Groombridge 34

Groombridge 34 is a binary star system that consists of two red dwarf stars in a nearly circular orbit with a separation of about 147 astronomical units. Both stars in this pair exhibit variability due to random flares, and they have been given variable star designations. (The brighter member Groombridge 34 A is designated GX And, and the other member is designated GQ And).(Wikipedia)

Groombridge 34 is located about 11.6 light-years (ly) from our Sun, Sol, in the north central part of (Aab=00:18:22.89+44:01:22.63, B=00:18:25.7+44:01:44:C~, ICRS 2000.0) in Constellation Andromeda, the Chained Maiden -- northwest of the Andromeda galaxy (M 31) and two of its satellite galaxies (M32 and M110). This binary star system was listed by Stephen Groombridge (1755-1834), whose "A Catalog of Circumpolar Stars" was published posthumously in 1838, but its large proper motion was not discovered and measured until 1860. Both visual members of the system are flare stars that have been given the variable star designations GX and GQ Andromedae.(Solstation)

Sources:

• http://www.solstation.com/stars/groomb34.htm
• http://en.wikipedia.org/wiki/Gl_15_Star_System
• Lippincott, S. L., "Parallax and orbital motion of the 2 nearby long-period visual binaries Groombridge 34 and ADS 9090", 1972, Astronomical Journal, 77, 165.

Luyten 726-8 AB (UV Ceti)

"Luyten 726-8 AB is the sixth closest system to Sol. The system is located only about 8.7 light-years away in the southwestern part (01:39:01-17:57.0:C~, ICRS 2000.0) of Constellation Cetus, the Whale -- northeast of Deneb Kaitos or Diphda (Beta Ceti). However, this binary system of red dwarf stars is too faint to be seen with the naked eye. The stars were discovered in 1949 by Willem Jacob Luyten (1899-1994), who found the proper motions of over 520,000 stars despite the loss of sight in one eye since 1925 by building an automated photographic plate scanner and measuring machine. Although both stars have been found to be flare stars, the fainter member of the system is considered to be an extreme example; it has been given the variable star designation UV Ceti but is also referred to as "Luyten's Flare Star."(Solstation)

Credit:Arnold O. Benz, Institute of Astronomy, ETH Zurich

UV Ceti (L 726-8 B) A flaring UV Ceti was resolved as at least two evolving spatial components that were separated by four to five stellar radii, in this VLBA/VLA radio image at 3.6 cm wavelength. The components changed in appearance over about six hours of observation but stayed aligned along the axis of the binary orbit, as shown by the arrow.

UV Ceti (L 726-8 B) as the dimmer of the pair is main sequence red dwarf star (M6.0 Ve) may have only 10 percent of Sol's mass (Geyer et al, 1988; and RECONS estimate), 14 percent of its diameter (Johnson and Wright, 1983, page 649), and less than 4/100,000th of its luminosity. However, 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 more slowly back down to normal luminosity within two or three minutes before flaring suddenly again after several hours. In 1952, UV Ceti was observed flaring to 75 times its normal brightness in only 20 seconds.(Solstation)

Sources:

• http://www.solstation.com/stars/luy726-8.htm

Tau Ceti

Tau Ceti star is located only about 11.9 light-years (ly) away. It lies in the south central part (01:44:04.08-15:56:14.93, ICRS 2000.0) of Constellation Cetus, the Whale -- southwest of Baten Kaitos (Zeta Ceti) and northeast of Deneb Kaitos or Diphida (Beta Ceti). While smaller and cooler than our own Sun, Sol, Tau Ceti is somewhat more like a sister star than nearby Epsilon Eridani. In Earth's night sky, it is clearly visible to the naked eye. In July 2004, astronomers announced that they had imaged a relatively large and dense disk of cold dust around this star (further discussion below -- RAS press release). As Tau Ceti has become one of the top 100 target stars for NASA's planned Terrestrial lanet Finder (TPF), images of this star and its position relative to the Milky Way in Earth's night sky are now available from the TPF-C team.(Solstation>

A relatively dense and large, cold dust disk of cometary and asteroid material has been detected around Tau Ceti . SCUBA Image: Image of the disc of dust particles around the star Tau Ceti, taken with the submillimetre-wavelength camera SCUBA. The false colours show the brightness of the disc. Its diameter is slightly larger than the Solar System. Credit: James Clerk Maxwell Telescope

Tau Ceti is a main sequence, yellow-orange dwarf (G8 Vp) that may be as much as 10 billion years old. It has about 81 to 82 percent of Sol's mass, around 77 percent its diameter (Pijpers et al, 2003), but only 59 percent of its luminosity (Saumon et al, 1996, page 17). The star does not appear to be as enriched as Sol in elements heavier than hydrogen ("metals") because it has only 22 to 74 percent of Sol's abundance of iron (Cayrel de Strobel et al, 1991, page 6). Dust has been detected around Tau Ceti, as has been found in the Solar System (Kuchner et al, 1998 -- in pdf). There also may be an optical companion star seen in telescopes that is not actually bound by gravity to Tau Ceti, and the star does not appear to have a dim stellar or substellar companion based on astrometric measurements (Lippincott and Worth, 1980) or radial velocity variations (Campbell et al, 1988). (Solstation)

Tau Ceti, only 12 light years away, is the nearest sun-like star and is easily visible without a telescope. It is the first star to be found to have a disk of dust and comets around it similar in size and shape to the disk of comets and asteroids that orbits the Sun. But the similarity ends there explains Jane Greaves, Royal Astronomical Society Norman Lockyer Fellow and lead scientist: 'Tau Ceti has more than ten times the number of comets and asteroids that there are in our Solar System. We don't yet know whether there are any planets orbiting Tau Ceti, but if there are, it is likely that they will experience constant bombardment from asteroids of the kind that is believed to have wiped out the dinosaurs. It is likely that with so many large impacts life would not have the opportunity to evolve.'(Joint Astronomy Centre, 2004)

Sources:

• http://www.solstation.com/stars/tau-ceti.htm
• http://outreach.jach.hawaii.edu/pressroom/2004-tauceti/

Epsilon Eridani

This star is located only about 10.5 light-years (ly) away in the northeastern part (03:32:55.84-09:27:29.74, ICRS 2000.0) of Constellation Eridanus, the River -- west of Rana (Delta Eridani) and northwest of Zaurak. Somewhat smaller and cooler than our own Sun, Sol, Epsilon Eridani is also less luminous. In Earth's night sky, however, it is clearly visible to the naked eye as the third closest star viewable without a telescope. On August 7, 2000, astronomers announced the discovery (a possible confirmation of earlier detections) of a Jupiter-like planet around this Sun-like star (press release -- details below). On October 9, 2006, a team of astronomers (led by G. Fritz Benedict and Barbara E. McArthur) working with the Hubble Space Telescope announced "definitive evidence" for the existence of a Jupiter-class planet around Epsilon Eridani using astrometic measurements (NASA press release -- more below). (Solstation)

This main sequence, orange-red dwarf (K2 V) is a relative young star that may be only around 660 to 800 million years old, within the range between 500 million to a billion years old (Benedict et al, 2006; and Saffe et al, 2005). It may have about 83 (+/- 0.05) to 85 percent of Sol's mass (Benedict et al, 2006; and RECONS), 84 percent of its diameter (Johnson and Wright, 1983, page 653), but only about 27.8 percent of its luminosity (Saumon et al, 1996, page 17). The European Space Agency has used ultraviolet spectral flux distribution data to determine stellar effective temperatures and surface gravities, including those of Epsilon Eridani.(Solstation)

The measured effective temperature for Epsilon Eridani was 5500K

On October 9, 2006, a team of astronomers (led by G. Fritz Benedict and Barbara E. McArthur) working with the Hubble Space Telescope announced "definitive evidence" for the existence of a Jupiter-class planet around Epsilon Eridani using astrometic measurements combined with those made at the University of Pittsburgh's Allegheny Observatory (NASA press release). Coming astrometric with radial-velocity measurements to determine the tilt of the planet's orbit, the astronomers were able to estimate the "true mass" of planet b as about 1.55 +/- 0.24 times the mass of Jupiter. Located at an average distance (semi-major axis) of 3.39 +/- 0.36 AUs from Epsilon Eridani, the planet takes about 6.9 (6.85 +/- 0.03) years to complete its highly eccentric orbit (e= 0.702 +/- 0.039). Approaching as close as 2.4 and as far as 5.8 AUs from its host star, the planet's orbit is also tilted about 30.1 +/- 3.8 degrees from Earth's line of sight (Benedict et al, 2006). Although the planet's orbit takes it so far from Epsilon Eridani that oceans on any moons would freeze, life could potentially survive on a moon if it is massive enough to hold onto a dense heat-trapping atmosphere like Saturn's moon, Titan, according to astronomer G. Fritz Benedict. Astronomers hope to image the planet in 2007, when its orbit is closest to Epsilon Eridani and may reflect sufficient starlight for an image.(Solstation)

Planet "b" moves around Epsilon Eridani within its dusk disk at about the same orbital plane (more). Credit:Greg Bacon, Benedict et al, STScI, ESA, NASA

Sources:

• http://www.solstation.com/stars/eps-erid.htm
• http://ines.laeff.esa.es/Ines_PCentre/Demos/Fluxdist/eeridani.html
• http://hubblesite.org/newscenter/archive/releases/2006/32/