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0.967 ± 0.010 [8] M ☉ Radius: ... The two stars are currently separated by 14.5 arc seconds, corresponding to a projected separation of 2016 ...
By the 2015 definition, 1 au of arc length subtends an angle of 1″ at the center of the circle of radius 1 pc. That is, 1 pc = 1 au/tan(1″) ≈ 206,264.8 au by definition. [9] Converting from degree/minute/second units to radians,
1.5 ± 0.1 [7] M ☉ Radius: ... Based upon an annual parallax shift of 0.02628 arc seconds as measured by the Hipparcos satellite, [1] this system is 124 light-years ...
A truly dark sky has a surface brightness of 2 × 10 −4 cd m −2 or 21.8 mag arcsec −2. [9] [clarification needed] The peak surface brightness of the central region of the Orion Nebula is about 17 Mag/arcsec 2 (about 14 milli nits) and the outer bluish glow has a peak surface brightness of 21.3 Mag/arcsec 2 (about 0.27 millinits). [10]
Position measurements of the pair made with the Hipparcos spacecraft show an annual parallax shift of 0.0935 seconds of arc, which is equivalent to a distance of about 35 ly (11 pc) from the Sun. [2] It is a binary star system with an angular separation of 4.0 arcseconds. [12]
The star has a relatively high proper motion, traversing the celestial sphere at the rate of 0.153 arc seconds per annum. [9] It is moving closer to the Earth with a heliocentric radial velocity of −38 km/s. [2] This is an aging giant star with a stellar classification of K0.5 III, [4] having exhausted the hydrogen at its core and expanded.
The second mark shows each's distance from Sol, with the concentric circles indicating the distance in steps of one ly. Lalande 21185 is a typical type-M main-sequence star with about 39% of the mass and radius of the Sun. It is also much cooler than the Sun with a surface temperature of 3,550 K.
The star system has a relatively high proper motion of 2.9 arc seconds per year, [13] and is moving away from the Solar System at a velocity of 11.6 km/s. [6] It achieved perihelion some 15,000 years ago when it came within 11 ly (3.5 pc) of the Sun. [ 13 ]