78 Ursae Majoris
Binary star system in Ursa Major with a 105-year orbit.
78 Ursae Majoris is a binary star located in the northern circumpolar constellation Ursa Major. To the naked eye, it appears as a dim star with a combined apparent visual magnitude of 4.93. Based on Hipparcos parallax measurements, the system lies about 83 light-years (25 parsecs) away, though it is moving closer to Earth at a radial velocity of −5 km/s. It is considered a possible member of the Ursa Major Moving Group.
The binary nature of the system was first reported by S. W. Burnham in 1894. The two stars orbit each other with a period of 105 years, an eccentricity of 0.39, and a semimajor axis of 1.2 arcseconds. The orbital plane is inclined at 47 degrees.
The primary star, component A, has a magnitude of 5.02 and is an F-type main-sequence star with a classification of F2V. It is 785 million years old, rotates with a projected rotational velocity of 92 km/s, and has 1.34 times the Sun’s mass. Its photosphere radiates 5.75 times the Sun’s luminosity at an effective temperature of 6,908 K.
The secondary star, component B, has a visual magnitude of 7.88. It is a G-type main-sequence star classified as G6V and is a suspected variable.
- Combined apparent visual magnitude
- 4.93
- Distance
- 83 light-years (25 pc)
- Radial velocity
- −5 km/s
- Binary discovery announcer
- S. W. Burnham
- Orbital period
- 105 years
- Orbital eccentricity
- 0.39
- Semimajor axis angular size
- 1.2″
- Orbital inclination
- 47°
- Primary component designation
- A
- Primary magnitude
- 5.02
Lore & Background
The binary nature of 78 Ursae Majoris was announced by S. W. Burnham in 1894. The pair orbit each other with a period of 105 years and an eccentricity of 0.39. Their semimajor axis has an angular size of 1.2″ and the orbital plane is inclined by 47°. The system is a candidate member of the Ursa Major Moving Group.
The primary member, component A, has a magnitude of 5.02 and is an F-type main-sequence star with a stellar classification of F2V. It is 785 million years old and is spinning with a projected rotational velocity of 92 km/s. The star has 1.34 times the mass of the Sun and is radiating 5.75 times the Sun's luminosity from its photosphere at an effective temperature of 6,908 K.
The secondary, component B, has a visual magnitude of 7.88. It is a G-type main-sequence star with a class of G6V. The star is a suspected variable.
Reader's Guide
78 Ursae Majoris is significant as a binary system with a well-determined orbit, discovered by S. W. Burnham in 1894. Its orbital period of 105 years and eccentricity of 0.39, along with a semimajor axis of 1.2″ and inclination of 47°, provide a clear example of a long-period visual binary. The system's membership in the Ursa Major Moving Group is only candidate status, reflecting ongoing uncertainty. The primary star, an F2V main-sequence star, is relatively young at 785 million years and rotates rapidly at 92 km/s, while the secondary is a G6V star suspected of variability. Parallax estimates from Hipparcos place the system at 83 light-years, and its radial velocity of −5 km/s indicates it is drifting closer to Earth. The combined magnitude of 4.93 makes it a faint naked-eye target, but the system's legacy lies in its detailed orbital and stellar parameters, which contribute to the study of binary star evolution and moving group membership.
Did You Know?
- The primary star has a projected rotational velocity of 92 km/s.
- The secondary star is a suspected variable.
More in Binary and Multiple Stars, Part 2 1-24
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