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This dim star is the seventh closest to our Sun, lying at about 9.7 light-years (ly) away. It is located in the eastern part (18:49:49.36-23:50:10.44, ICRS 2000.0) of Constellation Sagittarius, the Archer -- northeast of Kaus Borealis (Lambda Sagittarii). However, Ross 154 is too faint to be seen by the naked eye. The star was discovered in 1925 by Frank Elmore Ross (1874-1960), who also took the first good infrared and ultraviolet photographs of Venus in 1923. Ross first reported on this star in his "Second List of New Proper-Motion Stars," Astronomical Journal (36:856).
Due to Ross 154's proximity to Sol, the star has been an object of intense interest among astronomers. It was selected as a "Tier 1" target star for NASA's optical Space Interferometry Mission (SIM) to detect a planet as small as three Earth-masses within two AUs of its host star (and so some summary system information and images of Ross 154 may still be available from the SIM Teams), but the SIM project manager announced on November 8, 2010 that the mission was indefinitely postponed due to withdrawal of NASA funding. Astronomers were also hoping to use the ESA's Darwin group of infrared interferometers to analyze the atmospheres of any rocky planet found in the "habitable zone" (HZ) around Barnard's Star for evidence of Earth-type life (Lisa Kaltenegger, 2005), until it was indefinitely postponed .
Medialab, ę ESA
Larger illustration of
the Darwin Mission.
Astronomers have identified
Ross 154 as a prime target
for NASA's optical SIM and the
ESA's infrared Darwin missions,
now both indefinitely postponed.
This cool and dim, main sequence red dwarf (M3.5 Ve) has around 17 percent of Sol's mass (RECONS estimate and NASA Star and Exoplanet Database, derived from Henry and McCarthy, 1993), 19-24 percent of Sol's diameter (Turon et al, 1992; Johnson and Wright, 1983, page 693; and NASA Star and Exoplanet Database, derived from Kenneth R. Lang, 1980), less than 5/10,000th of Sol's visual luminosity, and 0.4 percent of its bolometric luminosity (NASA Star and Exoplanet Database, derived from Kenneth R. Lang, 1980). With less than 20 percent of Sol's mass, Ross 154 is so small that it can transport core heat to its surface only through convection, unlike larger red dwarf stars like Gliese 752 A -- also known as Wolf 1055 A or Van Biesbroeck's Star (more). The star has only around 56 percent of Sol's metallicity based on the concentration of iron relative to hygrogen (Wargelin et al, 2008; and Olin J. Eggen, 1996).
Jeffrey L. Linsky,
Like Gliese 752 B, Ross 154 is so small,
with less than 20 percent of Sol's mass,
that it can transport core heat only through
convection, unlike larger larger red dwarf
stars like Gliese 752 A (more).
Ross 154 would be only one of many unremarkable stars except that it appears to be a flare star, with X-ray emission comparable or greater than that of Sol (Wargelin et al, 2008), as well as one of Sol's closest neighbors. This is not so surprising, as the star is a "moderating fast" rotator and relatively young at less a billion years old (Wargelin et al, 2008, from Johns-Krull and Valenti, 1996). Its variable star designation is V1216 Sagittarii. Useful catalogue numbers include: V1216 Sgr, Gl 729, Hip 92403, AC-24 2833-183, CD-23 14742, Hip 92403, LPM 688, LFT 1437, LTT 7462, LHS 3414, and 2MASS J18494929-2350101.
With a spectral type of M3.5, Ross 154 can be used as a rough proxy for Wolf 1055 A (M3.5 V). According to one type of model calculations performed for the NASA Star and Exoplanet Database, the inner edge of Ross 154's habitable zone should be located very close to the star, at around 0.065 AU from the star, while the outer edge lies around 0.126 AUs. Accounting for the great infrared output of M-stars like Ross 154, the equivalent orbital distance for an Earth-type planet be only around 0.096 AUs. At slightly a fourth of Mercury's orbital distance in the Solar System, however, the rotation of the planet could become tidally locked with the star so that one side would have perpetual daylight with the other in darkness. Assuming that Ross 154 has about 17 percent of Sol's mass, a small Earth-type rocky planet would complete its orbit the star in about 26 days.
High resolution and jumbo images (Benz et al, 1998).
Ross 154 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.
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 the 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 Ross 154) 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 planets must be located very close to dim red dwarfs to be warmed sufficiently by star light to have liquid water (about 0.022 AU with an orbital period of about 2.9 days for Ross 154), 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 and Dust Disks
Searches for faint companions using the Hubble Space Telescope and Palomar 200-inch Telescope failed to find supporting evidence for a large Jupiter or brown dwarf sized object (Tanner et al, 2010; and Schroeder et al, 2000). Efforts to detect dust at 11.7 microns were unsuccessful, but astronomers were optimistic about future efforts (Plavchan et al, 2005).
The following star systems are located within 10 light-years of Ross 154.
|Star System||Spectra &|
|Barnard's Star||M3.8 V||5.5|
|Lacaille 8760||K7-M2 Ve||7.4|
|CD-46 11540||M2.5-3 V||7.7|
|BD-12 4523 AB||M3.0 V |
|Alpha Centauri AB||G2 V |
|Proxima Centauri||M5.5 Ve||8.2|
|CD-44 11909||M3.5-5 V||8.7|
|Epsilon Indi||K3-5 Ve||8.9|
|70 Ophiuchi 2?||K0-1 Ve |
|Lacaille 9352||M0.5 Ve||9.6|
|EZ Aquarii3||M5.0-5.5 Ve |
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, the Research Consortium on Nearby Stars (RECONS) list of the 100 Nearest Star Systems, and the SIMBAD Astronomical Database. Additional information may be available at Roger Wilcox's Internet Stellar Database.
Sagittarius is Latin for "archer," often represented as a centaur wielding a bow and arrow since ancient times. The constellation also contains the Great Sagittarius Star Cloud, where a vast milky swarm of millions of stars mark the way to the center of the galaxy. For more information about the stars and objects in this constellation and an illustration, go to Christine Kronberg's Sagittarius. For another illustration, see David Haworth's Sagittarius.
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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