Double Star System
HD 201092, PPM 86049, HIP 104217, NSV 13546, TYC 03168-2798 1, GSC
Magnitudes: 5.2 + 6
requirements to view: any telescope under urban skies
This multiple star system is
among the most interesting in the sky. A mere 11 light years distant, the
stars of 61 Cygni are among our closest neighbors. The primary (A) is a
cool K5 main sequence star that is suspected to be variable (from 5.19 to
5.27 magnitude). The B component is a cool K7 dwarf that is also suspected
of variability (a tiny variation from 6.02 to 6.09 magnitude). The orbit
for these stars is indeterminate, but an orbital period of at least 700
years is indicated.
61 Cygni lies to the southeast
of Deneb. Look for a triangle of stars of similar brightness. 61 Cyg is
the westernmost. North is up and east is left.
In 2009 these stars are 31.3" apart, making them easy in any scope. The cool surface
temperatures give them a ruddy orange hue that really makes them stand out
in the field. The fainter star is a slightly darker red-orange. These
stars are near enough to us that their motion relative to the sun is
visible in just a few short years. When you look at this ruddy pair in the
eyepiece imagine them drifting to the northeast against the stationary
The view in a 6" at 152x.
The first image represents the appearance in August 2009. The second is
how things will appear in August 2019. North is down and east is to the
In addition to their motion across the sky, the two
stars orbit one another over a period of about 650 years. The pair
is currently widening slowly as they approach maximum separation in about
It is a testament to how how
much fainter the low-mass stars such as these are that the
components of 61 Cygni appear apprciably fainter than stars of similar
distance, such as Sirius (-1.5 magnitude) and Procyon (0.4 magnitude). The
low mass stars are also cooler which gives them a distinct reddish hue.
The mass of each of the 61 Cygni stars is around 1/2 that of the sun and
produce 1/6 as much light. Sirius, on the other hand, has a mass around
2.5 times that of our sun and is 40 times brighter.
The most massive stars start out
with much more fuel to burn, but they burn it so fast that they live very
short lives. The components of 61 Cygni will be around for tens of
billions of years--long after Sirius and our sun are gone. As such, this
pair of stars will be drifting across the sky long after our sky is