31 Orionis (HIP 25737)

31 Orionis is an orange giant star in the constellation Orion. At 500 light years away, it shines at an apparent visual magnitude of 4.71. It is moving through the Galaxy at a speed of 19.5 km/s relative to the Sun.

Sky position: RA 5h 29.7m, Dec -1° 5.5'

Common designations: 31 Orionis, CI Orionis, HR 1834, HD 36167, HIP 25737, Gaia DR3 3220275601329091584

31 Orionis has the spectral type K4III. K-type stars are orange stars with surface temperatures of about 3,900 to 5,300 K. Their spectra are rich in neutral metal lines and, in the cooler examples, molecular bands. Its luminosity class III marks it as a giant: an evolved star that has left the main sequence and swollen to many times its original size.

How to find 31 Orionis in the sky

31 Orionis lies in Orion. With a declination of -1° 5.5', it is visible from the entire Southern Hemisphere and from the Northern Hemisphere up to about latitude 89° N. It is bright enough to see with the naked eye under dark skies. It is highest in the sky at local midnight around January, which makes the weeks around that month the best time to observe it.

Tabulated data for 31 Orionis

Astrometry and spectroscopy

Right Ascension (J2000)	82.43325837° (5h 29.7m)
Declination (J2000)	-1.09223843° (-1° 5.5')
Galactic Longitude	204.30536990°
Galactic Latitude	-18.61710189°
Proper motion RA	0.75 mas/yr
Proper motion Dec	-25.46 mas/yr
Parallax	6.52 ± 0.14 mas
Spectral Type	K4III
Radial Velocity	6.02 km/s

Photometry

Band	Magnitude	Abs. Mag.
B	6.28	0.35
V	4.71	-1.22
J	1.75	-4.18
H	0.95	-4.98
K	0.81	-5.12

Colour index B-V: +1.57

Derivations

Heliocentric distance: 153.44 pc (500 light years) (2.1% margin of error)
Absolute visual magnitude: -1.22
Transverse velocity: 18.5 km/s
Space velocity relative to the Sun: 19.5 km/s
Galactic position (X, Y, Z): -132.5 pc, -59.9 pc, -49.0 pc
Galactic velocity (U, V, W): 4.0 km/s, -16.3 km/s, -9.8 km/s

Data: Extended Hipparcos Compilation (XHIP) and SIMBAD.