J. Herschel 5173 AB
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© Torben Krogh & Mogens Winther,
(Amtsgymnasiet and EUC Syd Gallery,
student photo used with permission)
J. Herschel 5173 A is an orange-red
dwarf star, like Epsilon Eridani
at left center of meteor. (See a 2MASS
Survey image of J. Herschel 5173 A
from the NASA Star and Exoplanet
This system is located about 19.7 light-years (ly) away from our Sun, Sol, at the southeast corner (20:11:11.9-36:6:4.4 for Star A and 20:11:12.2-36:6:6 C~ for Star B, ICRS 2000.0) of Constellation Sagittarius, the Archer -- southeast of Theta1 and Theta2 Sagittarii. The binary pair was first discovered by Sir John Frederick William Herschel (1792-1871), portrait), the son of Sir William Friedrich Wilhelm Herschel (1738-1822, portrait). An early pioneer in photography with interests in chemistry, mathematics, and law, Sir John Herschel's first major publication in astronomy was a 1824 catalogue of double stars presented to the Royal Society for which he received honours, the Lalande Prize in 1825, and the Astronomical Society's Gold Medal in 1826. Later travelling to the Cape of Good Hope in South Africa, he observed the return of Halley's Comet in 1835 and discovered that gas was evaporating from it and that a repulsive force was acting on it -- when eventually led to the discovery of the Solar wind. This system is heading towards the Solar System and will come as close as 6.7 light-years in about 41,100 years (Vadim V. Bobylev, 2010, see HIP 99461; and Mülläri and Orlov, 1996, see Gl 783).
As a nearby and so relatively bright star in Earth's night sky, Star A is also catalogued as Harvard Revised (HR) 7703, a numbering system derived from the 1908 Revised Harvard Photometry catalogue of stars visible to many Humans with the naked eye. The HR system has been preserved through its successor, the Yale Bright Star Catalogue -- revised and expanded through the hard work of E. Dorrit Hoffleit (1907-2007) and others. HR 7703 is also listed as HD 191408 in the Henry Draper (1837-82) Catalogue with extension (HDE), a massive photographic stellar spectrum survey carried out by Annie Jump Cannon (1863-1941) and Edward Charles Pickering (1846-1919) from 1911 to 1915 under the sponsorship of a memorial fund created by Henry's wife, Anna Mary Palmer. (More discussion on star names and catalogue numbers is available from Alan MacRobert at Sky and Telescope and from Professor James B. Kaler's Star Names.)
Larger illustration of NASA's
Space Interferometry Mission (SIM).
Astronomers have identified J. Herschel (HJ)
5173 A and B as "Tier 1" targets for NASA's
optical SIM mission, now indefinitely delayed.
Due to J. Herschel (HJ) 5173 AB 's proximity to Sol, the system has been an object of high interest among astronomers. The star has been selected as "Tier 1" target stars for NASA's optical Space Interferometry Mission (SIM). The mission will attempt to detect planets as small as three Earth-masses within two AUs of each star. Although some summary system information and images of HJ 5173 A and B may still be available from the SIM Teams, the SIM project manager announced on November 8, 2010 that the mission was indefinitely postponed due to withdrawal of NASA funding.
AB Binary Star System
HJ 5173 A has a binary stellar companion at an observed separation of about 43 AUs in 1949 (based on a separation of 7.1" and a HIPPARCOS distance estimate of 19.74 ly) -- just beyond the "average" orbital distance (semi-major axis) of Pluto in the Solar System -- in approach from a wider separation of 9.8" in 1880 (ARICNS of Astronomisches Rechen-Institut at Heidelberg; and Poveda et al, 1994, pages 36 and 38). The stars have two optical companions that are not gravitationally bound, which are themselves binary systems (Gl 783.1 A and B; and Gl 738.2 A and B).
The star is a orange-red main sequence dwarf star of spectral and luminosity type K3 V, that has been classified as orange as K2. This star has about 82 percent of Sol's mass (RECONS estimate), 71 to 80 percent of its diameter (Pasinetti-Fracassini et al, 2001), and less than 23 percent of its visual luminosity and around 27 percent of its bolometric luminosity (NASA Star and Exoplanet Database, based on Kenneth R. Lang, 1980). HJ 5173 A appears to be 85 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Cayrel de Strobel et al, 1991, page 35). It may be an old disk star (Olin Jeuck Eggen (1919-1998), 1979, page 3), which implies an age up to 10 billion years, but , based on chromospheric activity and rotational period alone, Stars A and B are likely to be 6.4 to 7.7 billion years old (Mamajek et al, 2008). Useful star catalogue numbers for this star include: HR 7703, Gl 783 A, Hip 99461, HD 191408, CD-36 13940, CP(D)-36 9037, SAO 211885, LHS 486, LTT 7988, LFT 1529, and HJ 5173.
Estimates provided by the NASA Star and Exoplanet Database indicate that the inner edge of Star A's habitable zone could be located around 0.457 AU from the star, while the outer edge lies around 0.904 AUs. Accounting for infrared heating, the distance from Star A where an Earth-type planet would be "comfortable" with liquid water is centered around only 0.680 AU -- just inside the orbital distance of Venus in the Solar System, where a planet's period would be about 226 days or just under two-thirds of an Earth year.
NASA -- larger image
HJ 5173 B is a dim red dwarf star, like Gliese
623 A (M2.5V) and B (M5.8Ve) at lower right.
HR 7703 B is a red main sequence dwarf of spectral and luminosity type M3.5-4.0 V, that also has been classified as a subdwarf star (VI/sd) and even a giant star (III) in a catalog referenced by the SIMBAD Astronomical Database (which may be using an older reference). This star has about 20 percent of Sol's mass (RECONS estimate), 28 percent of its diameter (Pasinetti-Fracassini et al, 2001), and less than 77/100,000th of its visual luminosity. HJ 5173 B appears to be only 26 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Fe/H = -0.58, Cayrel de Strobel et al, 1991, page 35). Useful star catalogue numbers for this star include Gl 783 B, CD-36 13940 B, LHS 487, LTT 7989, and LFT 1530.
Jeffrey L. Linsky,
Like Gliese 752 B, HJ 5173 B is so small,
with only around 20 percent of Sol's mass,
that it can transport core heat only through
convection, unlike larger larger red dwarf
stars like Gliese 752 A (more).
While the NASA Star and Exoplanet Database does not yet have estimates available for HJ 5173 B (spectral type of M3.5), one can use Ross 154 (also a M3.5) as a proxy. According to one type of model calculations performed for the NASA Star and Exoplanet Database, the inner edge of Ross 154's habitable zone should be located very close to the star, at around 0.065 AU from the star, while the outer edge lies around 0.126 AUs. Accounting for the great infrared output of M-stars like Ross 154, the equivalent orbital distance for an Earth-type planet be only around 0.096 AUs. At slightly less than a fourth of Mercury's orbital distance in the Solar System, however, the rotation of the planet could become tidally locked with the star so that one side would have perpetual daylight with the other in darkness. Assuming that HJ 5173 B has about 20 percent of Sol's mass, a small Earth-type rocky planet would complete its orbit the star in just over 24 days.
Hunt for Substellar Companions
No circumstellar debris disk (Trilling et al, 2008, Table 1; and Beichman et al, 2006 in , Table 5) nor planetary or brown dwarf companions (Grether and Lineweaver, 2006) have been found around either HJ 5173 A or B, as of December 23, 2010.
The following star systems are located within 10 light-years of HJ 5173 AB.
|Star System||Spectra &|
|CD-49 13515 / Gl 832||M1.5 V||3.2|
|L 347-14||M4.5 V||4.5|
|CD-49 13515 / Gl 832||M1.5 V||7.1|
|Lacaille 8760||K7-M2 Ve||7.8|
|CD-27 14659||K0-3 V||9.8|
Up-to-date technical summaries on these stars can be found at: the Astronomiches Rechen-Institut at Heidelberg's ARICNS for Star A and Star B, the NASA Star and Exoplanet Database for Stars A and B, the SIMBAD Astronomical Database for Stars A and B, and the Research Consortium on Nearby Stars (RECONS) list of the 100 Nearest Star Systems. Additional information may be available at Roger Wilcox's Internet Stellar Database.
Sagittarius is Latin for "archer," often represented as a centaur wielding a bow and arrow since ancient times. The constellation also contains the Great Sagittarius Star Cloud, where a vast milky swarm of millions of stars mark the way to the center of the galaxy. For more information about the stars and objects in this constellation and an illustration, go to Christine Kronberg's Sagittarius. For another illustration, see David Haworth's Sagittarius.
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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