, Bibcode:2008ApJ...687.1264M, doi:10.1086/591785 
  10. ^ Cayrel de Strobel, G. (1996), "Stars Resembling the Sun", The Astronomy and Astrophysics Review 7 (3): 243–288, Bibcode:1996A&ARv...7..243C, doi:10.1007/s001590050006 
  11. ^ a b Porto de Mello, G. F.; da Silva, L. (1997), "HR 6060: The Closest Ever Solar Twin?", The Astrophysical Journal 482 (2): L89–L92, Bibcode:1997ApJ...482L..89P, doi:10.1086/310693 
  12. ^ A metallicity of [Fe/H] = 1.04 dex indicates that the star has 100.04 = 1.096, or 110% of the abundance of elements heavier than helium, compared to the Sun.
  13. ^ "The Colour of Stars", Australia Telescope, Outreach and Education (Commonwealth Scientific and Industrial Research Organisation), December 21, 2004, archived from the original on 2012-03-09, retrieved 2012-01-16 
  14. ^ Lockwood, G. W. et al. (May 2002), "Gauging the Sun: Comparative photometric and magnetic activity measurements of sunlike stars, 1984-2001" (PDF), Bulletin of the American Astronomical Society 34: 651, Bibcode:2002AAS...200.0709L 
  15. ^ a b Hall, J. C.; Lockwood, G. W. (2007), "The Sun-Like Activity of the Solar Twin 18 Scorpii", The Astronomical Journal 133 (5): 2206–2008, arXiv:astro-ph/0703450, Bibcode:2007AJ....133.2206H, doi:10.1086/513195 
  16. ^ Petit, P. et al. (2008), "Toroidal versus poloidal magnetic fields in Sun-like stars: a rotation threshold", Monthly Notices of the Royal Astronomical Society 388 (1): 80, arXiv:0804.1290, Bibcode:2008MNRAS.388...80P, doi:10.1111/j.1365-2966.2008.13411.x 
  17. ^ Hall, Jeffrey C.; Lockwood, G. W. (2000), "Evidence of a Pronounced Activity Cycle in the Solar Twin 18 Scorpii", The Astrophysical Journal 545 (2): L43–L45, Bibcode:2000ApJ...545L..43H, doi:10.1086/317331 
  18. ^ Meléndez, J.; Ramírez, I. (2007), "HIP 56948: A Solar Twin with a Low Lithium Abundance", The Astrophysical Journal 669 (2): L89–L92, arXiv:0709.4290, Bibcode:2007ApJ...669L..89M, doi:10.1086/523942 
  19. ^ Marcy, Geoffrey W. et al. (2005), "Five New Extrasolar Planets", The Astrophysical Journal 619 (1): 570–584, Bibcode:2005ApJ...619..570M, doi:10.1086/426384 
  20. ^ Lawler, S. M. et al. (November 2009), "Explorations Beyond the Snow Line: Spitzer/IRS Spectra of Debris Disks Around Solar-type Stars", The Astrophysical Journal 705 (1): 89–111, arXiv:0909.0058, Bibcode:2009ApJ...705...89L, doi:10.1088/0004-637X/705/1/89 

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