31 Leonis
Binary star in Leo with a red giant primary.
31 Leonis, known as Yunü, is a binary star system located in the equatorial constellation of Leo. To the unaided eye, it appears as a single point of light with a combined apparent magnitude of 4.39. Parallax measurements from Earth’s orbit yield an annual shift of 11.02 milliarcseconds, placing the system roughly 300 light-years away. Its brightness is reduced by 0.12 magnitudes due to interstellar extinction along the line of sight, and it is receding from the Sun at a radial velocity of +39.8 km/s.
The primary star, component A, is an evolved red giant of spectral type K3.5 IIIb Fe-1:, where the suffix indicates a deficiency of iron in its spectrum. It has swollen to 34 times the Sun’s radius and shines with about 283 times the Sun’s luminosity, with an effective temperature of 4,066 K. The secondary star, component B, has a magnitude of 13.6 and, as of 2008, lies at an angular separation of 7.9 arcseconds.
The traditional Chinese name Yunü (御女) refers to this star’s position among the southernmost stars of the Xuanyuan (轩辕) constellation, flanked by ο Leonis and ρ Leonis. On 18 July 2024, the IAU Working Group on Star Names officially approved Yunü for 31 Leonis A, and it is now listed in the IAU Catalog of Star Names.
- Combined apparent visual magnitude
- 4.39
- Distance estimate
- 300 light years
- Annual parallax shift
- 11.02 mas
- Interstellar extinction
- 0.12 magnitudes
- Radial velocity
- +39.8 km/s
- Primary classification
- K3.5 IIIb Fe-1:
- Primary luminosity
- 283 times Solar
- Primary effective temperature
- 4,066 K
- Primary radius
- 34 times Solar
- Secondary magnitude
- 13.6
Lore & Background
31 Leonis is a binary star system in the equatorial constellation of Leo. The system is visible to the naked eye in unresolved form, having a combined apparent visual magnitude of 4.39. An estimated distance of around 300 light years is obtained from the annual parallax shift of 11.02 mas as seen from Earth's orbit. At the current distance, interstellar extinction between Earth and 31 Leo diminished the apparent brightness by 0.12 magnitudes. It is moving away from the Sun with a radial velocity of +39.8 km/s.
The primary member of 31 Leonis, component A, is an evolved K-type red giant with a stellar classification of K3.5 IIIb Fe-1:, where the suffix notation indicates an underabundance of iron in the spectrum. It has expanded to 34 times the Solar radius and is radiating around 283 times the Sun's luminosity from its photosphere at an effective temperature of 4,066 K. The magnitude 13.6 secondary, component B, lies at an angular separation of 7.9 arcseconds, as of 2008.
This star has the traditional Chinese name Yunü (御女); it is in the middle of the southernmost stars of the Xuanyuan (轩辕) constellation (ο Leonis and ρ Leonis). The IAU Working Group on Star Names approved the name Yunü for 31 Leonis A on 18 July 2024 and it is now so entered in the IAU Catalog of Star Names.
Reader's Guide
31 Leonis serves as a notable example of a binary system where the primary is an evolved red giant with a measured iron underabundance, providing a case study for chemical peculiarities in late-type stars. Its traditional Chinese name Yunü, part of the Xuanyuan asterism, links the star to ancient Chinese astronomy, and its official recognition by the IAU Working Group on Star Names in 2024 formalizes this cultural heritage. The system's combined magnitude of 4.39 makes it accessible to naked-eye observers, while the faint secondary at magnitude 13.6 and 7.9 arcseconds separation offers a challenge for amateur telescopes. The measured parallax of 11.02 mas yields a reliable distance estimate of about 300 light years, and the radial velocity of +39.8 km/s indicates the system is receding from the Sun. The interstellar extinction value of 0.12 magnitudes quantifies the dimming effect of dust along the line of sight, contributing to the understanding of the local interstellar medium.
Did You Know?
- 31 Leonis has a combined apparent visual magnitude of 4.39, visible to the naked eye.
- The primary star is an evolved K-type red giant with an underabundance of iron in its spectrum.
Frequently Asked Questions
What is 31 Leonis (Yunü)?
31 Leonis, traditionally called Yunü, is a binary star pair residing in the constellation Leo. To the unaided eye it presents as a single point of light with a combined visual magnitude of about 4.39.
What kind of stars make up the 31 Leonis system?
The primary, component A, is an evolved red giant of spectral type K3.5 IIIb with a mildly iron-deficient spectrum. Its companion forms the secondary member of the binary, and together they are too close to split by the naked eye.
How far away is 31 Leonis and is it moving toward or away from us?
A parallax shift of roughly 11 milliarcseconds puts the system at about 300 light-years from Earth. It is receding from the Sun at a radial velocity of approximately +39.8 km/s.
Why do astronomers find 31 Leonis interesting?
Its red-giant primary offers a clear example of a star that has left the main sequence while sitting in one of the most recognizable constellations. The pair also experiences about 0.12 magnitudes of interstellar extinction, a small but measurable dimming along our line of sight.
Can I spot 31 Leonis without a telescope?
Yes—its combined magnitude of 4.39 makes it visible to the naked eye under dark-sky conditions. It simply looks like an ordinary star in Leo; resolving the two components requires at minimum a small telescope.
More in Binary and Multiple Stars, Part 2 1-24
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