Gl 338 AB
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© Torben Krogh & Mogens Winther,
(Amtsgymnasiet and EUC Syd Gallery,
student photo used with permission)
Gl 338 AB are orange-red dwarf
stars that are similar to, but
much dimmer and redder than,
Epsilon Eridani at left center
of meteor. (See a Digitized
Sky Survey image of Gl 338
from NASA's NStars Database.)
This multiple star system is located about 21.3 light-years (ly) away from our Sun, Sol, at the northwestern part (9:14:22.8+52:41:11.8 for Star A and 9:14:24.7+52:41:11.0 for Star B, ICRS 2000.0) of Constellation Ursa Major, the Great Bear, which also encompasses the Big Dipper or Plow (Plough) -- . The first observation and measurement of the system as binary stars was made in 1837 by F.G.W. Struve (page 186). According to radial velocity measurements which have been since disproved, both Gl 338 A and B were once suspected of each having an unseen companion (Helmut A. Abt, 1971 private communication to Kyogae Chang; Morbey and Griffin, 1987; and possibly L.G. Romanenko, 1989). All of the system's stars, however, are too dim to be viewed without a telescope in Earth's night sky. According to the SIMBAD Astronomical Database, both Gl 338 A and its companion Star B are flare stars. (See an animation of the orbits of Stars A and B and their potentially habitable zones, with a table of basic orbital and physical characteristics.)
Gl 338 AB were previously designated BD+53 1320 and BD+53 1321 in a catalogue that was originally published in 1863 by Friedrich Wilhelm August Argelander (1799-1875) on the position and brightness of 324,198 stars between +90° and -2° declination that were measured over 11 years from Bonn, Germany with his assistants Eduard Schönfeld (1828-1891) and Aldalbert Krüger (1832-1896). The catalogue became famous as the Bonner Durchmusterung ("Bonn Survey") and is typically abbreviated as BD. It was later expanded and extended during the early 20th Century with the Cordoba (observed from Argentina) then the Cape Photographic Durmusterung (observed from South Africa).
Gl 338 A and B are also listed as HD 79210 and HD 79211 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.)
Gl 338 A is a orange-red main sequence dwarf star of spectral and luminosity type K7-M0 V. This star has 43 to 60 percent of Sol's mass (Kyongae Chang, 1972); and RECONS estimate, respectively), probably between 66 to 69 percent of its diameter (Pasinetti-Fracassini et al, 2001), and about 2.9 percent of its visual luminosity. Other useful star catalogue designations for Gl 338 A include: Hip 45343, HD 79210, BD+53 1320, G 195-17, SAO 27178, LHS 260, LTT 12425, LFT 634, Vys/McC 541 A, STF 1231 A, and ADS 7251 A. Although Star A is now known to be a flare star, it does not appear to have any special variable star designations.
According to possibly premature orbit estimates still given in the Sixth Catalog of Visual Orbits of Binary Stars, stars A and B are separated on average by 103 AUs (16.675" of a semi-major axis) in an eccentric orbit (e= 0.28) that takes 975 years to complete. The orbit is inclined by 21.0° to Earth's line of sight (Kyongae Chang, 1972; Morbey and Griffin, 1987). (See an animation of the orbits of Stars A and B and their potentially habitable zones, with a table of basic orbital and physical characteristics.)
Gl 338 B is a orange-red main sequence dwarf star of spectral and luminosity type K7-M0 V. This star has almost 60 to 73 percent of Sol's mass (Kyongae Chang, 1972); and RECONS estimate, respectively), probably between 65 percent of its diameter (Pasinetti-Fracassini et al, 2001), and about 2.8 percent of its visual luminosity. Other useful star catalogue designations for Gl 338 B include: Hip 120005, HD 79211, G 195-18, BD+53 1321, SAO 27179, LHS 261, LTT 12426, LFT 635, Vys/McC 541 B, STF 1231 B, and ADS 7251 B. Although Star A is now known to be a flare star, it does not appear to have any special variable star designations.
Hunt for Substellar Companions
Since Gl 388 A and B are sort of like distant cousins to Sol, some speculate whether they might just be bright enough to support Earth-type life on a planet lucky enough to orbit in its water zone. The distance from Gl 388 A where an Earth-type planet would be "comfortable" with liquid water is centered around only 0.17 AU -- well inside Mercury's orbital distance in the Solar System. At that distance from the star, such a planet would have an orbital period of under 34 days -- just over a third of an Earth year. For Star B, the water-zone orbital distance would be 0.17 AU with a period of over 32 days. Astronomers would find it very difficult to detect using present methods.
Life Around a Flare Star
Many dim, red (M) and some orange-red (K) dwarf stars exhibit unusually violent flare activity for their size and brightness. These flare stars are actually common because red dwarfs make up more than half of all stars in our galaxy. Although flares do occur on the Sun every so often, the amount of energy released in a Solar flare is small compared to the total amount of energy that Sol produces. However, a flare the size of a solar flare occurring on a orange-red dwarf star (such as Gl 388 A or B) that normally has less than 21 percent of than Sol's luminosity would be more noticeable.
High resolution and jumbo images (Benz et al, 1998).
Gl 388 AB are flare stars, like UV Ceti (Luyten
726-8 B) shown flaring at left. UV Ceti is an extreme
example of a flare star that can boost its brightness by
five times in less than a minute, then fall somewhat slower
back down to normal luminosity within two or three
minutes before flaring suddenly again after several hours.
Flare stars erupt sporadically, with successive flares spaced anywhere from an hour to a few days apart. A flare only takes a few minutes to reach peak brightness, and more than one flare can occur at a time. Moreover, in addition to bursts of light and radio waves, flares on dim red dwarfs may emit up to 10,000 times as many X-rays as a comparably-sized Solar flare on our own Sun, and so flares would be lethal to Earth-type life on planets near the flare star. Hence, Earth-type life around flare stars may be less likely because their planets must be located very close to dim orange-red dwarfs to be warmed sufficiently by star light to have liquid water (less than 0. AU for Gl 388 A or B), which makes flares even more dangerous around such stars. In any case, the light emitted by late orange-red dwarfs may be too red in color for Earth-type plant life to perform photosynthesis efficiently.
The following star systems are located within 10 light-years of Gl 388.
|Star System||Spectra &|
|Groombridge 1618||K5-7 Ve||5.2|
|BD+44 2051 AB||M1 Ve |
|Ross 986 AB||M4.5 Ve |
|G 111-47||K-M4 V||9.5|
|G 250-31||M4.5 V||9.7|
Up-to-date technical summaries on these stars can be found at: the Astronomiches Rechen-Institut at Heidelberg's ARICNS Star A and Star B, NASA's NStar Database, 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.
Constellation Ursa Major is only visible from the northern hemisphere. The seven stars of the Big Dipper in this constellation are famous as the traveller's guide to Polaris, the North Star. For more information about the stars and objects in this constellation, go to Christine Kronberg's Ursa Major. For another illustration, see David Haworth's Ursa Major.
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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