6 Andromedae
A binary system with a dusty disk and a hidden secondary.
6 Andromedae, located in the northern constellation Andromeda, is an astrometric binary system. Its name originates from John Flamsteed’s star catalogue, which first appeared in 1712. The system has an apparent visual magnitude of 5.91, making it faintly visible to the naked eye under clear skies. Parallax measurements of 35.6 milliarcseconds place it about 92 light-years from the Sun, and it is moving toward us at a radial velocity of −32.4 km/s. Its proper motion across the sky is relatively high, at 0.272 arcseconds per year.
The system is a single-lined spectroscopic binary with a 9.2-year orbital period and an eccentricity of 0.3. Early classifications listed the visible star as a subgiant of spectral type F5 IV, as recorded in the Bright Star Catalogue. More recent data, however, classify it as an F-type main-sequence star (F5 V). This primary component is estimated to be 2.9 billion years old, with 1.3 times the Sun’s mass and 1.5 times its radius. It radiates 3.1 times the Sun’s luminosity from a photosphere at roughly 6,425 K. An infrared excess at 22 μm hints at a circumstellar disk of warm dust.
The unseen secondary has a mass at least equal to the Sun’s. If it were a normal star, it would be only one magnitude fainter than the primary and thus easily visible. The lack of strong ultraviolet emission rules out a white dwarf companion, so the secondary may itself be a binary system of two smaller stars, with an orbital period between a week and a year.
- Apparent visual magnitude
- 5.91
- Distance from sun
- 92 light years
- Parallax
- 35.6 mas
- Radial velocity
- −32.4 km/s
- Proper motion
- 0.272 arc seconds per annum
- Orbital period
- 9.2 years
- Eccentricity
- 0.3
- Spectral classification
- F5 V (modern); earlier F5 IV
- Mass of primary
- 1.3 solar masses
- Radius of primary
- 1.5 solar radii
Lore & Background
6 Andromedae was first cataloged by John Flamsteed in his star catalogue published in 1712. Early observations gave it a subgiant luminosity class of F5 IV, as recorded in the Bright Star Catalogue, but more modern measurements classify the visible component as an F-type main-sequence star of spectral type F5 V. The primary star is estimated to be 2.9 billion years old, with 1.3 times the mass of the Sun and 1.5 times its radius, radiating 3.1 times the Sun's luminosity at an effective temperature of about 6,425 K.
The system is a single-lined spectroscopic binary with an orbital period of 9.2 years and an eccentricity of 0.3. The mass of the secondary component is roughly at or above that of the Sun; if it were a single ordinary star, it would be readily visible as it would be just one magnitude fainter than the primary. However, the lack of conspicuous ultraviolet emission appears to rule out a white dwarf companion. The article suggests the secondary may itself be a binary system consisting of two smaller stars having an orbital period between a week and a year.
6 Andromedae displays an infrared excess at a wavelength of 22 μm, which may indicate a circumstellar disk of warm dusty debris. The system is moving closer to the Sun with a radial velocity of −32.4 km/s and has a relatively high proper motion, advancing across the celestial sphere at 0.272 arc seconds per annum.
Reader's Guide
6 Andromedae is significant as an astrometric and spectroscopic binary that illustrates the challenges of identifying faint companions. The primary star's spectral classification has shifted from F5 IV (subgiant) to F5 V (main-sequence) with more modern measurements, reflecting improved stellar modeling. The infrared excess at 22 μm points to a circumstellar debris disk, making the system a candidate for studying planetary system formation around binary stars. The secondary component's nature remains uncertain: its mass suggests it should be visible, but its absence of ultraviolet emission rules out a white dwarf, leading to the hypothesis that it is itself a close binary of two smaller stars. This unresolved ambiguity highlights the complexity of binary star systems and the need for further observation. The system's relatively high proper motion and radial velocity provide kinematic context, while its Flamsteed designation ties it to early 18th-century stellar cataloging. Overall, 6 Andromedae serves as a case study in the detection and characterization of stellar companions and debris disks.
More in Binary and Multiple Stars, Part 3 1-24
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