The International Astronomical Union (IAU) divides the sky into 88 constellations with clear boundaries, mapping every part of the sky to
one of these. Each constellation is grouped within one of eighty-eight constellation families. In three-dimensional space the stars in the constellations have
little or no relation to each other but, as viewed from Earth, appear grouped together against the night sky. Other patterns of stars are well known but
are not constellations, an example of which is The Plough - a pattern formed from the seven brightest stars within the constellation
of Ursa Major. Such
patterns of stars are called asterisms and may contain stars from one or more constellations.
Providing a way to segment the sky, the constellations are used to describe and find the location of objects. One of the first tasks for an observer is
to learn the constellations, at what time of year they are visible and in which constellations interesting objects are found. To begin recognising
constellations it is useful to take well-known reference points. The Plough is probably the best-known reference point in the northern hemisphere where it
is visible all year round. Within The Plough, Mizar is a double star, with a companion called Alcor, sometimes visible to the naked eye. There are numerous
galaxies lying within the constellation of Ursa Major, plus other deep sky objects, to be explored by telescope.
Looking at Constellation Map 1, a straight line drawn from The Plough`s two pointer stars Merak and Dubne extended five times as far again,
finds Polaris, the north star, within the constellation of Ursa Minor. If an arc is followed through the Plough`s handle stars of Megrez, Mizar and Alkaid
the bright star of Arcturus in the constellation of Bootes is found. Dropping through the īpan` of The Plough, between Phecda and Merak,
will bring you to the constellation of Leo, within the Zodiacal family of constellations. Within Leo the Leonids meteor shower
can be seen radiating out during November (at their best from the 16th to 18th).
Andromeda is another highlight of the winter sky in the northern hemisphere. When you have found it look through binoculars at the huge
Andromeda Galaxy which is our nearest large galaxy. It is visible to the naked eye and through binoculars will look like a small ball although it is
250,000 light years across, more than twice the size of the Milky Way. The spiral Andromeda Galaxy has two companion galaxies
known as M32 and M110, which are bright dwarf elliptical galaxies.
Another well-known reference point is the constellation of Orion, with three prominent and evenly spaced stars making
up his belt. During winter, Orion is clearly visible and is one of the brightest constellations.
Orion is home to one of the most beautiful sights in the sky, the Orion Nebula. With a magnitude of 4 it is one of the brightest deepsky
objects. It is situated below Orionīs belt as the middle of three stars that form Orionīs sword. If you look at this area through a pair of binoculars, you
can see not one, but many stars. Through a telescope you can see some of the giant gas clouds. The Hubble telescope has discovered that at least half of the
young stars have proto-planetary discs, which are considered to be the formation of solar systems. The high numbers are used as strong evidence that solar
systems are common in the Milky Way. This nebula is part of the huge Orion Molecular Cloud, composed of gas and dust, which extends throughout the
constellation of Orion and includes other nebulae.
Looking at Constellation Map 2, a line through Orionīs belt to the left arrives at Sirius in the constellation of Canis Major.
Sirius is the brightest star visible in our night sky. A line through the belt to the right, passes through the body of Taurus
to the Pleiades, also known as the Seven Sisters. This is an open star cluster that resembles a small shopping trolley.
The Pleiades is dominated by hot blue stars that have formed within the last
100 million years. Clouds of dust around the stars reflect the blue light. The cluster is 12 light years in diameter and contains about 500 stars of which
up to 14 can be seen by the naked eye, depending on eyesight and local conditions. It also contains many brown dwarfs, objects with such a small mass that
they are not heavy enough for nuclear fission reactions to begin in their cores which would allow them to become proper stars.
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