Struve 2398 AB

NASA -- larger image

Struve 2398 AB are probably dim red dwarf stars, like
Gliese 623 A (M2.5V) and B (M5.8Ve) at lower right.
(See a Digitized Sky Survey field image around
Struve 2398 at the Nearby Stars Database.)

System Summary

This system is located about 11.4 light-years (ly) away from our Sun, Sol. It lies in the northeastern part (18:42:46.69+59:37:49.42, ICRS 2000.0) in Constellation Draco, the Dragon -- northeast of Grumium (Xi Draconis) and southwest of Altais or Nodus Secundus (Delta Draconis). Struve 2398 is too dim to be seen with the naked eye.

There are two stars, Struve 2398 A and B, that orbit each other at an "average" distance of about 56 times the Earth-Sun distance -- 42 astronomical unit (AUs) of an orbital semi-major axis -- which is roughly Pluto's orbital distance in the Solar System. The separation of the two components was first measured in 1832 by Friedrich Georg Wilhelm von Struve (1793-1864), who became director of Russia's Dorpat Observatory in 1817 and founded and directed the Pulkovo Observatory in 1837, surveyed 120,000 stars from 1819 to 1827, published an extensive monograph of Halley's Comet based on observations in 1835 and his findings on 2,640 double stars in 1837, and measured the parallax of Vega from 1835 to 1838. While now often named in his honor, Struve's stars were originally numbered after the Greek letter "Sigma," so that this binary pair was designated as Sigma 2398.

Due to Struve 2398 AB's proximity to Sol, the system has been an object of high interest among astronomers. Stars A and B have 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. (Some summary system information and images of Star A and Star B are available from the SIM Teams).

JPL, CalTech, NASA

Larger illustration of NASA's
Space Interferometry Mission.

Astronomers have identified
Struve 2398 A and B as prime targets
for NASA's optical SIM mission.

Struve 2398 A

This star is probably a main-sequence red dwarf star of spectral and luminosity type M3.0 V, although it is still recorded as a K5 in some catalogues. The star has about 36 percent of Sol's mass, 54 percent of its diameter, and 27/10,000th of its luminosity. However, Struve 2398 A has been given the New Suspected Variable designation of NSV 11288.

Although the star and its stellar companion B are known to have an eccentric orbit, the specific orbital elements may still be uncertain. In 1958, Wilhelm F. Rabe (1893-1958) derived a period of 346 years with an average distance of 42 AUs (based on updated parallax) and an eccentricity of 0.55, so that the stars swing between 19.0 and 65.3 AUs apart. Using photographic plates from 1918 to 1987, however, (Wulff Dieter Heintz, 1987) subsequently calculated a period of 408 years with an eccentricity of 0.53, and an average distance of 56 AUs where the pair swings between 26 and 86 AUs apart. (See an animation of the orbits of the two stars and their potentially habitable zones, with a table of basic orbital and physical characteristics.)

Accounting for infrared heating, the distance from Struve 2398 A where an Earth-type planet would be "comfortable" with liquid water is centered around only 0.17 AU. At that distance from the star, such a planet would have an orbital period of around 98 days, and so it may become tidally locked in orbit so that one side stays in perpetual daylight while the other lies in darkness. Useful star catalogue numbers for Struve 2398 A include: ADS 11632, Gl 725 A, Hip 91768, HD 173739, BD+59 1915 A, SAO 31128, G 227-46, G 229-13, LHS 58, LTT 15522, LDS 1466, and LFT 1431.

Struve 2398 B

This star is probably a main-sequence red dwarf star of spectral and luminosity type M3.5 V, although it is still recorded as a K5 in some catalogues. It has about 30 percent of Sol's mass, 55 percent of its diameter, and about 13/10,000th of its luminosity. Struve 2398 B is a flare star.

Arnold O. Benz, Institute of Astronomy, ETH Zurich

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

Struve 2398 B is a flare star, 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.

Accounting for infrared heating, the distance from Struve 2398 B where an Earth-type planet would be "comfortable" with liquid water is centered around only 0.15 AU. At that distance from the star, such a planet would have an orbital period of only around 39 days and so would likely be tidally locked with perpetual daylight on one side. Useful star catalogue numbers for Struve 2398 B include: Gl 725 B, Hip 91772, HD 173740, BD+59 1915 B, SAO 31129, G 227-47, and LHS 59.

Life Around a Flare Star

Many dim, red (M) 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 our Sun every so often, the amount of energy released in a solar flare is small compared to the total amount of energy Sol produces. However, a flare the size of a solar flare occurring on a red dwarf star (such as Struve 2398 B) that is more than ten thousand times dimmer than our Sun would emit about as much or more light as the red dwarf itself, doubling its brightness or more.

Flare stars erupt sporadically, with successive flares spaced anywhere from an hour to a few days apart. A flare only takes a 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 unlikely because their planets must be located very close to dim red dwarfs to be warmed sufficiently by star light to have liquid water (about 0.038 AU for Struve 2398 B), which makes flares even more dangerous around such stars. In any case, the light emitted by red dwarfs may be too red in color for Earth-type plant life to perform photosynthesis efficiently.

Hunt for Substellar Companions

A recent search for faint companions using the Hubble Space Telescope found no supporting evidence for a large Jupiter or brown dwarf sized object (Schroeder et al, 2000).

Closest Neighbors

The following star systems are located within 10 light-years of Struve 2398 AB.

Star System	Spectra & Luminosity	Distance (light-years)
BD+68 946 AB	M3.0 V ?	4.2
V1581 Cygni 2	M5.5 Ve ?	5.8
61 Cygni 2	K3.5-5.0 Ve K4.7-7.0 Ve	6.1
Kruger 60 AB	M3 V M4V	6.2
Sigma Draconis	K0 V	7.9
LP 44-113	DXP9/VII	9.2
Barnard's Star	M3.8 V	9.5
Hip 85605	?	9.8
Groombridge 34 Aab,B	M1.3 V ? M3.8 Ve	9.9

Other Information

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 Nearby Stars 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 Draco is associated with the dragon slain by Cadmus, the brother of Europa. It is a large and extended constellation of the northern hemisphere and is one of the few constellations which really resemble the object they were named after. For more information about the stars and objects in this constellation and an illustration, go to Christine Kronberg's Draco. For another illustration, see David Haworth's Draco.

For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.