101 SKY OBJECTS
WWW.ASTRONOMY.COM 4154 The Double Cluster
Known since the earliest days of naked-eye astronomy, the Double
Cluster in Perseus appears in the star catalog of Greek astronomer
Hipparchus in 130 B.C. The two clusters are now designated h and
Chi (χ) Persei (NGC 869 and NGC 884, respectively) and look like neb-
ulous 4th-magnitude stars. One of astronomy history’s real mysteries is
why Charles Messier listed the Pleiades (M45) and the Beehive (M44)
clusters, but not the Double Cluster, in his catalog of non-cometary
objects — oops! The earliest use of the Double Cluster moniker seems
to be George F. Chambers’ 1867 edition of Descriptive Astronomy.
Finding the Double Cluster is only challenging when light pol-
lution is significant. Locate Epsilon (ε) and Delta (δ) Cassiopeiae
and look southeast about 50 percent farther than the gap between
those stars. Even with some light pollution, the Double Cluster
should be visible in a finder scope or binoculars.55 The Owl Cluster
The names of the members of our solar system date back to antiquity, derived from gods and
goddesses. Sometimes an object’s name connects to its constellation — the Orion Nebula and
Andromeda Galaxy are well-known examples. Other times, an object’s shape dictates its name: The
Ring Nebula and Whirlpool Galaxy have become proper nouns. And the combination of photography
and imagination has birthed even more names, like the North America Nebula or Horsehead Nebula.
But sometimes shape is more subtle and many observers assign the same object different names.
This seems to happen a lot these days, mostly because amateur astronomers assign various moni-
kers. NGC 457 is one such example. This rich open cluster in Cassiopeia was named the Owl Cluster
by David J. Eicher in the October 1980 issue of this magazine. It is also called the E.T. Cluster, the
No. 5 Cluster (for the robot in the movie Short Circuit), and the Stick Man Cluster, among others.
Many of Cassiopeia’s clusters lie in the Perseus Arm of the Milky Way. (We are in the Orion Spur of
the Sagittarius Arm.) The Owl Cluster is slightly more than 7,900 light-years away. It is 13' across and
one of the brightest open clusters in Cassiopeia at magnitude 6.4. This brightness makes it surprising
that Messier overlooked NGC 457, as M52 and M103 are both magnitude 7.
The Owl Cluster’s eyes are Phi (φ) Cassiopeiae and HD 7902 (magnitude 5 and 7, respectively).
While Phi has been considered a cluster member — and is one of the most luminous stars known —
astronomer James Kaler found evidence that this star is between 2,300 and 4,500 light-years away.
The Owl’s legs and arms are strings of stars that stand out from both cluster members and foreground
CRAIG & TAMMY TEMPLEstars. — A.G.CHRIS SCHURGAÉTAN THIBAULT
About 7,500 light-years away, these are some of the most distant
naked-eye star clusters. The light you're seeing today comes from
a time about 3,000 years earlier than the pyramids in Egypt, just
as humans first began smelting copper in the Neolithic period and
before the invention of the wheel.
The Double Cluster certainly defines “rich” in terms of star fields.
Even the smallest telescopes easily resolve both clusters. Yet larger
apertures don’t overwhelm the view. Increasing magnification will
push one of the clusters out of the field, but its stars are simply
replaced by more in the single cluster left.
Both clusters contain young, hot stars sprinkled in with red
supergiants like Antares and Betelgeuse. The color contrast of blue
and red — observable in many clusters with varying difficulty — is
ridiculously easy here. Whether you are using a 3-inch lens or a
12-inch mirror, NGC 869 and NGC 884 are an amazing sight that will
impress non-stargazing friends in the backyard after a barbecue. — A.G.