All About Space Astronomer Book - 2014 UK

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Introduction

[8] The beautiful double star Albireo in the constellation Cygnus.

[9] The Markarian Chain, part of the Virgo Galaxy Cluster, taken by Ian Sharp.

[10] The Pleiades open cluster (Messier 45), taken by Ian Sharp

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is woefully short and the myriad of such objects now known to exist is
better covered by the far more extensive New General Catalogue (NGC)
and Index Catalogue (IC) lists.
As an aside, PM was once in his observatory observing Jupiter, and
when Jupiter was obscured by a cloud, amused himself by looking at
some of the clusters and nebulae not listed by Messier, either because
they could not possibly be confused with comets or because they were
too far south in the sky to be seen from France. Light-heartedly he
compiled a catalogue of more than 100 objects and called it the Caldwell
catalogue. Caldwell being part of his surname – a hyphenated one,
Caldwell-Moore. To his surprise, these “C” numbers are now widely used.
If we observe nebulae, Latin for clouds, through a small telescope, they
look like patches of light in the sky, and are of two main types. Some,
such as the Great Nebula in the constellation of Orion, look as if they are
made of gas, and in fact they are. In nebulae of this kind, new stars are
being formed from the interstellar material. Other nebular objects, such
as M31 in the constellation of Andromeda, are galaxies in their own right
containing millions of stars.
Our Galaxy has about 100,000 million stars, and from above would look
like a spiral. Beyond our Galaxy, many millions of light years away, we
see other galaxies, some spiral, some elliptical and some irregular. Each
contains its quota of stars, and the total number of stars in the universe
is staggeringly great. Moreover we know that many of these stars are
attended by planets in the same way as our own Sun. This means there
must be millions upon millions of planets like Earth, and who can tell if
they support life? This again is a subject to which we will return later. It is
very hard to believe we are unique.
Also, we cannot see right to the edge of our universe – assuming it has
an edge, which is far from certain. The most distant objects we can see are
13.7 thousand million light years away. What happens even further out, we
do not yet know.

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