Gliese 105 / HR 753 ABC
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Larger and whole field images,
with Star C at upper right.
The Gliese 105 system has
an orange-red (K3) and two
red dwarf stars (M3.5 and
M7). Star C is near the mass
minimum of 0.08 Solar for
a star (more from STScI and
Astronomy Picture of the Day).
Also known as HR 753 for its relative bright primary, this triple star system is located 23.5 (ly) away from Sol. It lies in the northern edge (2:36:4.9+6:53:12.7 for Star A, and 2:36:15+6:52.3 for Star B, and 2:35:58.8+6:52.0 for Star C, ICRS 2000.0) of Constellation Cetus, the Whale -- northwest of Mira (Omicron Ceti) and Baten Kaitos (Zeta Ceti), north of Theta Ceti, and southwest of Alrescha (Alpha Piscium). Many astronomers now refer to this star system by its designation in the famous Gliese Catalogue of Nearby Stars (CNS, now ARICNS database) of Wilhelm Gliese (1915-93), who was a longtime astronomer at the Astronomiches Rechen-Institut at Heidelberg (even when it was at Berlin).
As a relatively bright star in Earth's night sky, Star A is catalogued as Harvard Revised (HR) 753, a numbering system derived from the 1908 Revised Harvard Photometry catalogue of stars visible to many Humans with the naked eye. The HR system has been preserved through its successor, the Yale Bright Star Catalogue -- revised and expanded through the hard work of E. Dorrit Hoffleit and others. (More discussion on star names and catalogue numbers is available from Alan MacRobert at Sky and Telescope and from Professor James B. Kaler's Star Names.)
Larger discovery image of Star C, with
dark circular coronagraphic stop at center
of Gl 105 A for suppressing light scatter.
Star C (protruding yellow spot at upper right)
was confirmed as Gl 105 A's fainter companion
with Palomar's 60" telescope (more).
Star A was found to have a distant, proper motion companion "B" by Adriaan van Maanen (1884-1946) before March 1938 (object Number 3 in Table 2 on page 33). However, a second, closer but unseen companion "C" to Star A had been on the astrometric program at Sproul Observatory (Swarthmore College) since 1937 (Sarah Lee Lippincott, 1973). In 1994, David Golimowski and collaborators at the Palomar Observatory in California visually detected the third stellar member to Gliese 105 (Golimowski et al, 1995; and May, 1995). While Star C is a much closer stellar companion of Star A than Star B, it apparently lies near the theoretical minimum mass limit of 0.75 to 0.80 Solar for core fusion of hydrogen (more from Palomar Adaptive Optics System Results). (See an animation of the orbits of Stars A and C and their potentially habitable zones, with a table of basic orbital and physical characteristics.)
Star A is a main sequence, orange-red dwarf of spectral and luminosity type K3 V. It may have around 81 percent of Sol's mass, 85 percent of its diameter, and only 21 percent of its luminosity. Some alternative useful star catalogue numbers for this star are: HR 753* A, Gl 105 A, Hip 12114, HD 16160, BD+06 398, SAO 110636, FK5 1073, G 73-70, G 76-11, LHS 15, LTT 10858, and LFT 217.
© Torben Krogh & Mogens Winther,
(Amtsgymnasiet and EUC Syd Gallery,
student photo used with permission)
Gliese 105 A is an orange-red dwarf
star, like Epsilon Eridani at left
center of meteor. (See a Digitized
Sky Survey image of Gl 105
(HR 753) from the Nearby Stars
Star A's late spectral type and dim luminosity puts it possibly close to the lower limit of habitability for (multicellular) Earth-type plant and animal life, given the redness of its light and the increased risk of tidal locking from the closeness of the orbit necessary for liquid water on a planetary surface. The distance from Star A where an Earth-type planet would be "comfortable" with liquid water is centered around 0.45 AU -- within the orbital distance of Mercury in the Solar System. At that distance from the star, such a planet would have an orbital period of about 124 days, or around a third of an Earth year.
A 12-magnitude star "B" was discovered to share the same common proper motion with Star A around 1938 (Adriaan van Maanen, 1938). Stars A and B, however, have a wide separation of some 1,200 AUs -- 165" at a HIPPARCOS distance estimate of 23.5 ly (Golimowski et al, 2000, in ps; Heintz and Cantor, 1994; Phillip A. Ianna, 1992; Martin and Ianna, 1975; and Sarah Lee Lippincott, 1973). On the other hand, Star C is currently separated from the AB pair by only about 24 AUs (3.39"). The new Sixth Catalog of Orbits of Visual Binaries presents outdated 1994 orbit estimates for Stars A and C.
Relative astrometric measurements taken over three years of the period of Star A's orbital perturbation suggest that Star C's orbit must have a high eccentricity (e= 0.75) and a semi-major axis of 15 AUs, to satisfy both the observed orbital motion and Gl 105A's astrometric period. Measurements of Gl 105A's radial velocity over 12 years show a linear trend and slope which is consistent with these orbital constraints and a nearly face-on orbit. No other stellar or brown dwarf sized objects have been detected that can cause the observed perturbation. (Golimowski et al, 2000, in ps).
NASA -- larger image
Gliese 105 B and C are dim red dwarf stars, like
Gliese 623 A (M2.5V) and B (M5.8Ve) at lower right.
Gliese 105 B or BX Ceti
This star is a red dwarf of spectral and luminosity type M3.5 Vn. It has about 21 percent of Sol's mass, 28 percent of its diameter, and 1/1000th of its brightness. The SIMBAD Astronomical Database identifies Star B as a variable star with the designation BX Ceti. Despite its variability, the star has "anomalously low" chromospheric and coronal activity (Doyle et al, 1998). Some alternative useful star catalogue numbers for the star are: BX Ceti, HR 5568 B, Gl 105 B, HD 16160 B, BD+06 398 B, G 73-71, G 76-12, LHS 16, LTT 10859, LFT 218, Vys or McC 396, and Rob 153.
For Star B, the water-zone orbit lies around 0.031 AU. As a result, a planet in such an orbit would have a period of only about 4.3 days. Tidal locking of such a closely orbiting planet would resulting in perpetual day on one side and perpetual night on the other.
An extremely dim red dwarf, Star C is of spectral and luminosity type M7 V with only about 8.2 percent of Sol's mass, (Golimowski et al, 2000, in ps; and 1995). In addition, Star C may have a diameter not much larger than Jupiter's and only around 7.5 one-millionth of its visual luminosity. Some alternative useful star catalogue numbers for the star are: HR 753 C and Gl 105 C.
For Star C, the water zone orbit lies only around 0.0027 AU, even closer than for Star B. A planet in such an orbit would have a period of only around 14 hours. Again, tidal locking of such a closely orbiting planet would resulting in perpetual day on one side and perpetual night on the other. (See an animation of the orbits of Stars A and C and their potentially habitable zones, with a table of basic orbital and physical characteristics.)
Hunt for Planetary Companions
Direct images of Gl 105A obtained with the Hubble Space Telescope (HST) and radial velocity analysis have failed to reveal substellar companions as faint and small as brown dwarfs lying within at least 10 AUs of the star (Golimowski et al, 2000, in ps; Schroeder et al, 2000; and Cummings et al, 1999). Hence, it is possible that small, terrestrial planets exist in inner orbits around each of the three stars of the Gliese 105 system. However, astronomers would find it very difficult to detect Earth-type planets around these stars using present methods.
The following star systems are located within 10 ly of Gliese 105.
|Star System||Spectra &|
|LP 469-206||M V||5.6|
|LP 469-67||M V||7.1|
|107 Piscium||K1 V||7.8|
|AC+25 7918||M3 V||7.9|
|Kappa Ceti||G5 Ve||8.2|
|L 1159-16||M4.5 Ve||9.6|
|L 730-18 ABC||M3 V |
Up-to-date technical summaries on this star can be found at: the Astronomiches Rechen-Institut at Heidelberg's ARICNS for Star A, Star B, and Star C, and the Nearby Stars Database. Additional information may be available at Roger Wilcox's Internet Stellar Database.
Constellation Cetus may be difficult to see from latitudes as far north as Scandinavia. In Greek mythology, Cetus is supposed to be the sea monster that would have devoured the "chained maiden," Andromeda, if Perseus had not come to the rescue. For more information on stars and other objects in Constellation Cetus, go to Christine Kronberg's Cetus. For another illustration, see David Haworth's Cetus.
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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