Mu Cassiopeiae 2

Mu Cassiopeiae A

This yellow-orange star of spectral and luminosity type G5 VIp appears to a subdwarf and a halo star. According to Drummond et al) (1995), Star A was found to have 74 percent of Sol's mass, 82 percent of its diameter, 46 percent of its bolometric luminosity, and to be around 10 billion years old based on a low estimate of helium content. Previous calculations based on older data indicated that Star A has at least 60 (+/- 20) percent of Sol's mass (Heintz and Cantor, 1994; and D.W. McCarthy, Jr., 1984), about 74 percent of its diameter, and around 42 of its visual luminosity. It is only 13 to 28 percent as enriched as Sol in elements heavier than hydrogen (metallicity), based on its abundance of iron (Cayrel de Strobel et al, 1991, page 6), and appears to be considerably older than Sol at about 7.9 billion years in age. (See an animation of the orbits of Stars A and B and their potentially habitable zones, with a table of basic orbital and physical characteristics.)

In the Sixth Catalog of Orbits of Visual Binary Stars, an erroneous "photocentric" semi-major axis appears to have been replaced with the correct "relative" value (Drummond et al, 1995; Heintz and Cantor, 1994; and Sarah Lee Lippincott, 1981). Star A appears to be separated from its companion by a semi-major axis of 7.61 AUs (a= 1.009 +/- 0.0016 arcseconds) and moving in a highly eccentric orbit (e=0.561 +/- 0.016) that takes 21.753 +/ 0.059 years to complete. Moving as close as 3.30 AUs but as far apart as 11.9 AUs, their orbit is inclined at 106.8° +/-0.9° from the perspective of an observer on Earth.

Mu Cassiopeiae A has a high velocity of motion within the galaxy. It has the New Suspected Variable designation NSV 405. Some useful catalogue numbers for this star are: Mu Cas, 30 Cas, HR 321, Gl 53 A, Hip 5336, HD 6582, BD+54 223, SAO 22024, FK5 1030, LHS 8, LTT 10460, and LFT 107.

NASA -- larger image
Mu Cassiopeiae B is probably older and bluer than
the red dwarf binary stars Gliese 623 A (M2.5V)
and B (M5.8Ve) -- at lower right.

Mu Cassiopeiae B

Detected through astrometric analysis and speckle interferometry, this dim red star (M V-VI) is probably a subdwarf like its primary (possibly M5 according to James Kaler). It appears to have only have 15 to 17 percent of Sol's mass (Drummond et al, 1995; Heintz and Cantor, 1994; and D.W. McCarthy, Jr., 1984); around 29 percent of its diameter; and only 0.62 percent of its bolometric luminosity.

Subdwarfs and Halo Stars

Subdwarfs, such as Mu Cassiopeiae 2, are more bluish than younger main-sequence dwarf stars and have a lower "metals" content of elements heavier than helium -- perhaps due to their birth in an earlier age (or region) of the galaxy when relatively few supernovae had as yet spewed their metals into surrounding dust clouds. Indeed, Mu Cassiopeiae A and B may have developed before the Milky Way's galactic disk was formed (Lippincott, 1981).

Most of the stars in the central bulge and in the globular clusters of the galactic halo are old, low metals stars, and halo stars account for only 0.1 to 0.2 percent of the stars near Sol. Halo stars are distributed somewhat spherically around the galactic core but most members of the halo lie far above or far below the galactic plane. Including the stars of the distant globular clusters, halo stars are among the galaxy's oldest, thought to be mostly 10 billion years and older. While halo stars are only very weakly concentrated towards the galactic plane, they exhibit a strong concentration towards and including the galactic nucleus but with highly eccentric orbits. As a group, these stars as a group show little if any net rotation around the galaxy, and so a halo member has a very negative V velocity ("retrograde direction"), since the Sun's motion around the galactic center is in the positive V direction.

Although subdwarfs contain a very low metals abundance relative to the sun (with a mean around two percent of Sol's), the Mu Cassiopeiae system's deficiency of metals is comparatively mild (Judith G. Cohen, 1968). While halo stars may only total as few as 0.1 percent of the stars in the solar neighborhood, they include local subdwarfs, Kapteyn's Star and Groombridge 1830 (a G8VIp with "superflares" that is now believe to be a single star -- without an M-type flare star companion). Also called Population II stars because of their later discovery, this group also includes RR Lyrae variables with periods greater than 12 hours, subdwarfs and other extremely metal-poor stars, and some red giants.

Hunt for Substellar Companions

At an average separation of 1.44 AUs (of a semi-major axis) but getting as close as 0.63 AU, this binary system has too tight and eccentric an orbit to support an Earth-type planet. Indeed, the distance from Mu Cassiopeiae A where an Earth-type planet would be "comfortable" with liquid water is centered around 0.68 AU -- just inside the orbital distance of Venus in the Solar System-- and would have an orbital period of about 176 days or less than half a year. However, the orbit of any developing planet at that distance from the star would likely have been disrupted by the gravitational pull of Mu Cassiopeiae B. For Star B, the liquid water zone would be centered around 0.079 AU with an orbital period would be around 19 days, but any planet in such a close orbit would be tidally locked. Astronomers would find it very difficult to detect Earth-sized planets around both stars using present methods.