21 Sagittarii
21 Sagittarii
21 Sagittarii is a binary star in the southern constellation of Sagittarius. It can be seen with the naked eye as a faint point of light with an overall apparent magnitude of about 4.81. The system is located roughly 410 light-years away from the Sun.
The pair is moving closer to us, with a radial velocity of about −11.8 km/s. In 2008, the two stars had an angular separation of about 1.7 arcseconds at a position angle of roughly 280 degrees.
Component A is the brighter star. It is a K-type bright giant with a spectral type of roughly K2II and a visual magnitude of about 5.03. It has around 7.9 times the Sun’s mass and shines with about 320 times the Sun’s luminosity. Its surface temperature is around 4,234 K, and it rotates slowly with a projected velocity of about 4.4 km/s. Component B is the fainter companion, magnitude 7.43, with a classification near A5:, though there is some uncertainty about its exact type.
The star has several other designations, including HD 169420, HIP 90289, HR 6896, BD −20°5134, and WDS AB, among others.
In fiction
The BBC science docu-drama First Contact: An Alien Encounter (2022) imagines how humanity might react to alien contact, and in the story an extraterrestrial artefact is said to originate from 21 Sagittarii.
Overall, 21 Sagittarii is a binary system in Sagittarius with a bright, evolved K-type giant as its primary star.
This page was last edited on 1 February 2026, at 18:43 (CET).