Menu

Tau Cygni — Double Star in Cygnus

Magnitude 3.6–3.8m DoubleStar Cygnus (Cyg) Visible
Star Map
+ List + Plan

Observe

1Physical Properties

Magnitude 3.72
Range 3.73 - 3.74
Variable Type Delta Scuti (Pulsating)
Spectral Type F1 IV subgiant
Star Color Yellow-white (B-V 0.39)
Distance 66 ly

2Position & Identifiers

RA 21h 14m 47.5s
Dec +38° 02' 44.0"
Constellation Cygnus (Cyg)
HR 8130
HIP 104887
HD 202444
SAO 71121
Bayer Tau
Flamsteed 65 Cyg
Variable ID Tau Cyg
Double Cat 14787
SIMBAD

3How easy to split?

Primary 3.7 mag Companion 6.6 mag Separation 1.0″
Sign in and configure your equipment and default location to see a personalized row.
Telescope Bortle 3 Bortle 4 Bortle 5
80mm refr. V. hard V. hard V. hard
150mm Newt. V. hard+ V. hard+ V. hard+
C8 203mm Hard Hard Hard
Easy Medium Hard Very hard Impossible

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

To reach "Medium" at Bortle 3, you'd need at least a 330 mm reflector.

4Visibility

Set a location in User Settings to see visibility data.

Best season Jul – Sep (peak: Aug)

5Multiple Star System Septuple C,G,I,K: optical

Components 7 (septuple)
Component IDs AB
Separation 1.0″
Companion Mag 6.6
Companion Sp F7V
Position Angle 170°
Star Colors A: Yellow-white B: Yellow
Discoverer AGC 13
AB 3.82 F0IV, 6.42 G1V, 49.80y, a = 0.85". Components C, 13v at 29" and P, 12v at 72", both optical. Q, 12v at 90", | physical. R, 13v at 90", CPM.

Separation over time

Period: 49.5 y Eccentricity: 0.245 Now: 0.95", PA 158° -0.25" in 5 years
0.00" 0.29" 0.59" 0.88" 1.2" 2025 2030 2035 2040 2045 0.95"

Apparent separation over time, computed from ORB6 orbital elements. Steep curves indicate fast-changing pairs — catch them while they're splittable.

Eyepiece View

Log in to set your own equipment
80x Dawes: 1.9″ TFOV: 0.6°
Realistic = true angular size
N E 170°

A: 3.7 · B: 6.6 · Sep: 1.0″ · PA: 170° · N up, E right

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

Explore

7

Size Comparison

Querying VizieR for stellar data…
8

Compare Stars

9

Spectral Classification

10

Hertzsprung-Russell Diagram

Loading HR diagram…
Read: The Hertzsprung-Russell Diagram — Reading Stars Like a Map
11

Stellar Lifecycle

12

Blackbody Spectrum

13

Stellar Absorption Spectrum

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

14

Stellar Fusion

Discover

15Stellar Notes

Delta Sct 3.65 - 3.84V. 2-3 hrs.? Possibly variable period.
0.143d not confirmed.
Color excess E(B-V) = +0.09.
0.046".
16

Light Travel Time Machine

17

Relativistic Travel

Nearby in the Sky

Other targets within a few degrees — pan your scope a little and keep exploring.

Visibility scores assume a 150 mm Newton at Bortle 4.

61 Cyg
Easy
Easy double, sep 32.1″
Double Star mag 5.21 1.7° Cygnus
NGC 7063
Easy
Open cluster
Open Cluster mag 7.0 2.5° Cygnus
IC 1392
Hard+
Elliptical galaxy
Galaxy mag 13.0 4.9° Cygnus
V460 Cyg
Easy
Semi-regular variable, range 5.6–7.0
Variable Star mag 6.07 6.0° Cygnus
Show on Map

Explore Nightbase

Related knowledge, tools, and stories — no observation planning required.

Double Stars — A Guide for Observers
Two suns where one appeared — how to find, split, and appreciate the night sky's finest pairs. From Albireo's gold-and-blue to the Sirius B challenge.
Article
Cygnus
Mythology, bright stars, and deep-sky highlights.
Constellation
Mars
Polar caps, dust storms, and observing its oppositions.
Deep dive
}