Struve 1321 / Gl 338 AB

According to radial velocity measurements which have been since disproved, both Struve 1321 A and B, each, were once suspected of having an unseen companion (Helmut A. Abt, 1971 private communication to Kyongae Chang; Morbey and Griffin, 1987; and possibly L.G. Romanenko, 1989). In any case, this system's stars are too dim to be viewed without a telescope. Although, according to the SIMBAD Astronomical Database, both Struve 1231 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.)

Struve 1321 A

Star A is a orange-red main sequence dwarf star of spectral and luminosity type K7-M0 Ve. The 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 3.1 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 1321 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 that still the only ones available in the Sixth Catalog of Visual Orbits of Binary Stars, stars A and B are separated on average by 108 AUs (16.675" of a semi-major axis using an NSTARS' average of 21.11 ly) 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; and 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.)

Struve 1321 B

Struve 1321 B is a orange-red main sequence dwarf star of spectral and luminosity type K7-M0 Ve. This star has almost 60 to 73 percent of Sol's mass (Kyongae Chang, 1972); and RECONS estimate, respectively), around 65 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 B include: Gl 338 B, Hip 120005, HD 79211, G 195-18, BD+53 1321, SAO 27179, LHS 261, LTT 12426, LFT 635, Vys/McC 541 B, STF 1321 B, and ADS 7251 B. Although Star B is now known to be a flare star, it does not appear to have any special variable star designations.

Hunt for Substellar Companions

Since Struve 1321 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 Struve 1321 A where an Earth-type planet would be "comfortable" with liquid water is centered around only 0.175 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 about 34.5 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 about 30 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 Struve 1321 A or B) that normally has less than 21 percent of than Sol's luminosity would be more noticeable.

Arnold O. Benz, Institute of Astronomy, ETH Zurich

High resolution and jumbo images (Benz et al, 1998).

Struve 1321 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 0.2 AU for Struve 1321 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.