Collins Night Sky. Wil Tirion

only some of the brightest stars were given individual names. Some are of Greek or Roman origin (such as Sirius and Polaris), but many were devised by medieval Arab astronomers (Betelgeuse, Deneb, Zubenelgenubi, etc.). For hundreds of years fainter stars tended to be identified as ‘the first in Orion’s club’ or ‘the right foot of Hercules’, and other similar, ambiguous descriptions. The first step in introducing a sensible method of identifying stars was taken by the German cartographer, Johannes Bayer, who published the first true stellar atlas, the Uranometria in 1603.

      Bayer designated each of the brightest stars in every individual constellation by a Greek letter, arranged approximately in decreasing order of brightness, so that the brightest was labelled α (alpha), the second brightest β (beta), and so on. When Bayer exhausted the Greek alphabet, he employed lower-case Roman letters (a, b, c ...), and in just a few instances, when he had also exhausted those, he used upper-case Roman letters (A, B, C ... Q). Although we now know that there are exceptions to the correct order, in general all these names have been retained to the present day. Astronomers tend to use the Bayer letters in preference to the proper names. The Greek names are followed by the genitive of the Latin constellation name, so the star Alpheratz, for example, is α Andromedae or, abbreviated, α And. Occasionally, the Greek letters have been supplemented by adding a superscript number, usually where stars are close together. The extreme example of this is in Orion, where, on the western side, there is a chain of stars designated π1 to π6.

      With advances in astronomy – and particularly with the advent of the telescope – it became important to give designations to many fainter stars. The British astronomer, John Flamsteed was the first to carry this out systematically, and his catalogue of 1725 gave numbers to all the stars in a constellation in increasing right ascension – i.e., from west to east – including stars that Bayer had labelled. These Flamsteed numbers continue to be used, but to prevent confusion, the older, Bayer letters are generally given where applicable. For example, α And is also Flamsteed 21 Andromedae, usually written ‘Fl 21’ or ‘21 And’. If a number appears on a chart, in the absence of any information to the contrary, it may be assumed to be a Flamsteed number.

      With the advent of photography, tens and hundreds of thousands of stars had to be catalogued, and so all later catalogues use a numerical format. Such numbers are rarely required when dealing with the majority of objects visible with the naked eye or small instruments.

      Variable stars have a complex system of names, but the brightest in any constellation will generally have a single, upper-case, Roman letter designation in the range R to Z – the letters Bayer did not use. Other variables will have a double letter designation (e.g., CH Cygni), or be known by the letter ‘V’ followed by a number (e.g., V465 Cassiopeiae). The Bayer, Greek-letter designations are retained for those stars that have subsequently proved to be variable, for consistency with older records.

      MESSIER, NGC & IC NUMBERS

      Non-stellar objects frequently have names consisting of the letter ‘M’ followed by a number in the range 1–110. This stands for the designation in the catalogue of non-stellar objects prepared by Charles Messier in the late 18th century to help in the search for comets. Messier catalogued 103 objects, M104 to M110 being added by later observers.

      Similar numbered catalogues prepared in the 19th century were the New General Catalogue (NGC) and the Index Catalogue (IC), and these abbreviations are frequently encountered. These catalogues included objects in the Messier list, so alternative designations are sometimes encountered. The great Andromeda Galaxy, M31, for example, is also NGC 224.

MAGNITUDES

      The brightness of a star, planet, or satellite is known as its magnitude. Originally, when this system was first devised, the brightest stars were judged to be of first magnitude, slightly fainter ones as second magnitude, and so on, down to sixth magnitude, which were the faintest visible to the naked eye.

      This very crude system survived until the nineteenth century, when it was set on a firm scientific basis. A first magnitude star was defined as being 100 times as bright as a sixth magnitude star, leading to a precise mathematical relationship between magnitudes. A first-magnitude star is 2.512 times as bright as a second-magnitude star. This apparently rather odd relationship – a logarithmic ratio – actually closely matches the way in which the eye perceives brightness.

      There was no problem in extending the scale to the innumerable fainter stars that could be seen with telescopes, but it was found that a few stars (and certain planets at particular times) were brighter than the star that was selected as the zero point of the scale. So the scale was extended to negative magnitudes. The brightest star in the sky, Sirius, in Orion, is of magnitude -1.44, Jupiter may reach magnitude -2.5, Venus magnitude -4.6, while the Full Moon is about -13.0.

      The sizes of stars on charts are carefully related to their actual magnitudes.

      The faintest star that is visible with the naked eye, or with a particular instrument is known as the limiting magnitude. Under perfect conditions, with an absolutely black, pollution-free sky, the limiting magnitude for the naked eye is about magnitude 6.5. The colour of a particular star can affect the way in which it is detected by the eye, and this will be discussed later.

      Stars of different magnitudes are shown on most star charts as dots of differing sizes: the larger the dot, the brighter the star. If care is taken in choosing the correct dot sizes, most people find it relatively easy to relate stars on a chart to stars of different brightness in the night sky. Depending on the scale of the charts and their intended purpose, their limiting magnitudes will vary. The individual constellation charts given later in this book reach magnitude 6.5 and thus show all the stars visible to the naked eye. Individual finder charts for certain specific objects have fainter limiting magnitudes.

      SEE ALSO

       colours of stars

      LUMINOSITIES

      The magnitudes that we have discussed are the apparent magnitudes (m), that is, the magnitudes as they appear in the sky, taking no account of the stars’ very differing distances.

      A telescopic view of Jupiter, the second brightest planet, often reaching magnitude –2.5.

      If we know the distance to a star, we can calculate the magnitude that it would have at some specific distance and this is chosen to be 10 parsecs (32.616 light-years). Magnitudes calculated for this standard distance are known as absolute magnitudes (M) and are a measure of the actual luminosity of stars. These luminosities are found to cover an extremely wide range, from thousands of times the luminosity of the Sun, to thousandths of its luminosity.

      Although apparent magnitudes are often quoted for non-stellar objects, such as planets, comets, nebulae, and galaxies, they need to be treated with some caution, because they do not apply to point sources. The magnitude is taken over the whole extent of the object, so it is far more difficult to see a 7th-magnitude comet or nebula than a 7th-magnitude star.

      Ed Grafton

       Steve Massey

      The magnitudes quoted for extragalactic objects such as M88 (in Coma Berenices) are often deceptive, because they include the light from the outer regions, which are difficult to detect.

PHOTOGRAPHING THE NIGHT SKY

      It is not difficult to take photographs of the night sky, and most of those shown in this book have been obtained with relatively simple equipment. Because