:rho Boötis
{{Short description|Star in the constellation Boötes}}{{distinguish|R Boötis}}
{{Starbox begin
| name = ρ Boötis
}}
{{Starbox image
| image =
| caption = Location of ρ Boötes (circled)
}}
{{Starbox observe
| epoch = J2000
| constell = Boötes
}}
{{Starbox character
| type =
| r-i = 0.65{{cn|date=June 2025}}
| v-r =
| variable = RS CVn.
}}{{Starbox astrometry
| radial_v = {{Val|−13.57|0.19}}
| prop_mo_ra = –100.531
| prop_mo_dec = +119.870
| parallax = 19.8550
| p_error = 0.1587
}}
{{Starbox detail
| mass = {{val|1.29|0.12}}
| radius = {{Val|20.58|0.19}}
| luminosity = {{Val|128.9|6.8}}
| temperature = {{Val|4285|54|fmt=commas}}
| metal_fe = {{Val|−0.19|0.10}}
| age_gyr = {{val|4.31|1.17}}
}}{{Starbox catalog
| names = {{odlist | B=ρ Boo | F=25 Boötis | BD=+31°2628 | FK5=534 | HD=127665 | HIP=71053 | HR=5429 | SAO=64202 | WDS=J14318+3022A | GC=19597 | PPM=77975 }}
}}{{Starbox reference
| Simbad=HD+127665
| ARICNS =
}}
{{Starbox end}}
File:RhoBooLightCurve.png for Rho Boötis, plotted from Hipparcos data]]
Rho Boötis is a single, orange-hued star in the northern constellation of Boötes. Its name is a Bayer designation that is Latinised from ρ Boötis, and abbreviated Rho Boo or ρ Boo. This star is visible to the naked eye with an apparent visual magnitude of 3.59. Based upon parallax measurements, it is located at a distance of approximately {{Convert|164|ly|pc|abbr=off|lk=on}} from Earth. It is moving closer to the Sun with a radial velocity of −13.6 km/s. There is an optical companion, a magnitude 11.5 star, located 34.7 arcseconds away along a position angle of 345° (as of 2013).
This is an evolved K-type giant star, currently on the red-giant branch, with a stellar classification of K4 III and an estimated age of 4 billion years. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. With around 1.23 times the mass of the Sun, it has expanded to 20.6 times the Sun's girth. The star is radiating 129 times the Sun's luminosity from its enlarged photosphere at an effective temperature of about {{val|4285|ul=K|fmt=commas}}. Rho Boötis is classified as a RS Canum Venaticorum variable.{{Dubious|reason=RS CVn are binary star systems of dwarfs or subgiants|date=June 2025}} Koen and Eyer examined the Hipparcos data for this star, and found that it varied with a period of 5.214 days, and an amplitude of 0.0027 magnitudes.
Nomenclature
Rho Boötis is known by several different names, including ρ Boo, 25 Boötis, BD+31° 2628, FK5 534, HD 127665, HIP 71053, HR 5429, and SAO 64202. In Chinese, {{lang|zh|梗河}} ({{lang|zh-Latn|Gěng Hé}}), meaning Celestial Lance, refers to an asterism consisting of ρ Boötis, ε Boötis and σ Boötis. Consequently, the Chinese name for ρ Boötis itself is {{lang|zh|梗河三}} ({{lang|zh-Latn|Gěng Hé sān}}, {{langx|en|the Third Star of Celestial Lance}}).
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{{Stars of Boötes}}
{{DEFAULTSORT:Rho Bootis}}
Category:RS Canum Venaticorum variables