names, such as Microscopium (the Microscope) and Telescopium (the
Telescope). This is particularly true for the stars in the far south of the sky
which couldn’t be seen from Europe when the first maps were made.
Earth spins on its axis once in 24 hours and does so from west to
east. This means the entire sky seems to move round from east to west
carrying the Sun, Moon and planets with it. In a northward direction
the axis points to the north celestial pole, closely marked by Polaris in
the constellation of Ursa Minor, the Little Bear. This means that Polaris
stands almost stationary while everything appears to move round it
every 24 hours. South of the equator Polaris can't be seen. The southern
equivalent, Sigma Octantis, is further from the south celestial pole, and is
much fainter.
There are stars of many kinds. Most shine for year after year, century
after century, but some brighten or fade over periods of days, weeks or
months. These are the variable stars. Stars very often appear in pairs,
with components that may be equal or unequal in brightness. These
double stars are of two kinds: optical doubles are line-of-sight effects,
with one more-or-less behind the other. However, in most cases the
components are connected and move round their common centre of
gravity. These are the binary stars. Rather surprisingly they are more
common than optical doubles. Our solitary Sun is exceptional.The Milky Way
All the stars we can see at night with the naked eye are members of our
own galaxy. Our Galaxy is a flattened star system containing 100,
million stars. When we look along the plane of the Galaxy we see many
stars in almost the same direction and these form the band in the sky
known as the Milky Way. The stars of the Milky Way are so numerous
they appear almost to be touching. Appearances are deceptive, as so
often in astronomy, and on average the stars are light years apart.
We also see clusters of stars, and here again they are of two kinds –
some are pure line-of-sight effects while others are genuinely associated.
The best known example is that of the Seven Sisters or Pleiades,
prominent in northern hemisphere winter skies, in the evening. People
with average eyesight can see at least seven stars, while those with keensight can see more – the record is 19; the whole cluster contains several
hundred stars, which were created in the same area at the same time.
Rather different are the globular clusters, huge spherical systems that
may contain up to a million stars. They are so far away few are visible
with the naked eye. The best example in the northern hemisphere at the
latitude of the UK or the northern United States is Messier 13, M13, the
Hercules Cluster.Cataloguing the Universe
In 1781, the French astronomer Charles Messier drew up a catalogue
of over 100 star clusters and nebulae. He did this not because he was
interested in the objects themselves, but rather because he wanted
to avoid them. He was a comet hunter and it is easy to confuse a faint
nebula or cluster with a comet. In modern times, Messier’s catalogue
is still used to list out some of the brightest and best deep-sky objects
in the sky. However, from an astronomical point of view, the catalogue56Astronomer Book
7[5] Comet C/2006 P1 McNaught photographed from the UK in January 2007.
[6] A long-exposure photograph of the Plough asterism results in star trails.
[7] Nebulosity within Orion’s Sword including the impressive Orion Nebula (Messier 42).