When is the best time to observe 31 Cygni?

31 Cygni is best observed from June through August, with peak visibility during July when it transits around midnight in the Northern Hemisphere.

Where is 31 Cygni located in the sky?

31 Cygni lies in the constellation Cygnus at right ascension 20h 14m and declination +46° 44′.

What telescope do I need to split 31 Cygni?

Splitting 31 Cygni requires an aperture of roughly 60 mm or larger — the components are separated by about 336.7″ at magnitudes near 3.8. Larger apertures and steady seeing help resolve the pair cleanly.

31 Cygni — Double Star in Cygnus

Observable Double Star Excellent (63/100)

Sep: 336.7", Companion: mag 4.8

DoubleStar Cygnus (Cyg) Visible

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Observe

1Physical Properties

Magnitude 3.79

Range 3.73 - 3.89

Period 3784 days

Variable Type Eclipsing Binary (Algol-type)

Spectral Type K2II+...

Star Color Red (B-V 1.28)

Distance 871 ly

2Position & Identifiers

RA 20h 13m 37.9s

Dec +46° 44' 29.0"

Constellation Cygnus (Cyg)

HR 7735

HIP 99675

HD 192578

Flamsteed 31 Cyg

Variable ID V695 Cyg

Double Cat 13554

SIMBAD

3How easy to split?

Primary 3.8 mag Companion 4.8 mag Separation 336.7″

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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 Jun – Aug (peak: Jul)

5Light Curve

6Multiple Star System Sextuple

Components 6 (sextuple)

Component IDs AD

Separation 336.7″

Companion Mag 4.8

Companion Sp B3V

Position Angle 322°

Star Colors A: Red D: Blue

Discoverer STFA 50

Notes

Component C, 6.99V, -0.14(B-V), -0.58(U-B) B5V, vsini : 25k/s, at 107" fixed. AP probably single star.

Separation over time

Measured 1800 → 2016 (216 y)

Separation drift 338.0" → 336.7" (-1.30")

Rate -0.0060" / y

PA drift 325° → 322° (-3°, -0.014°/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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32x Dawes: 1.9″ TFOV: 1.6°

Seeing

Scale Realistic = true angular size

A: 3.8 · B: 4.8 · Sep: 336.7″ · PA: 322° · N up, E left

Resolved · 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

ADS 13554A, EA 3.77 - 3.88V, 3784.3d. Amp. varies strongly with wavelength. C = NSV 12931, 6.98V.

Spectroscopic Binary

ADS 13554A, 3784.3d, K 14.0k/s, V0 -7.7k/s, msin3i 9.19, asini 711. Unresolved by speckle interferometry. SKYLAB | indicates cool star with hot companion. vsini of secondary 80k/s.

Spectral

Also classified K4Iab + B4IV-V.

Alternative Names

See HR 7730.

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