Eta Cassiopeiae (Achird) 2
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Also called Achird, Eta Cassiopeiae (Eta Cas) is located about 19.4 light-years (ly) from our Sun, Sol. It can be found in the central part (00:49:06.29+57:48:54.67, ICRS 2000.0) of Constellation Cassiopeia, the Lady of the Chair -- northeast of Schedar (Alpha Cassiopeiae) and southwest of Mu Cassiopeiae. This well known binary star system was discovered in 1779 by Sir William Friedrich Wilhelm Herschel (1738-1822, portrait), who subsequently discovered the planet Uranus in 1781 -- which led to his appointment in 1782 as private astronomer to the King of England.
Due to Eta Cassiopeiae's relative proximity and similarity of spectral type to Sol, the star has been an object of intense interest among astronomers. Eta Cassiopeiae became one of the top 100 target stars for NASA's proposed Terrestrial Planet Finder (TPF), but the project has been postponed indefinitely. It was also 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 Eta Cassiopeiae A and B 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 have identified Eta Cas A
as a prime target for the Terrestrial Planet
Finder (TPF), and the Space Interferometry
Mission (SIM), now both indefinitely postponed.
AB Binary Star System
According to a 1969 article by Kaj Aage Gunnar Strand (1907-2000; obit), Star A and B are separated by an "average" distance of 71 times the Earth-Sun distance (AU) (of a semi-major axis). They move in an eccentric orbit (e= 0.497) of about 480 years, so that the two stars get as close as 36 AUs and as far away as 107 AUs. Their orbit is inclined by almost 35 degrees from Earth's line of sight. Based on chromospheric activity and rotational period alone, both stars may be around 2.9 and 5.8 billion years old (Mamajek et al, 2008, Table 13). Star A, however, is no longer thought to have a spectroscopic binary companion with a nine-day orbital period (Morbey and Griffin, 1987; and Helmut Abt, 1987).
|AB Mass Center||0.0||...||...||...||...||...||...||...||...|
|Inner H.Z. Edge A?||0.90||0.81||0||34.76||...||...||...||...||...|
|Outer H.Z. Edge A?||1.80||2.29||0||34.76||...||...||...||...||...|
|Inner H.Z. Edge B?||0.59||0.59||0||34.76||...||...||...||...||...|
|Outer H.Z. Edge B?||1.18||1.65||0||34.76||...||...||...||...||...|
The main sequence yellow-orange primary (G3 V) may have about 90 to 111 percent of Sol's mass (RECONS; Allende Prieto and Lambert, 1999; and Daniel M. Popper, 1980), almost the same diameter -- 98 to 101 percent (Johnson and Wright, 1983, page 647; and Daniel M. Popper, 1980; or Petr Harmanec, 1988), and 1.2 times its luminosity. It is about 63 to 68 percent as enriched as Sol in elements heavier than hydrogen (metallicity), based on its abundance of iron (Cayrel de Strobel et al, 1991, page 5). Some useful catalogue numbers for this star are: Eta Cas, 24 Cas, HR 219, Gl 34 A, Hip 3821, HD 4614, BD+57 150, SAO 21732, LHS 123, LTT 10287, LFT 74, Wolf 24, Struve 60, and ADS 671 A.
Estimates provided by the NASA Star and Exoplanet Database indicate that the inner edge of Eta Cassiopeiae's habitable zone could be located around 0.90 AU from the star, while the outer edge edge lies around 1.80 AUs. The distance from Star A where an Earth-type planet could have liquid water on its surface is centered around 1.35 AU -- between Earth's and somewhat short of Mars' orbital distance of 1.5 AUs in the Solar System. At that distance from Star A and assuming that it has 1.1 Solar-mass, such a planet would have an orbital period of just under 1.5 years.
This cooler and dimmer, main sequence orange-red dwarf star (K7 V) may have 56 to 60 percent) of Sol's mass (RECONS; and Daniel M. Popper, 1980), 66 percent of its diameter (Johnson and Wright, 1983, page 647), and only around three percent of its luminosity. Radial velocity variations have been detected (Andrei A. Tokovinin, 1992). Useful catalogue numbers for this star include: Gl 34 B, LHS 122, and ADS 671 B.
With a spectral type of K7, 41 Arae B can be used as a rough proxy for Eta Cass B (K1). According to calculations performed for the NASA Star and Exoplanet Database, the distance from 41 Arae B where an Earth-type rocky planet may have liquid water on its surface has been estimated to be between 0.593 and 1.176 AU -- between the orbital distances of Mercury and Earth in the Solar System. In that distance range from the star, such a planet would have an orbital period shorter an Earth year. For an Earth-type planet, the orbital distance where it would have liquid water zone on its surface would be around 0.884 AU, where the orbital period would be 392 days (1.073 years) if the star actually does have around 60 percent of a Solar-mass.
An attempt to find large planets from December 1986 to February 1987 failed to detect large periodic variations in radial velocities (McMillan and Smith, 1987; more discussion at Hatzes et al, 2004). A subsequent search ruled out close-orbiting giant planets and similar objects at least as large as 0.878 Jupiter-mass in circular orbits within three AUs of Star A (Wittenmyer et al, 2006, Table 5).
The following star systems are located within 10 ly of Eta Cassiopeiae 2.
|Star System||Spectra &|
|BD+56 2966||K3 V||4.9|
|Mu Cassiopeiae 2||G5 IV-VIp|
|EV Lacertae||M3.5 Ve||7.8|
|Kruger 60 AB||M3 V |
|Groombridge 34 Aab,B||M3.3 V |
|Stein 2051 AB||M4 V |
Up-to-date technical summaries on this star can be found at: the Astronomiches Rechen-Institut at Heidelberg's ARICNS for Star A and Star B, 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.
With its stars shaped in a "W," this northern constellation was named by the Ancient Greeks for the mother of Andromeda who claimed to be more beautiful than the daughters of Nereus, a god of the sea. Cassiopeia's vanity so angered the sea god Poseidon that he had Andromeda chained to a rock of the coast as a sacrifice for Cetus (the monstrous whale) until Perseus rescued her. For more information on stars and other objects in this Constellation and a photograph, go to Christine Kronberg's Cassiopeia. For an illustration, see David Haworth's Cassiopeia.
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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