While visual estimates of brightness don't achieve the accuracy of those measured with instrumentation,
they do indeed have true scientific value. Though a handful of estimates would be very limited in their usefulness, much larger numbers of estimates accumulated over years, can produce a very accurate description of a star's behavior. The AAVSO, or American Associaion of Variable Star Observers, maintains
the largest, and longest running database in the world. At present, several hundred amateur astronomers
worldwide contribute roughly 300,000 observations a year. Amateurs are able to follow a greater number
of candidate stars than can professionals, who are limited by telescope time and competing projects. Careful monitoring of variable sources by amateur astronomers can produce timely alerts to changes, of interest to professionals, who can then employ more sophisticated methods.
for the Mira type variable Chi
Cygni. Individual observations
appear as dots. The large amp-
litude variation in the curve indi-
cates a maximum brightness of
about 5, to fainter than 13. Since
the magnitude scale is expo-
nential, this translates to a factor
of greater than 1500. In practical
terms, at it's peak Chi Cygni is
barely visible to the naked eye.
When near minimum, a six to
eight inch telescope is required
to see it at all.
of the AAVSO through it's
Director, Janet Mattei.
star between two comparison stars. These comparison stars ideally span a range that somewhat exceeds the variable at it's maximum
and minimum. These stars do not change in brightness and have
had their magnitudes determined photoelectrically. By glancing back and forth between variable and comparison stars, the observer finds
one star slightly brighter and one a little fainter than the subject star.
Thus if the variable appears to fit halfway between 7.8 and 8.2 com-
parison stars, its estimate would be 8.0.
The chart at right is the inner portion of an AAVSO chart for variable
VX Andromedae. The magnitudes of the comparison stars range from 7.5 to 11.9 . The decimals are omited to avoid confusion with faint stars.
making an estimate. Red stars tend to have a cumulative effect on the retina, appearing to brighten if watched
for a prolonged period. This is known as the Purkinje Effect. The presence of moonlight can also introduce a bias,
in this case due to the increase in contrast between the red star and the bluer sky. Two techniques are used to
minimize these errors, the quick glance, and the out of focus method. Looking at the variable for only brief intervals
avoids red light build up, minimizing the Purkinje Effect. The second method has two attributes. Defocusing a red
star makes the disc appear fainter, as its light is spread out over a greater area. The fainter surface brightness forces
the eye to perceive it in "black and white" mode, eliminating a color difference with the comparison star. Additionally,
by stretching the light into evenly illuminated discs, small brightness differences become more apparent. An accuracy
of a tenth magnitude is generally accepted to be within the skills of an experienced observer.
more, once a month is sufficient. Weekly or biweekly estimates are desireable for stars with short periods, say several months,
or semiregular variables, whose behavior is more erratic. To aid observers, the AAVSO issues a yearly bulletin with the predicted dates of maxima and minima for hundreds of long period variables.
Recommended reading:
Starlight Nights by Leslie Peltier. Autobiographical account of an outstanding amateur astronomer and variable star observer.
In and out of print, it may be a little difficult to find, but is well worth the search.
Observing Variable Stars by David Levy. A practical observing guide, with details and descriptions of many individual stars.