16 Cygni 3
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Although stars hosting planets have been found to be generally richer in elements heavier than hydrogen and helium ("metals"), a team of astronomers submitted a paper on July 28, 2011, which suggested that when such heavy elements are integrated into the cores of planets around a star, their host stars may end up less metal rich than they would otherwise be without planets. 16 Cygni has been confirmed to be a triple-star system, which includes two Sol-type stars A (or Aa) and B of spectral type G in a widely separated orbit of nearly 900 AUs, as well as a red dwarf C (or Ab) of spectral type M in a comparatively closer orbit with Star A of 80 to 150 AUs. While Star B has been found in 1996 to host a Jupiter-class planet, no planets have been detected around Star A or C as of July 2011. Despite its birth from apparently the same molecular cloud as Star A, however, Star B appears to be slightly less enriched in many of 22 elements sampled than Star A in one 2011 study, although another study released two weeks prior found the differences to be insignificant for 15 elements. On the other hand, a 2009 study found that our Sun is around 20 percent deficient in refractory metals than similar stars of similar composition and age ("Solar twins") (Jon Voisey, Universe Today, August 1, 2011; Ramirez et al, 2011; Schuler et al, 2011; and Melendez et al, 2009).
The 16 Cygni system has two widely separated bright stars, and possibly a dim stellar companion around Star A. The system is located about 70.5 light-years from Sol, in the northwestern corner (19:41:49.0+50:31:30.2 for Stars A and 19:41:52.0+50:31:3.1 for Star B, ICRS 2000.0) of Constellation Cygnus, the Swan -- northwest of Deneb (Alpha Cygni), Sadr (Gamma Cygni), and Delta Cygni, and east of Theta Cygni.
© James B. Kaler, UIUC -- more information
Photo from Stars, Planet Project, and
16 Cygni B (used with permission).
In 1996, astronomers announced the discovery of a Jupiter-like planet around this Sun-like star (Cochran et al, 1997 -- details below). (See an animation of the planetary and potentially habitable zone orbits of the 16 Cygni B system, with a table of basic orbital and physical characteristics.)
16 Cygni A is a yellow-orange main sequence dwarf star of spectral and luminosity type G1.5 Vb, with about 1.05 +/- 0.02 of a Solar-mass (Ramirez et al, 2011, Table 3, page 7; and Baliunas et al, 1997), 1.3 to 1.4 times its diameter (NASA Star and Exoplanet Database, and nearly 1.7 times its luminosity. One analysis of isochrones suggests that the star may be 9.5 billion years old, more than twice Sol's age (Guinan et al, 1999), while a more recent estimate indicates 6.8 +/- 0.4 billion years based on analysis of oscillations of stars A and B using asteroseismic observations from the Kepler Mission (Ken Croswell, 2012; and Metcalfe et al, 2012). Despite its presumed greater age, the star may be slightly more enriched (112 percent) as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Friel et al, 1993).
It has a widely separated, stellar companion "B", and possibly a closer companion "C" (at 70" from Star A, according to the Yale Bright Star Catalogue, 1991 5th Revised Edition notes entry for HR 7503). Stars A and B are separated by about 840 AUs (39" at a distance of 70.5 light-years), while A and C may be separated by 80 to 150 AUs, if C is not a background star. Useful catalogue numbers and designations for the star include: 16 Cyg, HR 7503*, Gl 765.1 A, Hip 96895, HD 186408, BD+50 2847, SAO 31898, LTT 15750, USNO 889, Struve 46, and ADS 12815 A.
NASA -- larger image
Stellar companion "Ab" is a dim red dwarf, like
Gliese 623 A (M2.5 V) and B (M5.8Ve) at lower right.
Star "B" is a yellow-orange main sequence dwarf star of spectral and luminosity type G 2.5 V, but it also has been classified as G3 and G5. It has around the same mass (1.00 +/- 0.01) as Sol (Ramirez et al, 2011, Table 3, page 7; and 16 Cyg B at exoplanets.org), 1.2 times its diameter (NASA Star and Exoplanet Database, and more than 1.3 times its luminosity. As with Star A, one analysis of isochrones suggests that 16 Cygni B may be 9.5 billion years old, more than twice Sol's age (Guinan et al, 1999), while a more recent estimate indicates 6.8 +/- 0.4 billion years based on analysis of oscillations of stars A and B using asteroseismic observations from the Kepler Mission (Ken Croswell, 2012; and Metcalfe et al, 2012). Despite its presumed greater age, Star B may be more enriched (123 percent) than Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (exoplanets.org). Useful catalogue numbers and designations for the star include: 16 Cyg B, HR 7504, GJ 765.1 B, Hip 96901, HD 186427, BD+50 2848, SAO 31899, LTT 15751, USNO 890, and ADS 12815 B.
|16 Cygni B||0.0||...||...||...||330,000||130||...||...||1.23|
|Inner H.Z. Edge?||0.79||0.71||0||...||...||...||...||...||...|
|Outer H.Z. Edge?||1.59||2.0||0||...||...||...||...||...||...|
Estimates based on one type of model calculations performed for the NASA Star and Exoplanet Database indicate that the inner edge of 16 Cygni B's habitable zone could be located around 0.793 AU from the star, while the outer edge edge lies around 1.586 AUs. Based on a luminosity of 1.115 Solar, the orbital distance from 16 Cygni B where an Earth-type planet may have liquid water on its surface is centered around 1.112 AU -- just beyond the orbital distance of Earth in the Solar System -- where a planet probably would have an orbital period of about 428 days or around 1l7 percent of an Earth year. Unfortunately, the highly eccentric orbit of planetary candidate "b" (discussed below) would disrupt the orbit of a planet that orbits Star B wholley within this liquid water zone.
In 1996, two team of astronomers announced the discovery of a Jupiter-class planet around Star B using highly sensitive radial-velocity methods (Cochran et al, 1997). Planet "b" has at least 1.68 times of Jupiter's mass. It moves around Star B at an average distance of 1.66 AUs (a semi-major axis between the orbital distances of Mars and the Main Asteroid Belt) in a highly elliptical orbit (e=0.68) that takes almost 2.2 years to complete (16 Cyg B and Multiple Planet Systems at exoplanets.org).
Whatmough -- larger image of "b"
(Artwork from Extrasolar Visions, used with permission)
Despite an eccentric orbit, some hoped that planet b could have
a moon with a habitable "Spring," as imagined by Whatmough.
Subsequent astrometric analysis, however, suggests that planet b may have 14 times the mass of Jupiter with an inclination of 173.0° from Earth's line of sight (Han et al, 2000). Thus, the planet could be an extremely dim brown dwarf, substellar companion of 16 Cygni B. The authors consider their analysis to be preliminary, needing confirmation with additional astrometric as well as other observations.
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.
University of California at Berkeley astronomer Ben R. Oppenheimer, who helped to discover a nearby brown dwarf, 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 16 Cygni B.
|Star System||Spectra &|
|Theta Cygni 2?||F4 V-IV |
|* plus bright stars *||. . .|
|BD+57 1961||G8 V-IV||11|
|BD+56 2272||G0 V||12|
|HR 7294 AB||G4-6 V |
|HR 7451||F7 V||14|
|BD+41 3799 AB||G9 V |
|HR 7683 AB||G5 V-IV |
|HR 7123 AB||G9 IVa||19|
|HR 6847||G2 V||20|
Try Professor Jim Kaler's Stars site for other information about 16 Cygni at the University of Illinois' Department of Astronomy. The late John Whatmough created illustrated web pages on this system in Extrasolar Visions. For another illustrated discussion, see Christoph Kulmann's web page on 16 Cygni.
Up-to-date technical summaries on these stars can be found at: Jean Schneiders's Extrasolar Planets Encyclopaedia; the Astronomiches Rechen-Institut at Heidelberg's ARICNS (Star A and Star B), and the NASA Star and Exoplanet Database for Star A and B. Additional information may be available at Roger Wilcox's Internet Stellar Database.
The Swan lies between Constellations Pegasus and Draco and is often shown with outstretched wings flying parallel with the Milky Way. Deneb, the tail star, along with Vega in Constellation Lyra and Altair in Constellation Aquila form the Summer Triangle. While the mythology of Constellation Cygnus is not known with certainty, the Greek myth of Leda and the Swan involves Zeus, the King of the Gods, who fell in love with Leda and ravaged her disguised as a swan. Subsequently, Leda laid an egg which bore Helen, the most beautiful woman in the world, and Castor and Pollux, the Gemini twins. For more information about the stars and objects in this constellation and an illustration, go to Christine Kronberg's Cygnus. For an illustration, see David Haworth's Cygnus.
For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.
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