Upsilon2 Cassiopeiae

Lore & Background

Upsilon2 Cassiopeiae is an aging giant star that has exhausted its core hydrogen and expanded off the main sequence, now having 10 times the girth of the Sun. It is classified as a red clump giant on the horizontal branch, meaning it is currently fusing helium in its core. The star has 1.44 times the mass of the Sun and radiates 55 times the Sun's luminosity from its enlarged photosphere at an effective temperature of 4,937 K.

The star is a barium star, showing an excess of barium in its spectrum. This excess may result from mass transfer from a more-evolved companion that later became a white dwarf, though no companion has been detected. The star's suffix notation indicates a mild underabundance of iron in its spectrum.

The name Castula appears in Bayer's Uranometria and is Latin for a type of woman's tunic. The IAU Working Group on Star Names approved the name Castula for this star on 5 September 2017, and it is now included in the List of IAU-approved Star Names.

Reader's Guide

Upsilon2 Cassiopeiae holds significance as a named star with a long history, its proper name Castula dating back to Bayer's Uranometria. Its official recognition by the IAU in 2017 cemented its place in modern astronomical nomenclature. As a red clump giant, it represents a key stage in stellar evolution—a star that has left the main sequence and is now fusing helium in its core. Its status as a barium star adds astrophysical interest, as such stars often provide evidence of past binary interactions, even though no companion has been detected here. The star's mild iron underabundance and its measured distance of about 189 light years, combined with its radial velocity of −48 km/s indicating it is moving closer to the Sun, contribute to a fuller picture of its kinematics and composition. For observers, it is a faint naked-eye object in Cassiopeia, located near the brighter Gamma Cassiopeiae, and serves as a useful target for studying giant stars and chemical peculiarities.