36 Serpentis
Triple star system with two spectroscopic binaries and rapid rotation.
36 Serpentis is a triple star system in the equatorial constellation of Serpens, also designated b Serpentis (Bayer) and 36 Serpentis (Flamsteed). It is visible to the naked eye as a dim, white-hued point of light with a combined apparent visual magnitude of 5.09, located 162 light years away from the Sun. The system is notable for consisting of two spectroscopic binaries, with an inner pair (components A and B) and an outer pair (components AB and C), and for its estimated age of 710 million years.
- Bayer designation
- b Serpentis
- Flamsteed designation
- 36 Serpentis
- Combined apparent visual magnitude
- 5.09
- Distance from sun
- 162 light years
- Radial velocity
- −8 km/s
- Age of system
- 710 million years
- Orbital period of ab and c
- 52.8 years
- Eccentricity of ab and c orbit
- 0.83
- Spectral classification
- A2IV-Vn (Gray et al. 2017) or A3Vn (Cowley et al.)
- Primary component class
- A7 main sequence star
Lore & Background
The system comprises an inner spectroscopic binary pair (components A and B) and an outer pair (AB and C) that orbit each other with a period of 52.8 years and a high eccentricity of 0.83. The primary, 36 Serpentis A, is an A7-class main sequence star of visual magnitude 5.2, spinning rapidly with a projected rotational velocity of 229 km/s. It was once thought to be a Lambda Boötis star, but this is now disputed. The star has 1.97 times the Sun's mass, 2.25 times the Sun's radius, and radiates 19 times the Sun's luminosity at an effective temperature of 8,213 K.
The secondary, 36 Serpentis B, has been detected only by spectroscopy, with an orbital period of 1,073 days (2.94 years) and a high eccentricity of 0.7. Interferometric observations have failed to detect it, implying a luminosity-derived mass less than 0.50 solar masses, while radial velocity data suggest a mass of at least 0.37 solar masses. It may be a faint red dwarf or a white dwarf, and is likely the source of the X-ray emission detected from the system. The tertiary, 36 Serpentis C, is a G0 star of visual magnitude 7.8, with 1.27 times the Sun's mass, 1.26 times the Sun's radius, and an effective temperature of 6,555 K.
Reader's Guide
36 Serpentis serves as an example of a hierarchical triple star system where both the inner and outer pairs are spectroscopic binaries, offering a laboratory for studying orbital dynamics over long periods. The high eccentricities of both the inner (0.7) and outer (0.83) orbits are notable, as is the rapid rotation of the primary star (229 km/s), which contributes to the 'nebulous' spectral lines noted in its classification. The disputed status of the primary as a Lambda Boötis star highlights ongoing uncertainties in stellar classification. The secondary's undetected nature via interferometry, combined with its X-ray emission, suggests it may be a low-mass red dwarf or a white dwarf, making it a candidate for further study of faint companions. The system's age of 710 million years and its motion toward the Sun (radial velocity −8 km/s) provide context for its evolutionary stage and kinematics within the Milky Way.
Did You Know?
- The system consists of two spectroscopic binaries: an inner pair (A and B) and an outer pair (AB and C).
- The primary star, 36 Serpentis A, spins rapidly with a projected rotational velocity of 229 km/s.
- The secondary, 36 Serpentis B, has never been directly detected by interferometry and may be a red dwarf or white dwarf.
- The tertiary, 36 Serpentis C, is a G0 star with a visual magnitude of 7.8.
Frequently Asked Questions
What is 36 Serpentis?
36 Serpentis (also catalogued as b Serpentis by Bayer) is a triple star system located in the equatorial constellation Serpens. It appears to the unaided eye as a faint, white-tinted dot with a combined visual magnitude of about 5.09.
How is the 36 Serpentis system structured?
Rather than three independent stars orbiting a common center, the system is built from two spectroscopic binaries: an inner pair of components A and B, and an outer pair in which that AB unit orbits a third star, C. Each binary is detected through Doppler shifts in its spectral lines rather than through direct visual separation.
How far is 36 Serpentis from Earth and what is its radial velocity?
The system sits roughly 162 light years from the Sun. Its radial velocity of about −8 km/s indicates it is drifting slightly toward us along our line of sight.
How old is 36 Serpentis and what does that tell us?
Astronomers estimate the system's age at approximately 710 million years, placing it well past its earliest, most turbulent phase but still far from a red-giant stage. That middle-age status, combined with the rapid rotation of its components, makes it a useful benchmark for studying how close binary pairs evolve over hundreds of millions of years.
Why do multiple-star fans single out 36 Serpentis?
Its architecture—two nested spectroscopic binaries rather than a simple visual triple—offers a clean laboratory for testing how gravitational tides and angular-momentum exchange shape tightly packed stellar systems. At 162 light years it is also close enough that its spectral signatures remain sharp, making it a popular target for radial-velocity follow-up campaigns.
More in Binary and Multiple Stars, Part 2 1-24
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