Dr.
Robert Agnew (shown above sitting atop an ancient Mayan observatory in
Seeing
Double - In Living Color
For
centuries after the invention of the telescope, when two stars appeared
close
together in the eyepiece, they were thought to be simply aligned in the
same
path, though probably at great distances from each other. In the late
18th
century, however, a few astronomers began to speculate that some stars
were
part of a binary system of gravitationally bound stars.
We now
know
that a significant percentage of stars are in a binary system, and many
are
part of a multiple system of stars. Most of those double stars are too
close to
be seen independently by backyard telescopes, but there are hundreds
available
through a modest telescope and thousands with a large amateur telescope.
Perhaps
the
greatest joy for the amateur astronomer in the observation and
contemplation of
the double star is the perception of color. No other sights in the
night sky
reveal such a diversity of hues.
From my
journal of
“Struve
2544 (19h 37.1m / +08° 18’) contained a magnitude
8.59 white primary with a
magnitude 9.84 purple secondary. The separation was 14.6 arc-seconds
and the
magnification 80X. The contrast was fine on this colorful and beautiful
binary.”
So what
are
we dealing with in this journal entry? Wilhelm Struve was a cataloger
of double
stars, the numbers in parenthesis represent the position of the double
star in
the night sky, and the given magnitudes are the brightness of the two
stars.
The lower the magnitude, the brighter the star, with a magnitude six
star being
just perceivable with a good pair of eyes at a dark sky location. The
separation is the astronomical distance of the two stars. Contrast is
the
difference between the brighter star, usually termed the primary, and
the
dimmer star, usually called the secondary.
Here is
another color combination from my journal of
“Struve
1183 contains magnitude 6.23 and 7.93 stars at a distance of 30.8
arc-seconds.
The binary has a yellow primary with a haunting, vivid, blue
secondary.”
Perhaps
the
most amazing color perceived by the double star hunter is found in this
journal
entry from
“Gamma
Delphini (Struve 2727: 20h 46.6m / +16° 08’) is one
of those rare binary
systems in which the secondary appears green (an impossible color since
no star
can actually be perceived as green by the human eye). A magnitude 4.5
yellow
primary was accompanied by a magnitude 5.5 green secondary at a
separation of
9.6 arc-seconds. An amazing sight!”
Green
stars
do exist, except that when the temperature of a star is such that the
maximum
of the emissions is in the green wavelength, the brightness of the
other colors
are so close to that of green, that the star, in reality, appears
white. In
fact, red, yellow, white, and blue are the only colors perceivable
through the
eyepiece of a telescope. So how did I see green? There are several
possible
reasons, but the one usually cited is that given a bright primary and a
dim
secondary, and a color contrast between them, the dimmer star may
appear “off
color” thanks to the effect of the bright star. One of those
“off color” colors
is green. Purple is another, as well as shades of pink and orange. The
variety
is astounding!
Contrast
in
size makes for another interesting aspect of double stars, as does the
closeness of the two stars through the eyepiece. I have viewed over a
thousand
of them, and they remain one of my favorite pastimes with a telescope.
©
Article &
image Dr. Robert Agnew 2008/2009