78 Pegasi

Lore & Background

The primary member, component A, is a magnitude 5.07 giant star with a stellar classification of K0III, having exhausted the supply of hydrogen at its core and expanded to 10 times the Sun's radius. It is a red clump giant, indicating it is on the horizontal branch and generating energy through core helium fusion. It has 1.5 times the mass of the Sun and radiates 57 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,898 K. The secondary companion, component B, is magnitude 8.10.

The system is located approximately 224 light years from the Sun based on parallax, but is drifting closer with a radial velocity of −8 km/s. It is the 78th star numbered by Flamsteed in order of right ascension. In the 1795 French-language Fortin-Flamsteed edition of the Atlas Coelestis, 56 Pegasi is labelled with the letter 'i'.

Reader's Guide

78 Pegasi is notable as a binary star system discovered by A. G. Clark in 1862, with components orbiting each other over a 630-year period. The primary is a red clump giant of class K0III, a stage of stellar evolution where core helium fusion occurs after hydrogen exhaustion. Its physical parameters—1.5 solar masses, 10 solar radii, 57 solar luminosities, and an effective temperature of 4,898 K—provide a clear example of a horizontal-branch star. The system's radial velocity of −8 km/s indicates it is moving closer to the Sun. As the 78th Flamsteed star in Pegasus, it holds a place in historical star catalogues, though the 1795 Fortin-Flamsteed atlas labels 56 Pegasi with the letter 'i', not 78 Pegasi. The system's faint naked-eye visibility (magnitude 4.93) and orange hue make it accessible to amateur observers, while its binary nature and evolutionary state offer insights into stellar dynamics and post-main-sequence evolution.

Did You Know?

• 78 Pegasi was discovered to be a double star by A. G. Clark in 1862.
• The primary is a red clump giant, generating energy through core helium fusion.
• The system has an orbital period of 630 years with an eccentricity of 0.11.