5 Serpentis

Lore & Background

The primary star, component A, is an F8 IV subgiant that has exhausted its core hydrogen and is evolving toward becoming a red giant. It was once given the variable star designation MQ Ser based on observations made between 1975 and 1980, which reported random small brightness variations of less than 0.03 magnitude and three flare events that increased brightness by 0.1 magnitudes. However, these variations were later suggested to be possibly due to normal observational error rather than intrinsic variability.

The system has a common proper motion companion, component B, a magnitude 10.1 star at an angular separation along a position angle of 35°. Its estimated orbital period is 3,371 years. It has been suggested that if component B is itself a variable star, it might account for the brightness changes originally attributed to the primary.

The system made its closest approach to the Sun about 153,000 years ago at a separation of 20.7 parsecs, and is currently moving away with a heliocentric radial velocity of +54.3 km/s. The brighter member is an IAU radial velocity standard star.

Reader's Guide

5 Serpentis holds significance as an IAU radial velocity standard star, providing a reference point for spectroscopic measurements. Its history as a suspected variable star illustrates the challenges of distinguishing genuine stellar variability from observational noise. The initial reports of small-amplitude brightness changes and flare events, later questioned as possible observational errors, highlight the importance of careful verification in variable star astronomy. The system's wide binary nature, with an orbital period of thousands of years, offers a case study in long-period binary dynamics. The possibility that the companion star might be the actual variable source adds another layer of complexity. The star's designation MQ Ser remains in the variable star nomenclature even though the variability has been disputed. Its relatively close distance of 82 light years and naked-eye visibility make it accessible for amateur and professional study alike, while its evolutionary status as a subgiant provides insight into stellar evolution after core hydrogen exhaustion.

Did You Know?

• 5 Serpentis was once designated MQ Ser as a suspected BY Draconis variable, but was later found not to be variable.
• The companion star has an estimated orbital period of 3,371 years.