Eta Sagittarii

η Sagittarii
Location of η Sagittarii (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Sagittarius
Right ascension 18h 17m 37.63505s[1]
Declination −36° 45′ 42.0667″[1]
Apparent magnitude (V) +3.11[2]/+7.8
Characteristics
Spectral type M2 III[3] + F7 V[4]
U−B color index +1.71[2]
B−V color index +1.56[2]
Variable type Lb[5]
Astrometry
Radial velocity (Rv)+0.5[6] km/s
Proper motion (μ) RA: −129.56[1] mas/yr
Dec.: −166.26[1] mas/yr
Parallax (π)22.35 ± 0.24 mas[1]
Distance146 ± 2 ly
(44.7 ± 0.5 pc)
Absolute magnitude (MV)−4.90[7]
Details[8]
η Sgr A
Mass2.1±0.7 M☉
Radius66±12 R☉
Luminosity525±6 L☉
Surface gravity (log g)1.13±0.05 cgs
Temperature3638±125 K
Other designations
η Sagittarii, η Sgr, β Telescopii, β Tel, CCDM J18176-3646AB, CD−36°12423, CPD-36°8128, FK5 683, GC 24944, HD 167618, HIP 89642, HR 6832, IDS 18109-3647 AB, PPM 297508, SAO 209957, WDS J18176-3646AB.[9]
Database references
SIMBADdata

Eta Sagittarii (Eta Sgr, η Sagittarii, η Sgr) is a binary star system in the southern zodiac constellation of Sagittarius. Based upon parallax measurements, it is located at a distance of 146 light-years (45 parsecs) from Earth.[1] In India, where part of the constellation of Sagittarius represents an Elephant, this star forms the creature's tail.[10]

The primary component, η Sagittarii A, is a red giant star with a stellar classification of M2 III.[3] It is an evolved star that is currently at a stage called the asymptotic giant branch, having exhausted both the hydrogen and the helium at its core.[11] This star is classified as an oxygen-rich irregular variable,[7] as it undergoes small magnitude fluctuations between +3.08 and 3.12.[4] The measured angular diameter of this star is 11.9 ± 2.1 mas.[12] At the estimated distance of Eta Sagittarii,[1] this yields a physical size of about 57 times the radius of the Sun.[13]

The companion, η Sagittarii B, was first noted by American astronomer S. W. Burnham in 1879. The two stars share a common proper motion and hence are probably gravitationally bound to each other.[14] The secondary is likely an F-type main sequence star with an apparent magnitude of +7.77. It located at an angular separation of 3.6 arcseconds from the primary, along a position angle of 108°.[15] This star is at a projected distance of 165 Astronomical Units from the red giant primary and the pair take a minimum of 1,270 years to complete an orbit.[4]

Within the context of the Milky Way galaxy, this system is a member of the faint old disk group.[7] Because of proper motion, this star will move into constellation Corona Australis around 6300 CE.[16] Eta Sagittarii has two optical companions that are not physically associated with the system. The first is a 10th magnitude star at an angular separation of 93 arcseconds with a position angle of 303°. There is a fainter, 13th magnitude star at an angular separation of 33 arcseconds along a position angle of 276°.[14]

  1. ^ a b c d e f g Cite error: The named reference aaa474_2_653 was invoked but never defined (see the help page).
  2. ^ a b c Cite error: The named reference aass34_1 was invoked but never defined (see the help page).
  3. ^ a b Cite error: The named reference houk1979 was invoked but never defined (see the help page).
  4. ^ a b c Cite error: The named reference kaler was invoked but never defined (see the help page).
  5. ^ Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
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  7. ^ a b c Cite error: The named reference aaa374_968 was invoked but never defined (see the help page).
  8. ^ Cite error: The named reference kallinger was invoked but never defined (see the help page).
  9. ^ Cite error: The named reference SIMBAD was invoked but never defined (see the help page).
  10. ^ Cite error: The named reference omeara2011 was invoked but never defined (see the help page).
  11. ^ Cite error: The named reference aj104_1_275 was invoked but never defined (see the help page).
  12. ^ Cite error: The named reference aaa434_3_1201 was invoked but never defined (see the help page).
  13. ^ Cite error: The named reference lang2006 was invoked but never defined (see the help page).
  14. ^ a b Cite error: The named reference burnham1978 was invoked but never defined (see the help page).
  15. ^ Cite error: The named reference aas107_235 was invoked but never defined (see the help page).
  16. ^ Cite error: The named reference moore2011 was invoked but never defined (see the help page).