4 Sagittarii, also named Tianyue, is a suspected astrometric binary star system in the zodiac constellation of Sagittarius, located approximately 410 light-years away based on parallax. It is visible to the naked eye as a faint, blue-white hued star with an apparent visual magnitude of 4.74. The system is moving away from the Earth with a heliocentric radial velocity of +64 km/s.
The visible component is a B-type main-sequence star with a stellar classification of B9 V. It has a high rate of spin, displaying a projected rotational velocity of 149 km/s. This is giving it an oblate shape with an equatorial bulge that is an estimated 14% larger than the polar radius. 4 Sagittarii has 3.23 times the mass of the Sun and is radiating 240 times the Sun's luminosity from its photosphere at an effective temperature of 9,661 K.
In Chinese astronomy, this star is part of the constellation Tiān Yuè (, Celestial Keyhole). The IAU Working Group on Star Names adopted the name Tianyue for this star on 13 August 2026, after this Chinese constellation.
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