78 Virginis
First star beyond the Sun with a detected magnetic field.
78 Virginis, also known by the designations CW Virginis and o Virginis, is a faint white-hued star in the constellation Virgo, roughly 175 light-years from Earth. With an apparent magnitude of 4.92, it can be seen without a telescope. The star is moving toward the Solar System at a radial velocity of −10 km/s. It holds the distinction of being the first star beyond the Sun where a magnetic field was detected; that field has a dipole structure with a surface intensity of about . Classified as an Ap star of type ApEuCrSr, its spectrum shows strong lines of strontium, chromium, and other iron-peak elements. Its brightness varies as an Alpha2 Canum Venaticorum variable, ranging from magnitude 4.91 to 4.99 over a period of 3.722 days—a variability first noted by Kazimierz Stepień in 1967. The star is also a candidate rapidly oscillating Ap (roAp) star. With an estimated age of 435 million years, it has 2.16 times the Sun’s mass and 2.11 times its radius, radiating 27 times the Sun’s luminosity from a photosphere at 9,100 K.
- Bayer designation
- o Virginis
- Flamsteed designation
- 78 Virginis
- Variable star designation
- CW Virginis
- Apparent visual magnitude
- 4.92 (ranging from 4.91 to 4.99)
- Spectral type
- ApEuCrSr
- Distance
- 175 light-years
- Radial velocity
- −10 km/s
Lore & Background
78 Virginis is an Ap star with a stellar classification of ApEuCrSr, displaying strong lines of strontium, chromium, and other iron peak elements. In 1967, Kazimierz Stepień discovered that the star's brightness varies. It is classified as an Alpha2 Canum Venaticorum variable, ranging in magnitude from 4.91 down to 4.99 with a period of 3.722 days. The star displays a dipole magnetic field structure with a surface intensity of around (value not specified in notes). It is a candidate rapidly oscillating Ap (roAp) star.
The star is 435 million years old with 2.16 times the mass of the Sun and 2.11 times the Sun's radius. It is radiating 27 times the Sun's luminosity from its photosphere at an effective temperature of 9,100 K. 78 Virginis is moving closer to the Earth with a heliocentric radial velocity of −10 km/s.
Reader's Guide
78 Virginis holds a unique place in astrophysics as the first star other than the Sun to have a magnetic field detected. This discovery opened the door to the study of stellar magnetism beyond our solar system. Its classification as an Alpha2 Canum Venaticorum variable, with brightness variations tied to its rotation and magnetic field, makes it a key object for understanding the behavior of chemically peculiar Ap stars. The star's strong spectral lines of strontium, chromium, and other iron peak elements indicate significant surface abundance anomalies, which are linked to its magnetic field structure. As a candidate rapidly oscillating Ap star, it may also exhibit pulsations that provide further insight into its internal structure. The star's measured parameters—mass 2.16 times that of the Sun, radius 2.11 times solar, and luminosity 27 times solar—place it as an evolved main-sequence star. Its age of 435 million years and effective temperature of 9,100 K contribute to the understanding of stellar evolution in intermediate-mass stars. The detection of its magnetic field and variability has made 78 Virginis a benchmark for studies of magnetic fields in chemically peculiar stars.
More in Variable Stars, Part 5 1-24
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