Omega Ophiuchi
A magnetic Ap star that varies in brightness every 2.99 days.
Omega Ophiuchi, also designated ω Ophiuchi, is a variable star in the equatorial constellation Ophiuchus, sitting just north of the ecliptic near the constellation's western border with Scorpius. This white-hued star is just visible to the unaided eye, with an apparent visual magnitude that typically varies around 4.45. Based on parallax measurements, it lies roughly 168.6 light-years from the Sun and is moving away at a radial velocity of +2.5 km/s.
It is classified as an Ap star with the stellar type ApSrEuCr, indicating strong overabundances of strontium, europium, and chromium. A 1995 study by Abt and Morrell noted that it is an A-type main-sequence star whose singly-ionized calcium line shows both sharp and broad components. As an Alpha2 Canum Venaticorum variable, its brightness shifts between magnitude 4.44 and 4.51 over a 2.99-day cycle, and a powerful magnetic field has been detected on its surface.
The star is about 860 million years old, with a projected rotational velocity of 32.2 km/s and a rotation period of 2.3 days. It has 2.3 times the Sun's mass and 3.0 times its radius, radiating 35 times the Sun's luminosity from its photosphere at an effective temperature of 8,150 K. It also emits X-rays.
On 9 December 2017, Venus occulted Omega Ophiuchi as seen from the South Atlantic. The star's proper motion has changed significantly over time, hinting that it may be an astrometric binary. The suspected companion is likely a compact object, such as a white dwarf.
- Apparent visual magnitude
- 4.44–4.51
- Variable type
- Alpha2 Canum Venaticorum
- Variability period
- 2.99 days
- Distance
- 168.6 light years
- Spectral type
- ApSrEuCr
- Mass
- 2.3 solar masses
- Radius
- 3.0 solar radii
- Luminosity
- 35 solar luminosities
- Effective temperature
- 8,150 K
- Age
- 860 million years
Lore & Background
Omega Ophiuchi is an Ap star with a stellar classification of ApSrEuCr, displaying strong abundance anomalies of strontium, europium, and chromium. Abt and Morrell (1995) found a class showing an A-type main-sequence star with a Ca II line having both sharp and broad components. Its brightness ranges from magnitude 4.44 down to 4.51 over a period of 2.99 days, characteristic of an Alpha2 Canum Venaticorum variable, and a strong magnetic field has been measured on its surface.
The star is around 860 million years old with a projected rotational velocity of 32.2 km/s and a rotation period of 2.3 days. It has 2.3 times the mass of the Sun and 3.0 times the Sun's radius, radiating 35 times the solar luminosity from its photosphere at an effective temperature of 8,150 K. It is a source of X-ray emission. On 9 December 2017, it was occulted by Venus as viewed from the South Atlantic.
Omega Ophiuchi shows a significant change in its proper motion measurements over time, suggesting it may be an astrometric binary. The proposed companion is probably a compact star such as a white dwarf.
Reader's Guide
Omega Ophiuchi holds significance as a chemically peculiar Ap star with strong magnetic fields, making it a key object for studying stellar magnetism and elemental abundance anomalies. Its classification as an Alpha2 Canum Venaticorum variable links it to a class of stars whose brightness variations are tied to magnetic activity and surface spots, providing insights into the interplay between rotation, magnetic fields, and atmospheric chemistry. The star's measured strong magnetic field and X-ray emission further underscore its value for understanding magnetic activity in intermediate-mass stars. The possibility that it is an astrometric binary with a compact companion, likely a white dwarf, adds a layer of interest for studies of binary evolution and the fate of stars. Its occultation by Venus in 2017 offered a rare observational event, while its location near the ecliptic makes it accessible for future occultation studies. The star's well-determined physical parameters—mass, radius, luminosity, temperature, and age—serve as benchmarks for stellar models of A-type stars with peculiar abundances.
Did You Know?
- Omega Ophiuchi is an Alpha2 Canum Venaticorum variable with brightness ranging from magnitude 4.44 to 4.51 over 2.99 days.
- It has a strong magnetic field measured on its surface.
- The star may be an astrometric binary with a probable white dwarf companion.
More in Variable Stars, Part 6 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced