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ę Torben Krogh & Mogens Winther,
(Amtsgymnasiet and EUC Syd Gallery,
student photo used with permission)
Groombridge 1618 is an orange-red
dwarf star similiar to, but redder and
much dimmer, than Epsilon Eridani
at left center of meteor. (See a 2MASS
Survey image of Groombridge 1618
from the NASA Star and Exoplanet
This star is located about 15.9 light-years (ly) away from our Sun, Sol, in the northwestern corner of (10:11:22.1+49:27:15.2, ICRS 2000.0) of Constellation Ursa Major, the Great Bear, which also encompasses the Big Dipper or Plow (Plough) -- southwest of Merak (Beta Ursae Majoris), north of Tania Australis and Borealis (Mu and Lambda Ursae Majoris), and east of Talitha (Iota Ursae Majoris). The star was listed by Stephen Groombridge (1755-1834), whose "A Catalog of Circumpolar Stars, Reduced to January 1, 1810" [see page 54] was published posthumously in 1838. Although Groombridge 1618 is brighter and more orange or hotter at the surface than most nearby flare stars (which are typically classified as red dwarfs), it has also been relatively less active.
Due to Groombridge 1618'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 Groombridge 1618 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.
Groombridge 1618 is a orange-red main sequence dwarf star of spectral and luminosity type K7 Vne, that has been classed as orange as K5 by the SIMBAD Astronomical Database (which may be using an older reference) and as red as M0. This star has about 64 percent of Sol's mass (RECONS estimate), 59 to 68 percent of its diameter (Pasinetti-Fracassini et al, 2001), and about 4.6 percent of its visual luminosity and 13.3 percent of its theoretical bolometric luminosity, correcting for infrared output (NASA Star and Exoplanet Database, derived using exponential formula from Kenneth R. Lang, 1980). The star appears to be about 191 percent as enriched as Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Cayrel de Strobel et al, 1991, page 290). Groombridge 1618 is chromospherically active with star spots (Vassiliki Tsikoudi, 1989). Its status as a flare star and its average equatorial rotational velocity speed of 1.7 to 2.1 (+/- 0.5 to 0.8) km (1.1 to 1.3 miles) per second (Marcy and Chen, 1992; page 553) suggest that it may be a relatively youthful star that is over a billion years old. It is a New Suspected Variable star designated NSV 4765. Useful star catalogue numbers for Groombridge 1618 include: Gl 380, Hip 49908, HD 88230, BD+50 1725, G 196-9, SAO 43223, LHS 280, LTT 12732, and LFT 696.
Since Groombridge 1618 is sort of like a distant cousin to Sol, some speculate whether it might just be bright enough to support Earth-type life on a planet lucky enough to orbit in its water zone. Estimates provided by the NASA Star and Exoplanet Database indicate that the inner edge of Groombridge 1618's habitable zone could be located around 0.354 AU from the star, while the outer edge edge lies around 0.691 AUs. The distance from the star where an Earth-type planet could have liquid water on its surface is centered around 0.523 AU -- between the orbital distances of Mercury and Venus in the Solar System. At that distance from the star and assuming that it has 0.64 Solar-mass, such a planet would have an orbital period of nearly 173 days (or close to half an Earth year).
Hunt for Substellar Companions
No stellar or brown dwarf companion of at least seven Jupiter-masses with an orbital period of less than four years has been detected around Groombridge 1618 (Marcy and Benitz, 1989). The star was included in a subsequent imaging search with adaptive optics found that "with 90% confidence no more than 50% of stars like those in our survey have a 5 MJup or more massive planet orbiting between 30 and 94 AU, no more than 15% have a 10 MJup planet orbiting between 22 and 100 AU, and no more than 25% have a 20 MJup object orbiting between 8 and 100 AU" (Heinze et al, 2010).
Life Around a Flare Star
Many dim, red (M) and some of the dimmer 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 Groombridge 1618) that normally has less than five percent of the luminosity of the Sun would be quite noticeable.
High resolution and jumbo images (Benz et al, 1998).
Groombridge 1618 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.
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 unlikely because their planets must be located very close to dim orange-red dwarfs to be warmed sufficiently by star light to have liquid water (about 0.22 AU for Groombridge 1618), 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 Groombridge 1618.
|Star System||Spectra &|
|BD+44 2051 A |
WX Ursae Majoris
|M1 Ve |
|Struve 1321 AB||K7-M0 Ve |
|GJ 1116 AB||M5.5 Ve |
|AD Leonis||M3 Ve||8.1|
|DX Cancri||M6.5 Ve||8.2|
|Lalande 21185||M2.1 Ve||8.3|
|AC+79 3888||M3.5 V||9.0|
Up-to-date technical summaries on these stars can be found at: the Astronomiches Rechen-Institut at Heidelberg's ARICNS, the NASA Star and Exoplanet 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.
Note: Nominated as a "notable nearby star" by Erik Allan Edler.
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