Sky & Telescope - November 2023 - 75

Arizona observer
Jeremy Perez made
these sketches of
double stars using
a 6-inch Newtonian
telescope. Notice
the subtle color differences.
North is up
and east to left.
Gamma (γ) Andromedae
Gamma (γ) Delphini
in which each of the two stars eclipses
its mate once every orbit - which
astronomers know only because we
see them edge-on and the light of the
primary dims periodically. In spectroscopic
binaries, the two stars are so close
together that they're not resolvable in
any telescope. But using spectroscopy,
or the study of their light, astronomers
can sometimes tease apart stars that are
visually not separable. This is how they
determined, for instance, that Mizar is
a four-star system.
Dabs of Color
What many observers find most pleasing
about double stars is their color contrasts.
Take Alberio, for example, which
is many observers' favorite double star.
Also known as Beta (β) Cygni, Alberio
sits at the bill-end of Cygnus, the Swan.
Its bright orange primary and sapphireblue
secondary comprise arguably the
most famous optical double in the sky.
Another standout is Gamma
Andromedae, a binary in Andromeda.
Known as Almach, the pair's striking
golden and greenish-blue hues are obvious
even in a small scope. (Like Mizar,
Almach is also a quadruple-star system,
with the primary and secondary each
having its own orbiting star.)
Gamma Delphini, in Delphinus,
the Dolphin, has a deep-yellow primary
and a pale-green secondary. They are
easily split in small telescopes. Haas has
described the colors as " muted lemon
and lime green. "
Eta Cassiopeia has strikingly vivid
tints. There's a distinctive brightness
Eta (η) Cassiopeia
contrast as well, with four magnitudes
difference between the primary and secondary.
Many observers see the primary
as deep yellow and the secondary as a
reddish purple, which led S&T Contributing
Editor Jim Mullaney, author
of Double and Multiple Stars and How
to Observe Them, to call Eta Cass the
" Easter Egg double. "
Note that colors are subjective: Different
people often see different colors
when viewing the same double star.
When a double has a faint secondary,
tints can also be entirely illusory, the
effect of contrast with its much brighter
primary. That's because, when two stars
are close by each other, our brains naturally
enhance any perceived color differences
that may exist between them. In
reality, the components of a double star
North
0°
have the same colors as single stars of
the same type and temperature, which
range from bluish-white to orange.
Location, location
To describe double stars, astronomers
use two measurements: separation and
position angle. Separation of stars in a
multi-star system is given in arcseconds.
One arcsecond is 1/60th of an arcminute,
which is 1/60th of a degree. As
a guide, the Moon is about one-half of a
degree in diameter. Double star enthusiasts
delight in the challenge of splitting
close pairs - especially those near the
resolution limits of their telescopes.
Position angle, which tells you where
to look for a secondary star in relation
to the primary, is measured from north
around through east, south, and west.
This is true whatever the orientation
of your eyepiece view, which can differ
depending on your type of telescope (see
S&T: Aug. 2023, p. 76).
Armed with these two numbers,
East
90°
Position angle
in degrees
Primary
star
Separation
in arcseconds
Secondary
star
South
180°
Double stars are described by their separation
in arcseconds and the position angle of the
dimmer secondary with respect to the brighter
primary. Position angle is measured from north
around through east, south, and west, whatever
the orientation of the eyepiece view.
West
270°
experienced observers will not only
know where to look for the often fainter
secondary, but can also tell if they will
be able to split that particular double.
Note that because stars are continually
in motion, these two figures can change
significantly over time.
Thousands of Doubles
Want to dig deeper? Have a look at
Haas's or Mullaney's books. To go all
in, consult the definitive reference, the
Washington Double Star Catalog, which
lists over 100,000 double or multiple
stars (astro.gsu.edu/wds). ¢
sk yand tele scope .o r g * NOVEMBER 2023 75
DOUBLE STAR SKETCHES: JEREMY PEREZ / PEREZMEDIA.NET;
CHART: S&T ILLUSTRATION
http://www.PEREZMEDIA.NET http://astro.gsu.edu/wds http://skyandtelescope.com
Sky & Telescope - November 2023

Table of Contents for the Digital Edition of Sky & Telescope - November 2023
