The summer solstice having come and gone on June 21, the sun has reached
its northernmost point and is now heading south for the winter. Notice
where the sun lies on the horizon at sunrise and sunset, and follow its
latest annual migration southward day by day.
This month's star chart features the summer constellations from Cygnus
(the Swan, also the Northern Cross) through Lyra (the Lyre) and Hercules
to Corona Borealis (the Northern Crown). To my eye, Corona Borealis looks
more like a tiara than a crown. How about Tiara del Norte?
Hercules vaguely resembles a man with bent legs and outstretched arms.
The "Keystone" represents his chest and contains M13, the second largest
globular cluster in the sky. (Last month's star chart shows the largest
globular, Omega Centauri, in the southern sky.) M13 is readily visible
in binoculars and even to the naked eye under good conditions.
M13 is about 25,000 light years away. Its diameter is about 160 ly,
and it contains about 1 million stars. The stars appear very densely
packed in a scope, but are in fact widely separated even in the most densely
populated region about 100 ly in diameter. Burnham's Celestial Handbook
says the whole cluster has a volume of about 1 million cubic ly, so each
of the million stars occupies roughly a cubic ly of space.
Burnham offers an imaginary model of M13 where a grain of sand represents
each of the 1 million stars. The model would be about 300 miles in
diameter. Most of the sand grains would be about 3 miles apart, except
in the most densely populated central core of the model, where they would
be separated by a sizable fraction of a mile. Like most of the rest
of the universe, globular clusters are mostly nothing.
You can print a copy of this star
chart so you can take it outside.
