The Double Cluster
in the constellation
Perseus is one of the more interesting objects for small telescopes.
The twin star groups are easily found midway (and a little to
the north) between Mirfak and Marfak. Mirfak is the brightest
star in Perseus, and Marfak is Theta Cassiopeiae. Either of these
two clusters would be a fine one on its own, but together in one
field of view they are superb. They are designated NGC 869, to
the west, and NGC 884, which has several red stars, but fewer
stars overall than its counterpart. It is curious why these fine
objects were not included in Messier's catalog. They are considered
to be among the youngest associations of stars in our galaxy,
with thousands of hot blue and white stars likely only a few millions
of years old.
Perseus is the location
of a bright nova that flared forth at the first of this century,
in February 1901. At maximum it was as bright as Aldebaran and
Capella, and formed a nice triangle in the sky with these two
stars. It then dropped in brilliance, and underwent a long period
of variable intensity. Now, after nearly a hundred years, only
a shell of expanding gas marks the spot not far from the centerline
of the Milky Way.
Variable intensity is
the characteristic of another star in Perseus, the well-known
and often-feared Algol, the Blinking Demon. The Chinese called
it the Tseih She, which means piled-up corpses. Unfortunately,
astrologers claimed it to be an unfortunate and sinister star.
It is, however, a prime example of an eclipsing binary star, and
that is the reason for its three-day period of brightness fluctuations.
As the pair of stars—one a massive blue-white star, the other
a larger, dimmer, yellow star—circle each other in an orbit
that lies in a plane with the Earth, we observe an eclipse, first
of one, then the second. Each of these events causes a dip in
the light curve, and the result is the still blinking ‘evil
eye’ of the Gorgon Medusa, slain by the hero Perseus. The
period of Algol had been accurately determined by John Goodricke
by 1782. He suggested the mechanism for the phenomenon. The stars
are about 100 light years away from us, and the pair, only 6.5
million miles apart, are too close together to be resolved by
direct observation from Earth.
Perseus, with the Milky
Way running through it, is home to a number of other fine star
clusters including one Messier object, M34. It contains about
80 stars within a half-degree diameter circle. It is located about
15 degrees south of the Double Cluster, half way between Algol
and Gamma Andromedae, the easternmost bright star in that constellation.
A second Messier object
is here in Perseus, too. M76 is so faint a planetary nebula, however,
that it is remarkable that Messier knew about it with his 3-inch
telescope. It is about 12th magnitude, a small object showing
little detail. Another faint deep-sky object here is NGC 1499.
This diffuse nebula requires photography to bring out its shape
that gives it the name California Nebula.