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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).
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 ,
The largest star in the system, Gliese 667 A (GJ 667 A), is a K-type main-sequence star of stellar classification K3V. [2] It has about 73% [5] of the mass of the Sun and 76% [2] of the Sun's radius, but is radiating only around 12-13% of the luminosity of the Sun. [15] The concentration of elements other than hydrogen and helium, what astronomers term the star's metallicity, is much lower ...
The star system was discovered in 1948 by Willem Jacob Luyten in the course of compiling a catalog of stars of high proper motion; he noted its exceptionally high proper motion of 3.37 arc seconds annually and cataloged it as Luyten 726-8. [11] The two stars are of nearly equal brightness, with visual magnitudes of 12.7 and 13.2 as seen from Earth.
Phi Tauri has a magnitude 7.51 visual companion located at an angular separation of 48.80 arc seconds along a position angle of 258°, as of 2015. The pair form a yellow and blue double that is visible in small telescopes. [9] A fainter, magnitude 12.27 companion lies at a separation of 118.10 arc seconds along a position angle of 25°, as of ...
From the orbital elements, the total mass of Alpha Centauri AB is about 2.0 M ☉ [e] – or twice that of the Sun. [70] The average individual stellar masses are about 1.08 M ☉ and 0.91 M ☉, respectively, [5] though slightly different masses have also been quoted in recent years, such as 1.14 M ☉ and 0.92 M ☉, [96] totaling 2.06 M ☉.
Based on results from the Gaia telescope's second data release from April 2018, an estimated 694 stars will approach the Solar System to less than 5 parsecs in the next 15 million years. Of these, 26 have a good probability to come within 1.0 parsec (3.3 light-years) and another 7 within 0.5 parsecs (1.6 light-years). [ 3 ]
The largest asteroid to pass within 1 LD of Earth in 2021 was 2021 SG with an estimated diameter of around 68 meters and an absolute magnitude of 24.0. The fastest asteroid to pass within 1 LD of Earth in 2021 was 2021 UA 7 that passed Earth with a velocity with respect to Earth of 27.7 km/s (62,000 mph).