21 Comae Berenices
A white-hued, magnetic Ap star in the Melotte 111 cluster.
21 Comae Berenices, also known by its variable star designation UU Comae Berenices, is a white-hued star in the northern constellation Coma Berenices. It is just barely visible to the unaided eye, with an apparent visual magnitude that varies around 5.4. Parallax measurements place it roughly 270 light-years from the Sun. Though it is a single star, it is a confirmed physical member of the Melotte 111 open cluster.
The English orientalist Thomas Hyde, as noted by R. H. Allen, linked this star to the ancient title Kissīn, a name derived from a climbing plant such as bindweed or dog rose. Since 1953, the star has been studied extensively, yielding occasionally conflicting results—including hints of pulsational behavior or a binary companion. It is a weakly magnetic chemically peculiar star of type CP2, or Ap star, most likely still on the main sequence. Its stellar classification is A3p SrCr, indicating abundance anomalies of the iron-peak element chromium and strontium.
In 1950, Olin Jeuck Eggen reported that the star’s brightness varies. It is an Alpha2 Canum Venaticorum (ACV) variable, meaning its luminosity changes as it rotates due to surface spots created by a magnetic field. The brightness variation has an amplitude of 0.02 magnitude and a period just over two days. Samus et al. (2017) classified it as a low-amplitude Delta Scuti variable, though this classification is disputed.
The age of the Melotte 111 cluster, and thus this star, is between 400 and 800 million years. The star has a projected rotational velocity of 63 km/s, with a polar inclination of 64° or greater, giving a rotation period of 2.05 days. Stellar evolutionary models suggest a mass about 2.3 times that of the Sun and a radius about 2.6 times the Sun’s radius. It radiates 38 times the Sun’s luminosity from its photosphere at an effective temperature of 8,900 K.
- Variable star designation
- UU Comae Berenices
- Flamsteed designation
- 21 Comae Berenices
- Apparent visual magnitude
- ~5.4 (variable)
- Distance
- ~270 light years
- Stellar classification
- A3p SrCr
- Rotation period
- 2.05 days
- Mass
- ~2.3 solar masses
- Radius
- ~2.6 solar radii
- Luminosity
- 38 times solar
- Effective temperature
- 8,900 K
Lore & Background
According to R. H. Allen, English orientalist Thomas Hyde attributed the ancient title Kissīn to this star, a name that comes from a climbing plant – either bindweed or dog rose. This object has been studied extensively since 1953, producing some occasionally contradictory results such as hints of pulsational behavior or a binary companion. It is a weakly magnetic chemically peculiar star of type CP2, or Ap star, that is most likely on the main sequence. The stellar classification is A3p SrCr, where the suffix notation indicates abundance anomalies of the iron-peak element chromium, as well as strontium.
In 1950, Olin Jeuck Eggen reported that the brightness of 21 Comae Berenices is variable. This is an Alpha2 Canum Venaticorum (ACV) variable, which indicates it varies in luminosity as it rotates due to spots on its surface created by a magnetic field. The range of variation has an amplitude of 0.02 magnitude and a period of just over two days. Samus et al. (2017) have it classified as a low-amplitude Delta Scuti variable, although this is disputed.
The age of the Melotte 111 cluster, and therefore this star, lies in the range of 400–800 million years. The star has a projected rotational velocity of 63 km/s, with a polar inclination of 64° or greater, resulting in a rotation period of 2.05 days. Stellar evolutionary models yield a mass of around 2.3 times that of the Sun and 2.6 times the Sun's radius for this object. It is radiating 38 times the luminosity of the Sun from its photosphere at an effective temperature of 8,900 K.
Reader's Guide
21 Comae Berenices is notable as a chemically peculiar Ap star that is a confirmed member of the Melotte 111 open cluster, providing a benchmark for studying stellar evolution in a young cluster environment. Its variability, first reported in 1950, is characteristic of Alpha2 Canum Venaticorum variables, where magnetic spots on the rotating surface cause small brightness changes. The star's classification as a low-amplitude Delta Scuti variable by Samus et al. (2017) remains disputed, reflecting ongoing uncertainty about its pulsational behavior. The ancient name Kissīn, attributed by Thomas Hyde, links the star to classical botanical imagery, adding cultural depth. Its weak magnetic field and abundance anomalies of chromium and strontium make it a valuable object for understanding magnetic fields and chemical peculiarity in main-sequence stars. The star's rotation period of 2.05 days and polar inclination of at least 64° help constrain models of magnetic geometry and spot distribution. As part of the Melotte 111 cluster, which is 400–800 million years old, 21 Comae Berenices contributes to knowledge of cluster dynamics and the evolution of intermediate-mass stars. Its proximity at about 270 light years allows detailed study, despite its low-amplitude variability of only 0.02 magnitude.
Did You Know?
- The star has the ancient title Kissīn, attributed by Thomas Hyde, meaning a climbing plant such as bindweed or dog rose.
- Its variability was first reported in 1950 by Olin Jeuck Eggen.
- The star's rotation period is 2.05 days, with a projected rotational velocity of 63 km/s.
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