HD 5408

Did You Know?

• Its name denotes that it is the 5,408th object in the Henry Draper Catalogue, included within the first volume published in 1918.
• Alternate designations include HR 266 and HIP 4440.

Properties

HD 5408 is a multiple star system. Two of its stars are directly resolvable, shining at magnitudes 6.12 and 6.57. The brighter of these two is a massive B9V star, the secondary component, which takes 83 years to orbit the inner triple system on a moderately eccentric path. The tertiary is a dimmer 16th-magnitude red dwarf of type M4Ve. Within the inner trio, the most massive star, designated Aa, circles the other two—Ab1 and Ab2—every four years, also on a moderately eccentric orbit. The innermost pair, Ab1 and Ab2, whip around each other every four days in a more eccentric orbit. All these orbits lie roughly in the same plane. From the orbital motions, astronomers have calculated the masses of four stars: Aa, Ab1, Ab2, and B. Their individual spectral types are uncertain, but mass constraints suggest they could be B7V, B9IV, A7V, and B7V respectively. This pairing of a subgiant (B9IV) with a main-sequence star in such an eccentric inner orbit is surprising, since tidal forces should have circularized it over the system’s lifetime. Component Ab1 has also been classified as B9V, but the accepted overall spectral type for the four stars is B9IV. A more distant companion, UCAC4 752-011208, was identified using Gaia data, though its similar distance and proper motion had been noted since 2018. The whole system likely belongs to the gamma Cassiopeiae association, a group of over 140 stars near HD 5408 and gamma Cas. Some catalogs list a faint component C, possibly a double star itself, but Gaia shows it is unrelated—likely a background binary of two red dwarfs. One star in the system is a mercury-manganese star, a chemically peculiar late B-type main-sequence star with strong mercury and manganese lines. The primary of the innermost binary has been reported as type B9V HgMn.

Observational history

HD 5408 was first identified as a close double star in 1899 by Sherburne Wesley Burnham. Over time, astronomers noticed shifts in its radial velocity and gave it conflicting spectral labels, such as a possible mercury-manganese star or a fast rotator. These puzzles were solved when the system was recognized as a hierarchical multiple: an inner pair orbits every 4.2 days, a middle orbit takes 1,769 days, and an outer orbit spans about 84 years. It was not immediately clear which of the two visible stars hosted this multiplicity. Later studies, including resolving component A into two stars and using speckle interferometry, suggested that component A is an unusual triple. In that arrangement, the outermost star is the most massive, while one of the inner stars may be more evolved. Components Aa and B turned out to have equal mass and brightness. In 2024, an X-ray flare lasting more than an hour was detected near HD 5408’s position, though its origin in this system remains uncertain.