Alpha Sagittarii
 Location of α Sagittarii (circled) |
|
| Observation data Epoch J2000 Equinox J2000 |
|
|---|---|
| Constellation | Sagittarius |
| Right ascension | 19h 23m 53.17483s[1] |
| Declination | −40° 36′ 57.3705″[1] |
| Apparent magnitude (V) | +3.97[2] |
| Characteristics | |
| Spectral type | B8 V[3] |
| U−B color index | −0.33[2] |
| B−V color index | −0.10[2] |
| Astrometry | |
| Radial velocity (Rv) | −0.7[4] km/s |
| Proper motion (μ) | RA: +30.49[1] mas/yr Dec.: −119.21[1] mas/yr |
| Parallax (π) | 17.94 ± 0.22[1] mas |
| Distance | 182 ± 2 ly (55.7 ± 0.7 pc) |
| Details | |
| Radius | 2.49 R☉ |
| Luminosity | 60 L☉ |
| Temperature | 12,370 ± 500[5] K |
| Metallicity [Fe/H] | -0.02[6] dex |
| Other designations | |
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.[6] 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.[7]
Name and etymology
- The names Alrami and Rukbat, derived from the Arabic rukbat al-rāmī = the knee of the archer. It is not to be confused with Delta Cassiopeiae, which also is called Ruchbah or Rukbat, from the Arabic word ركبة rukbah meaning "knee".
- In Chinese, 天淵 (Tiān Yuān), meaning Celestial Spring, refers to an asterism consisting of α Sagittarii, β1 Sagittarii and β2 Sagittarii. Consequently, α Sagittarii itself is known as 天淵三 (Tiān Yuān sān, English: the Third Star of Celestial Spring.)[8]
- This star, together with β1 Sgr and β2 Sgr, were Al Ṣuradain (ألسردين), the two Surad, desert birds.[9]
See also
References
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- ↑ (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 2 日
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