When is the best time to observe Theta Orionis?

Theta Orionis is best observed from November through January, with peak visibility during December when it transits around midnight in the Northern Hemisphere.

Where is Theta Orionis located in the sky?

Theta Orionis lies in the constellation Orion at right ascension 05h 35m and declination -05° 25′.

What telescope do I need to split Theta Orionis?

Splitting Theta Orionis requires an aperture of roughly 60 mm or larger — the components are separated by about 52.3″ at magnitudes near 5.1. Larger apertures and steady seeing help resolve the pair cleanly.

Theta Orionis — Double Star in Orion

HIP 26235; Theta2 Orionis; 43 Orionis

Observable Double Star Excellent (63/100)

Sep: 52.3", Companion: mag 6.2

Magnitude 5.0–5.2^m DoubleStar Orion (Ori) Visible

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1Physical Properties

Magnitude 5.08

Spectral Type O9.5Vp

Star Color Blue (B-V -0.09)

2Position & Identifiers

RA 05h 35m 22.9s

Dec -05° 24' 58.0"

Constellation Orion (Ori)

HR 1897

HIP 26235

HD 37041

SAO 132321

Bayer Theta2

Flamsteed 43 Ori

Double Cat 4188

SIMBAD Wikipedia

3How easy to split?

Primary 5.1 mag Companion 6.2 mag Separation 52.3″

Telescope	Bortle 3	Bortle 4	Bortle 5
80 mm refractor 80mm refr.	Easy	Easy	Easy
150 mm Newton 150mm Newt.	Easy	Easy	Easy
Celestron C8 (203 mm SCT) C8 203mm	Easy	Easy	Easy

Easy Medium Hard Very hard Impossible

Bortle 3 = rural · 4 = outer suburbs · 5 = suburbs

4Visibility

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Best season Nov – Jan (peak: Dec)

5Multiple Star System 12 components

Components 12

Component IDs *

Separation 52.3″

Companion Mag 6.2

Companion Sp B3

Position Angle 93°

Star Colors A: Blue B: Blue

Discoverer STFA 16

Notes

ADS 4188AB fixed at 52". B is 6.38V, -0.05(B-V), -0.93(U-B), B0.5V; vsini 17k/s. See HR 1893.

Separation over time

Measured 1836 → 2019 (183 y)

Separation drift 52.8" → 52.3" (-0.50")

Rate -0.0027" / y

PA drift 92° → 93° (+1°, +0.005°/y)

Essentially fixed on human timescales — the same view your grandchildren will see.

Measured from the WDS observational archive. No orbital solution has been derived — most likely the period is too long to fit an orbit to the available measurement arc.

Eyepiece View

Telescope Eyepiece

80x Dawes: 1.9″ TFOV: 0.6°

Seeing

Scale Realistic = true angular size

A: 5.1 · B: 6.2 · Sep: 52.3″ · PA: 93° · N up, E right

Resolved · Rayleigh: 2.3″ · Dawes: 1.9″ · Eff: 2.3″

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Spectral Classification

Hertzsprung-Russell Diagram

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Blackbody Spectrum

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14Stellar Notes

Variable Star

5.05 - 5.19V. Component B is NSV 2326, 6.38 - 6.44V.

Spectroscopic Binary

ADS 4188A, 20.96722d, K 90.5k/s, V0 +32.1k/s, asini 25.6. Primary 20 solar masses, secondary over 14 solar. | Possibly the counterpart of X-ray binary 3U0527-05 with perhaps collapsed components.

Color

Abnormal reddening at 1500A.

Group/Cluster

Ori OB1d; sword of Orion; M42; HII region.

Spectral

Emission is nebular, to red of H beta.