Albireo (β Cyg) is a showpiece double star and one of the great small telescope night sky objects. To the unaided eye it's the fifth brightest star in Cygnus (combined mag. +2.9) and the lone star that marks the head of "the Swan" constellation. When viewed through a small telescope or even good 10x50 binoculars, Albireo is split into two components, a mag. +3.1 primary (Albireo A) and a mag. +5.1 secondary (Albireo B). These stars are separated by a generous 34.3 arc seconds but the most pleasing aspect of this grouping and what makes it such a special object is its beautiful colour contrast, a brilliant yellowish primary coupled with a soft blue secondary.

Location

Cygnus is one of the most recognizable constellations during the Northern Hemisphere summer and autumn months. It represents a Swan flying down the plane of the Milky Way. Containing many bright stars, positioned at the heart of the constellation is a prominent asterism in the shape of a large cross. Consequently it's often referred to as the Northern Cross. At the northern end of the cross is Deneb (α Cyg); a blue-white supergiant star (mag. +1.3) and the brightest star in the constellation. Deneb marks the "tail" of the Swan and located at the opposite side of the cross, Albireo, marks the "head" of the Swan.

Albireo is a striking double star through amateur telescopes (credit:- freestarcharts.com)

Finder Chart for Albireo (β Cyg)

Finder Chart for Albireo (β Cyg) - pdf format

Observations

Albireo is easily split using low powers through any small telescope or large binoculars (20x80mm for example). The surrounding field contains numerous fainter stars that add to the beauty of the view. At higher magnifications the separation of the two stars appears large. Larger scopes show more background stars and the colour contrast of the primary and secondary stars is strong and more evident.

Albireo System

Albireo is located about 430 light-years from the Earth. It's not known whether Albireo A and B are actually orbiting each other in a physical binary system. If they are their orbital period is probably at least 100,000 years. In 1976, it was discovered using speckle interferometry that Albireo A itself is a binary star. In the Washington Double Star Catalog, the pair are designated as components Aa and Ac with a current separation of 0.4 arc seconds. This is generally beyond the splitting capabilities of the average size amateur telescope. Those with a scope of 500mm (20-inch) diameter or larger may be able to split the Aa/Ac pair under exceptionally stable atmospheric conditions.

Summary

Albireo is a must see object and without doubt one of the finest double stars in the night sky. Its beauty comes from the striking colour contrast of the component stars, a brilliant yellow primary and a soft blue secondary. With a wide separation, the double is easily split with small telescopes or even good binoculars, as a result, it's a favourite target for amateur astronomers and one of the most observed double stars in the night sky.

Albireo Data Table

NameAlbireo
Apparent Magnitude2.90
ComponentsA (Aa and Ac) and B
Angular separation (arc secs)35.3 (AB), 0.4 (AaAc)
Position angle (degrees)54 (AB)
Other DesignationsBeta Cygni, β Cyg

Albireo A Components Data Table

NameAlbireo AaAlbireo Ac
Washington Double Star catalog (WDS)19307+2758Aa19307+2758Ac
HD183912183913
HIP9594795947
SAO8730187301
Flamsteed6 Cygni A6 Cygni A
ConstellationCygnusCygnus
RA (J2000)19h 30m 43.286s19h 30m 43.295s
DEC (J2000)+27d 57m 34.84s+27d 57m 34.62s
Apparent Magnitude3.185.82
Apparent Magnitude Aa and Ac Combined3.083.08
Absolute Magnitude-2.45-0.25
Distance (light-years)430430
Mass (Solar)5.03.2
Radius (Solar)703.5
Luminosity (Solar)1,200230
Temperature (K)4,08012,000
Spectral typeK2IIB8:p
Other DesignationsBeta Cygni A, β Cyg ABeta Cygni A, β Cyg A

Albireo B Component Data Table

NameAlbireo B
Washington Double Star catalog (WDS) 19307+2758B
HD183914
HIP95951
SAO87302
Flamsteed6 Cygni B
ConstellationCygnus
RA (J2000)19h 30m 45.395s
DEC (J2000)+27d 57m 55.00s
Apparent Magnitude5.09
Absolute Magnitude-0.35
Distance (light-years)400
Mass (Solar)3.7
Radius (Solar)2.7
Luminosity (Solar)230
Temperature (K)13,200
Spectral TypeB8Ve
Other DesignationsBeta Cygni B, β Cyg B

Sky Highlights - July 2017

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for July

Meteor Shower
Southern Delta Aquariids (Aquarids) meteor shower peaks on July 29

Northern Hemisphere
Evening
West:- Mercury (mag. -0.5 to +0.3) (second half of month)
Southwest:- Jupiter (mag. -2.0)
South:- Saturn (mag. +0.2)
Midnight
West:- Jupiter
South:- Saturn
East:- Neptune (mag. +7.8)
Morning
Southwest:- Saturn
South:- Neptune
Southeast:- Uranus (mag. +5.8)
East:- Venus (mag. -4.1)

Southern Hemisphere
Evening
West:- Mercury (second half of month)
Northwest:- Jupiter
East:- Saturn
Midnight
West:- Jupiter
North:- Saturn
East:- Neptune
Morning
West:- Saturn
North:- Neptune
Northeast:- Venus, Uranus

Deep Sky

Small telescopes:-
Messier 13 - M13 - Great Hercules Globular Cluster
Messier 92 - M92 - Globular Cluster
Messier 11 - M11 - The Wild Duck Cluster (Open Cluster)
Messier 7 - M7 - The Ptolemy Cluster (Open Cluster)
Messier 6 - M6 - The Butterfly Cluster (Open Cluster)
Messier 4 - M4 - Globular Cluster
Messier 8 - M8 - Lagoon Nebula (Emission Nebula)
Messier 16 - M16 - Eagle Nebula (Emission Nebula with Open Cluster)
Messier 20 - M20 - Trifid Nebula (Emission and Reflection Nebula)

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