21 Sagittarii

{{short description|Binary star system in the constellation Sagittarius}}

{{Starbox begin

| name = 21 Sagittarii

}}

{{Starbox image

|image=

{{Location mark

|image=Sagittarius constellation map.svg|alt=|float=center|width=280

|label=|position=right

|mark=Red circle.svg|mark_width=10|mark_link=21 Sagittarii

|x=532|y=240

}}

| caption=Location of 21 Sagittarii (circled)

}}

{{Starbox observe

| epoch = J2000

| constell = Sagittarius

| ra = {{RA|18|25|21.04075}}{{cite journal | arxiv=0708.1752 | bibcode=2007A&A...474..653V | doi=10.1051/0004-6361:20078357 | title=Validation of the new Hipparcos reduction | journal=Astronomy and Astrophysics | volume=474 | issue=2 | pages=653–664 | year=2007 | last1=Van Leeuwen | first1=F. | s2cid=18759600 }} [http://vizier.u-strasbg.fr/viz-bin/VizieR-S?HIP%2090289 Vizier catalog entry]

| dec = {{DEC|−20|32|30.0385}}

| appmag_v = 4.81{{cite journal | bibcode=2012AstL...38..331A | doi=10.1134/S1063773712050015 | arxiv=1108.4971 | title=XHIP: An extended hipparcos compilation | journal=Astronomy Letters | volume=38 | issue=5 | pages=331 | year=2012 | last1=Anderson | first1=E. | last2=Francis | first2=Ch. | s2cid=119257644 }} [http://vizier.u-strasbg.fr/viz-bin/VizieR-4?-source=V/137D/XHIP&HIP=90289 Vizier catalog entry] {{nowrap|(5.03 + 7.43)}}{{cite journal | bibcode=2008MNRAS.389..869E | doi=10.1111/j.1365-2966.2008.13596.x | arxiv = 0806.2878 | title=A catalogue of multiplicity among bright stellar systems | journal=Monthly Notices of the Royal Astronomical Society | volume=389 | issue=2 | pages=869 | year=2008 | last1=Eggleton | first1=P. P. | last2=Tokovinin | first2=A. A. | doi-access=free | s2cid=14878976 }} [http://vizier.u-strasbg.fr/viz-bin/VizieR-4?-source=J/MNRAS/389/869/table1&mHR=6896 Vizier catalog entry]

}}

{{Starbox character

| class = {{nowrap|K2II + A5:}}{{cite journal | bibcode=1995yCat.5050....0H | title=VizieR Online Data Catalog: Bright Star Catalogue, 5th Revised Ed. (Hoffleit+, 1991) | journal=VizieR On-line Data Catalog: V/50. Originally Published in: 1964BS....C......0H | volume=5050 | last1=Hoffleit | first1=D. | last2=Warren | first2=W. H. | year=1995 }}

| b-v = +1.30{{cite journal | bibcode=2014JAVSO..42..443M | title=Sloan Magnitudes for the Brightest Stars | journal=The Journal of the American Association of Variable Star Observers | volume=42 | pages=443 | last1=Mallama | first1=A. | year=2014 | issue=2 }}[http://vizier.u-strasbg.fr/viz-bin/VizieR-4?-source=J/other/JAVSO/42.443/ugriz&HD=169420 Vizier catalog entry]

| u-b = +0.93

}}

{{Starbox astrometry

| radial_v = −11.80{{Cite journal | doi=10.1134/S1063773706110065 | bibcode=2006AstL...32..759G| title=Pulkovo Compilation of Radial Velocities for 35 495 Hipparcos stars in a common system| journal=Astronomy Letters| volume=32| issue=11| pages=759–771| year=2006| last1=Gontcharov| first1=G. A.|arxiv = 1606.08053 | s2cid=119231169}}

| prop_mo_ra = +8.46

| prop_mo_dec = −28.07

| parallax = 7.95

| p_error = 0.72

| parallax_footnote =

| absmag_v = −0.67

}}

{{Starbox detail

| component1 = 21 Sgr A

| mass = 7.9{{cite journal | bibcode=2011MNRAS.410..190T | doi=10.1111/j.1365-2966.2010.17434.x | arxiv=1007.4883 | title=A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun | journal=Monthly Notices of the Royal Astronomical Society | volume=410 | pages=190–200 | year=2011 | last1=Tetzlaff | first1=N. | last2=Neuhäuser | first2=R. | last3=Hohle | first3=M. M. | issue=1 | doi-access=free | s2cid=118629873 }} [http://vizier.u-strasbg.fr/viz-bin/VizieR-4?-source=J/MNRAS/410/190/table1&HIP=90289 Vizier catalog entry]

| temperature = 4,234{{cite journal | bibcode=2012MNRAS.427..343M | doi=10.1111/j.1365-2966.2012.21873.x |title=Fundamental parameters and infrared excesses of Hipparcos stars| journal=Monthly Notices of the Royal Astronomical Society | volume=427 | pages=343–357 | year=2012 | last1=McDonald | first1=I. | last2=Zijlstra | first2=A. A. | last3=Boyer | first3=M. L. | issue=1 | doi-access=free |arxiv = 1208.2037 | s2cid=118665352 }} [http://vizier.u-strasbg.fr/viz-bin/VizieR-4?-source=J/MNRAS/427/343/table2&HIP=90289 Vizier catalog entry]

| luminosity = 320

| rotational_velocity = 4.4{{cite journal | bibcode=2005yCat.3244....0G | title=VizieR Online Data Catalog: Catalog of Stellar Rotational Velocities (Glebocki+ 2005) | journal=VizieR On-line Data Catalog: III/244. Originally Published in: 2005csss...13..571G; 2005yCat.3244....0G | volume=3244 | last1=Glebocki | first1=R. | last2=Gnacinski | first2=P. | year=2005 }} [http://vizier.u-strasbg.fr/viz-bin/VizieR-4?-source=III/244/catalog&HIP=90289 Vizier catalog entry]

}}

{{Starbox catalog

| names = {{odlist | F=21 Sgr | BD=−20°5134 | HD=169420 | HIP=90289 | HR=6896 | SAO=186794 | CCDM=J18254-2033AB | GC=25132 | GSC=06274-01663 | WDS=J18254-2033AB }}{{cite simbad | title=21 Sgr | access-date=2019-06-08}}

}}

{{Starbox reference

| Simbad = 21+Sgr

}}

{{Starbox end}}

21 Sagittarii is a binary star system in the southern zodiac constellation of Sagittarius. It is visible to the naked eye as a faint point of light with a combined apparent visual magnitude of 4.81. The system is located approximately 410 light years away from the Sun based on parallax. It is moving closer to the Earth with a heliocentric radial velocity of −11.80 km/s.

As of 2008, the pair had an angular separation of {{Val|1.7|ul=arcsecond}} along a position angle of 280°. The brighter member of the pair, designated component A, is a K-type bright giant with a stellar classification of K2II and visual magnitude 5.03. It has eight times the mass of the Sun and is radiating 320 times the Sun's luminosity from its photosphere at an effective temperature of 4,234 K Its companion, component B, is magnitude 7.43 with type reported as A5:, where the ':' indicates some uncertainty about the classification.{{cite journal | bibcode=2001AJ....122.3466M | doi=10.1086/323920 | title=The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog | journal=The Astronomical Journal | volume=122 | issue=6 | pages=3466 | year=2001 | last1=Mason | first1=Brian D. | last2=Wycoff | first2=Gary L. | last3=Hartkopf | first3=William I. | last4=Douglass | first4=Geoffrey G. | last5=Worley | first5=Charles E. | doi-access=free }} [http://vizier.u-strasbg.fr/viz-bin/VizieR-4?-source=B/wds/wds&WDS=18254-2033 Vizier catalog entry]