Alpha Sagittarii

Alpha Sagittarii (Alpha Sgr, α Sagittarii, α Sgr) is a star in the Sagittarius constellation. It has the traditional names Alrami and Rukbat.

Alpha Sagittarii is a blue, class B dwarf star. It does not appear particularly bright in the sky to the naked eye, with a visual apparent magnitude of +3.97. However, this is due to its distance; in reality, the star is twice as hot as the Sun and considerably more massive, with a luminosity in visible wavelengths about 40 times greater than that of the Sun. Based on an excess emission of infrared radiation, it may have a debris disk, much like Vega. It is unclear why Bayer designated this star as the alpha, rather than ε Sgr or σ Sgr. This led some old star charts to occasionally depict Alpha and Beta Sgr as much brighter than they are in reality, as they are invisible from northern Europe, being too far south to see there.

This is a single-lined spectroscopic binary system. The ROSAT All Sky Survey discovered that Alpha Sagittarii is emitting an excess flux of X-rays, which is not expected to originate from a star of this spectral class. The most likely explanation is that the companion is an active pre-main sequence star or else a star that has just reached the main sequence.