HD 125248
A magnetic peculiar star with extreme europium and chromium variations.
HD 125248, designated CS Virginis in variable star nomenclature, is a binary star system located in the equatorial constellation Virgo. It appears as a faint point of light to the naked eye, with an apparent visual magnitude that fluctuates between 5.84 and 5.95. Based on parallax measurements, the system lies roughly 280 light-years from the Sun and is moving closer at a heliocentric radial velocity of −8 km/s.
Originally classified in the Henry Draper Catalogue (1918–1924) as a peculiar A0p star due to strong lines of ionized silicon in its spectrum, its variability was discovered in 1931 by W. W. Morgan. He found that the spectrum changed over several days, with lines of ionized chromium and europium varying dramatically in intensity—from strong to very faint—similar to the behavior of α² Canum Venaticorum. These two sets of lines vary in opposite directions: chromium lines are weakest when europium lines are strongest, and vice versa. In 1947, A. J. Deutsch determined a period of 9.295 days for this variation.
Also in 1947, H. W. Babcock used Coudé spectrograms to detect a general magnetic field around the star, with a strength at the poles that was, at the time, the strongest ever observed in a star. The magnetic field was found to be variable, showing opposite polarity when europium lines were at minimum versus maximum. Subsequent observations confirmed that both the period and amplitude of this magnetic variation remained stable over time. In 1950, D. W. N. Stibbs proposed the oblique rotator model to explain these properties, suggesting the magnetic field is fixed at an angle to the star's rotation axis.
Radial velocity variations indicate that HD 125248 is a single-lined spectroscopic binary with an orbital period of 4.4 years and an eccentricity of 0.21. The visible component is classified as A1p SrCrEu, a magnetic peculiar Ap star with pronounced abundance anomalies of strontium, chromium, and europium in its atmosphere. It has about twice the mass and nearly twice the radius of the Sun, is estimated to be 234 million years old, and rotates once every 9.3 days. Its photosphere radiates 42 times the Sun's luminosity at an effective temperature of 9,850 K.
- Apparent visual magnitude
- 5.84 to 5.95
- Distance
- approximately 280 light years
- Radial velocity
- −8 km/s
- Spectral type
- A1p SrCrEu
- Rotation period
- 9.3 days
- Mass
- double the mass of the Sun
- Radius
- nearly twice the radius of the Sun
Lore & Background
HD 125248 was classified as peculiar with spectral type A0p in the Henry Draper Catalogue, based on the strength of ionized silicon lines. In 1931, W. W. Morgan discovered that the spectrum varied over several days, with lines of ionized chromium and europium changing in intensity in opposite directions. A period of 9.295 days for the variation was found by A. J. Deutsch in 1947. H. W. Babcock examined the star using Coudé spectrograms in 1947 and found a general magnetic field with a strength of around at the poles, then the strongest magnetic field observed in a star. The magnetic field varied, showing opposite polarity when europium lines were at minimum versus maximum. In 1950, D. W. N. Stibbs first proposed the 'oblique rotator model' to explain the star's properties, with the magnetic field locked at an angle to the rotation axis. The star displays radial velocity variations suggesting it is a single-lined spectroscopic binary with a period of 4.4 years and an orbital eccentricity of 0.21. The visible component has a stellar classification of A1p SrCrEu, indicating a magnetic peculiar Ap star with abundance anomalies of strontium, chromium, and europium. It has double the mass and nearly twice the radius of the Sun, is estimated to be 234 million years old, and spins with a rotation rate of 9.3 days. It radiates 42 times the luminosity of the Sun at an effective temperature of 9,850 K.
Reader's Guide
HD 125248 holds a significant place in stellar astrophysics as the star for which the oblique rotator model was first proposed. This model, introduced by D. W. N. Stibbs in 1950, explains the periodic variations in spectral lines and magnetic field strength as a result of a magnetic field that is not aligned with the star's rotation axis. The star's magnetic field, when first measured by H. W. Babcock in 1947, was the strongest ever observed in a star, and its stable period and amplitude of variation over time made it a benchmark for studying magnetic Ap stars. The anticorrelated variations of chromium and europium lines, along with the polarity reversal of the magnetic field, provided key evidence for the oblique rotator geometry. As an Alpha2 Canum Venaticorum variable, its brightness modulation is tied to rotation, and magnetic Doppler imaging has shown that its magnetic field deviates strongly from a simple dipole. The system's binary nature, with a 4.4-year orbital period, adds further complexity. HD 125248 remains a foundational object for understanding magnetic fields in chemically peculiar stars and the rotational modulation of their atmospheres.
Did You Know?
- HD 125248 was the first star for which the oblique rotator model was proposed.
- Its magnetic field was the strongest observed in any star when measured in 1947.
- The chromium and europium spectral lines vary in opposite directions.
Frequently Asked Questions
What is HD 125248?
HD 125248, also known as CS Virginis, is a binary star system in the constellation Virgo that sits about 280 light-years from the Sun. It is a magnetic peculiar A-type star roughly twice the Sun's mass, showing a slow brightness swing between magnitudes 5.84 and 5.95 over a 9.3-day rotation cycle.
What makes HD 125248's spectrum so unusual?
Its spectrum is classified A1p SrCrEu, meaning it displays extreme overabundances of strontium, chromium, and europium that are far beyond normal stellar compositions. These anomalies are driven by strong surface magnetic fields that lock different elements into distinct latitudinal bands, so the observed line strengths shift as the star rotates.
Is HD 125248 getting closer to Earth?
Yes. Parallax-based distance puts the system at roughly 280 light-years, and its heliocentric radial velocity of −8 km/s indicates it is slowly drifting toward the Solar System. At that rate the change in distance over human timescales is negligible, but the sign of the velocity confirms the inward drift.
When was HD 125248's variability first noticed?
The star's variability was identified in 1931, several years after it had already been entered in the Henry Draper Catalogue (1918–1924) as a peculiar A0p star because of unusually strong ionized-silicon absorption lines. The 1931 discovery confirmed that the spectral oddities were tied to a real, periodic brightness change rather than a one-off measurement artifact.
Why do variable-star fans care about HD 125248?
It is one of the clearest textbook examples of a chemically peculiar magnetic rotator, where surface inhomogeneities in europium and chromium produce a measurable photometric cycle every 9.3 days. Because it is bright enough (around magnitude 5.9) for small telescopes, it remains a popular target for amateur spectroscopists tracking those extreme elemental band changes over successive rotations.
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