Delta Coronae Borealis

Delta Coronae Borealis, Latinized from δ Coronae Borealis, is a variable star in the constellation Corona Borealis. Its apparent magnitude varies regularly between apparent magnitude 4.57 and 4.69, and it is around 167 light-years distant. It is visible to the naked eye.

A visual band light curve for Delta Coronae Borealis, plotted from data published by Fernie (1991).^[9] The 59 day rotation period is shown in red.

δ Coronae Borealis

Location of δ Coronae Borealis (circled)
Observation data Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation	Corona Borealis
Right ascension	15h 49m 35.64700s[1]
Declination	+26° 04′ 06.2143″[1]
Apparent magnitude (V)	4.57 - 4.69[2]
Characteristics
Spectral type	G5III-IV Fe-1[3]
U−B color index	+0.32[4]
B−V color index	+0.78[4]
Variable type	RS CVn?[2]
Astrometry
Radial velocity (Rv)	−20.356±0.035[5] km/s
Proper motion (μ)	RA: −78.907[1] mas/yr Dec.: −64.894[1] mas/yr
Parallax (π)	19.4965±0.0850 mas[1]
Distance	167.3 ± 0.7 ly (51.3 ± 0.2 pc)
Absolute magnitude (MV)	+1.18[6]
Details
Mass	2.4[7] M☉
Radius	7.4[7] R☉
Luminosity	34.3[7] L☉
Surface gravity (log g)	3.29[6] cgs
Temperature	5,180[7] K
Metallicity [Fe/H]	−0.12[6] dex
Rotation	59[7] days
Rotational velocity (v sin i)	4.56[6] km/s
Age	851[6] Myr
Other designations
δ CrB, 10 Coronae Borealis, BD+26°2737, HD 141714, HIP 77512, HR 5889, SAO 84019[8]
Database references
SIMBAD	data

δ Coronae Borealis is a yellow giant star of spectral type G3.5III that is around 2.4 times as massive as the Sun and has swollen to 7.4 times its radius. It has a surface temperature of 5180 K.^[10]

For most of its existence, Delta Coronae Borealis was a blue-white main sequence star of spectral type B before it ran out of hydrogen fuel in its core. Its luminosity and spectrum suggest it has just crossed the Hertzsprung gap, having finished burning core hydrogen and now begun burning hydrogen in its shell.^[11][1] However, one recent paper gives a 64% chance that it has begun fusing core helium and is on the horizontal branch.^[12] It is a strong source of X-rays due to its hot corona.^[7]

In 1989, it was noticed that the brightness of δ Coronae Borealis is not constant. Approximately every 45 days, its brightness changes sinusoidally between 4.57 and 4.69, too small to be noticed without close monitoring. The evolutionary state of the star and its probable rotation period mean that the variations may be due to its rotation with different parts of the surface having spots or different temperatures. This would make it an RS Canum Venaticorum variable.^[13] The period has since been refined to 59 days and this is now accepted as the star's rotation period.^[7]