BD−22 5866

BD−22 5866 is a quadruple-star system located 166 light years from Earth. The four stars are each about half the mass of the Sun and are approximately 500 million years old. The system is unusual in how closely the four stars are orbiting each other; one pair has an orbital separation of at most .04 astronomical units and an orbital period of about two days,[4] the other pair has a separation of at most .26 astronomical units and a period of about 55 days, and the two pairs are separated by 5.8 AU and have an orbital period of less than nine years.[6]

BD−22 5866

A light curve for BD-22 5866 plotted from TESS data[1]
Observation data Epoch J2000      Equinox J2000
Constellation	Aquarius
Right ascension	22h 14m 38.3637s[2]
Declination	−21° 41′ 53.199″[2]
Apparent magnitude (V)	10.1[3]
Characteristics
Spectral type	K7 + K7 + M1 + M2[4]
Variable type	Eclipsing binary[4]
Astrometry
Radial velocity (Rv)	-14.07[3] km/s
Proper motion (μ)	RA: 155.485[5] mas/yr Dec.: 184.484[5] mas/yr
Parallax (π)	25.9941 ± 0.1768 mas[5]
Distance	166 ly (51[4] pc)
Absolute magnitude (MV)	12.3
Orbit[4]
Primary	Aa
Companion	Ab
Period (P)	2.21107 ± 0.000004 days
Semi-major axis (a)	0.0351 ± 0.0024
Eccentricity (e)	0.0 ± 0.01
Inclination (i)	85.5 ± 1.0°
Periastron epoch (T)	2453937.59
Argument of periastron (ω) (secondary)	82°
Details
A
Mass	0.59 / 0.59 M☉
Radius	0.61 / 0.61 R☉
Other designations
NLTT 53279, LP 875-68, RBS 1834, 2MASS J22143835-2141535, CPD-22 8173, PPM 273545, TYC 6384-505-1[3]
Database references
SIMBAD	data

Since current theories of star formation indicate that stars like these could not form in such close proximity to each other, a favored explanation is that there may have been a single gaseous disk that forced them into such small orbits within the first 100,000 years of their evolution. The two pairs are currently moving farther apart due to tidal interaction, indicating that they were once even more closely associated than today.[6]