HD 5788 and HD 5789

Lore & Background

The primary component, HD 5789, is a B-type main-sequence star classified as B9.5Vnn, with the 'n' indicating nebulous spectral lines due to rapid rotation. It has been listed as a Lambda Boötis star, though this classification is disputed. The star spins at a projected rotational velocity of 249 km/s, has 2.7 times the mass of the Sun, and radiates 86 times the Sun's luminosity from its photosphere at an effective temperature of 9,977 K.

The secondary component, HD 5788, is an A-type main-sequence star with a class of A2 Vn. It shows a projected rotational velocity of 270 km/s, also has 2.7 times the Sun's mass, and shines with 73 times the Sun's luminosity at an effective temperature of 9,840 K. As of 2016, the pair had an angular separation along a position angle of 195°. Although both stars share similar proper motion and parallax, there remains no proof that they are gravitationally bound.

Reader's Guide

The HD 5788/5789 system is notable as a pair of massive, rapidly rotating main-sequence stars in Andromeda, both with nearly identical masses but differing in temperature and luminosity. The primary's disputed classification as a Lambda Boötis star adds to its interest, as such stars are chemically peculiar and relatively rare. The high rotational velocities of both components—among the fastest for their spectral types—make them valuable for studying stellar rotation and its effects on spectral line broadening. The lack of confirmed gravitational binding, despite shared kinematics, leaves the system's true nature open: it may be a wide binary or merely an optical double. This ambiguity, combined with the stars' similar masses and ages, provides a useful case study for binary star formation and dynamics. The system's location in Andromeda and its moderate brightness (visible with small telescopes) make it accessible for amateur and professional follow-up observations, particularly to resolve the binary status through long-term astrometric monitoring.

Did You Know?

• The primary star HD 5789 has a projected rotational velocity of 249 km/s.
• The secondary star HD 5788 is slightly cooler at 9,840 K compared to the primary's 9,977 K.