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| Observation data Epoch J2000 Equinox J2000 |
|
|---|---|
| Constellation (pronunciation) |
Corona Borealis |
| Right ascension | 15h 34m 41.3s |
| Declination | +26° 42′ 53″ |
| Apparent magnitude (V) | 2.21 (2.24 / 7.1) |
| Characteristics | |
| Spectral type | A0 V / G5 V |
| U-B color index | −0.02 / |
| B-V color index | −0.02 / |
| Variable type | Eclipsing binary |
| Astrometry | |
| Radial velocity (Rv) | +1.7 km/s |
| Proper motion (μ) | RA: 120.38 mas/yr Dec.: −89.44 mas/yr |
| Parallax (π) | 43.65 ± 0.79 mas |
| Distance | 75 ± 1 ly (22.9 ± 0.4 pc) |
| Absolute magnitude (MV) | 0.18 / 5.0 |
| Details | |
| Mass | 3/0.9 M☉ |
| Radius | 2.7/1.0 R☉ |
| Luminosity | 60/0.7 L☉ |
| Temperature | 9,750/5,400 K |
| Metallicity | ? |
| Rotation | 133 km/s |
| Age | 3.14 × 108 years |
| Other designations | |
| Database references | |
| SIMBAD | data |
Alpha Coronae Borealis (α CrB / α Coronae Borealis) is a binary star in the constellation Corona Borealis. It also has the traditional names Gemma or Alphekka or Alphecca.
α CrB is an eclipsing binary system similar to Algol (β Per). Its period is 17.36 days, with magnitude varying from +2.21 to +2.32, which is hardly noticeable to the unaided eye.
α CrB is believed to be a stream star member of the Ursa Major Moving Group.
A large disc of dust and material has been shown to exist around Alphecca, prompting speculation of a similar planetary/proto-planetary system to that currently assumed around Vega. How the binary-star dynamic would affect such a system is the subject of intense debate.
The name Gemma is Latin for "jewel". The other traditional name comes from Arabic al-fakkah, "the broken" (ring of stars), from فكّ fakk "unsealed"). In astrology it is one of the Behenian fixed stars and has the symbol 
.
References
- Tomkin, J.; Popper, D. M., 1986, "Rediscussion of eclipsing binaries. XV - Alpha Coronae Borealis, a main-sequence system with components of types A and G", Astronomical Journal, vol. 91, June 1986.
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