65 Ursae Majoris

65 Ursae Majoris, abbreviated as 65 UMa, is a star system in the constellation of Ursa Major. With an apparent magnitude of about 6.5, it is at the limit of human eyesight and is just barely visible to the naked eye. Trigonometric parallax measurements made by the Hipparcos spacecraft put it at a distance of about 690 light years (210 parsecs); this is in close agreement with the dynamical parallax value of .
65 Ursae Majoris is a sextuple star system. It contains six stars in a hierarchical orbit where each star orbits the central system. Such systems are fairly uncommon, with only a few sextuple stars known. Higher-multiplicity star systems are uncommon because they are less stable than their simpler counterparts, and often decay into smaller systems.
Multiplicity.
Hierarchy of orbits in the 65 Ursae Majoris system
The central pair of stars are both A-type main-sequence stars. These are relatively bright, white-colored stars that typically have masses from to . 65 Ursae Majoris Aa1 and Aa2 both have relatively low masses for an A-type main sequence star and have spectral types of A7V. Both components periodically pass in front of each other while orbiting, so they form an eclipsing binary with the variable star designation DN Ursae Majoris. Therefore, the apparent magnitude of the system varies between 6.6 and 6.7 magnitudes. Its orbital period of 1.73 days.
The eclipsing binary pair 65 Ursae Majoris Aa is orbited by another star, designated 65 Ursae Majoris Ab. It is a spectroscopic binary: while the pair cannot be resolved, periodic Doppler shifts in their spectra indicate that there must be orbital motion. 65 Ursae Majoris orbits the inner pair with a period of 641 days (1.76 years) and an eccentricity of 0.169.
65 Ursae Majoris B orbits the three every 118 years. It is separated from the triple by 0.18″, and an astrometric orbit has been calculated. 65 Ursae Majoris C and D are common proper motion companions and are separated 4″ and 63″ away from the central system, respectively. 65 Ursae Majoris D also appears to be a chemically peculiar star with higher amounts of chromium, strontium, and europium than normal. Because of its unusual composition, determination of its stellar parameters is difficult; the effective temperature of this star may be 9300 or 9500 K, with the radius and the surface gravity of the star depending on the effective temperature.
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