73 Leonis

Lore & Background

The binary nature of 73 Leonis was initially suggested in 1920, based on spectroscopic observations. The system was confirmed to be a binary by two studies from 1926 and 1928. The most modern determination of the orbital elements, as of 2025, combines spectroscopy, astrometry, and direct observations from interferometry. It finds that the stars orbit with a period of 8.1042 years, an eccentricity of 0.4251, and an inclination of 58.03°. The angular semi-major axis is 45.470 mas, which at the distance of 73 Leonis corresponds to 6.535 astronomical units. Given the relatively high eccentricity, the separation between components varies from 9.322 AU at apoastron to 3.749 AU at periastron. The combined mass of the pair is 4.24±0.37 M☉.

The primary star dominates the energy output of the system. Its spectrum matches a spectral class of K2 III, with the luminosity class III indicating it is a giant star that has exhausted the hydrogen at its core. This star has around 26 times the radius of the Sun, radiating 195 times the Sun's luminosity from its photosphere at an effective temperature of 4,271 K, giving it the orange hue typical of a K-type star.

The secondary star is poorly characterized. It is 3.62 magnitudes fainter than the primary, and based on this difference it is estimated to have a spectral type of F6V, although it was never characterized spectroscopically to accurately determine its spectral type.

Reader's Guide

73 Leonis is notable as a binary system whose orbital elements have been determined through a combination of spectroscopy, astrometry, and interferometry, yielding precise parameters such as an 8.1042-year period and an eccentricity of 0.4251. The primary star, a K2 III giant, is well-characterized with a radius of 26 R☉ and a luminosity of 195 L☉, while the secondary remains poorly characterized, its spectral type only estimated as F6V from its magnitude difference. The system's distance of 460 light-years, derived from Gaia parallax, places it within reach of detailed study. The combined mass of 4.24±0.37 M☉ and the varying separation from 3.749 AU to 9.322 AU provide a clear picture of the binary's dynamics. The legacy of 73 Leonis lies in its role as a testbed for combining multiple observational techniques to derive orbital solutions, and in highlighting the challenges of characterizing a faint secondary component in a binary system.

Did You Know?

• The binary nature of 73 Leonis was first suggested in 1920 based on spectroscopic observations.
• The primary star is a K2 III giant with 26 times the Sun's radius and 195 times its luminosity.
• The secondary star is 3.62 magnitudes fainter than the primary and is estimated to be spectral type F6V.
• The separation between the two stars varies from 3.749 AU at periastron to 9.322 AU at apoastron.