When is the best time to observe Eta Orionis?

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

Where is Eta Orionis located in the sky?

Eta Orionis lies in the constellation Orion at right ascension 05h 24m and declination -02° 24′.

What telescope do I need to split Eta Orionis?

Splitting Eta Orionis requires an aperture of roughly 150 mm or larger — the components are separated by about 1.8″ at magnitudes near 3.4. Larger apertures and steady seeing help resolve the pair cleanly.

Eta Orionis — Double Star in Orion

Observable Double Star Fair (27/100)

Sep: 1.8", Companion: mag 4.9

Magnitude 3.3–3.6^m DoubleStar Orion (Ori) Visible

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Observe

1Physical Properties

Magnitude 3.36

Range 3.31 - 3.60

Period 8.0 days

Variable Type Eclipsing Binary (Algol-type)

Spectral Type B1 V +B2.

Star Color Blue (B-V -0.17)

Distance 1,113 ly

2Position & Identifiers

RA 05h 24m 28.6s

Dec -02° 23' 49.0"

Constellation Orion (Ori)

HR 1788

HIP 25281

HD 35411

SAO 132071

Bayer Eta

Flamsteed 28 Ori

Variable ID Eta Ori

Double Cat 4002

SIMBAD

3How easy to split?

Primary 3.4 mag Companion 4.9 mag Separation 1.8″

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

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)

5Light Curve

6Multiple Star System Quintuple

Components 5 (quintuple)

Component IDs AB

Separation 1.8″

Companion Mag 4.9

Companion Sp B2e

Position Angle 77°

Star Colors A: Blue B: Blue

Discoverer DA 5

Notes

ADS 4002A is SB triple system, abc with periods 9.2y and 7.989d. Speckle interferometry gives 9.219y, a = 0.036" for | for ab x c. Visual components AB, binary, 3.8, 4.8v, sep. 1.650"; C, 9.4v at 115". The total mass of the quintuple | system exceeds 50 solar masses, the most massive component being about 17 solar masses.

Separation over time

Measured 1848 → 2022 (174 y)

Separation drift 0.90" → 1.8" (+0.90")

Rate +0.0052" / y

PA drift 87° → 77° (-10°, -0.057°/y)

Slow change over generations — observable in lifetime comparisons.

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

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80x Dawes: 1.9″ TFOV: 0.6°

Seeing

Scale Realistic = true angular size

A: 3.4 · B: 4.9 · Sep: 1.8″ · PA: 77° · N up, E right

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

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Size Comparison

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Spectral Classification

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Hertzsprung-Russell Diagram

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Stellar Lifecycle

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Blackbody Spectrum

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Stellar Absorption Spectrum

Simulated absorption spectrum based on spectral type. Hover over lines to identify elements.

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Stellar Fusion

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16Stellar Notes

Variable Star

EA + Beta C 3.31 - 3.60V, 3.14 - 3.35B, 7.989268d. Also sinusoidal pulsation period, amp. 0.05V, 0.30197 or 0.30145d.

Spectroscopic Binary

ab, 7.9841d, K 145.2k/s, V0 +35.9k/s, asini 15.9. abc, 9.2y, K 17.5k/s, V0 +19.5k/s, asini 805. Resolved by | speckle interferometry at 4-meter Mayall telescope, sep. 0.04"; vsini secondary 39k/s.

Group/Cluster

Ori OB1a; Orion belt; cluster CR 70.

Dynamical

0.005".

Radial Velocity

Expanding circumstellar shell.

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