1.1 The magnitude scale

If you have been able to make your own observations of constellations such as Orion you will have seen the main outlines of the constellation marked out by first and second magnitude stars, which are bright enough to be seen even from a site with some light pollution. In order to see the far fainter fifth and sixth magnitude stars in the constellation, you would need to be located at a good dark observing site with clear skies and to have your eyes fully dark adapted. Faint stars are more numerous than bright stars in the sky, and at a really dark site such as the observatory at Tenerife, so many stars are visible on a clear night that it can actually be difficult to make out the constellations!

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Figure _unit4.2.1 Figure 1 The constellation of Orion, as seen from the Teide observatory. The four bright stars at each corner and the three stars of Orion’s belt range from magnitude 0.15 (Rigel) to magnitude 2.4 (Mintaka). Many fainter stars can be seen within and around Orion, with brightnesses ranging from third to sixth magnitude.

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The constellation of Orion as seen from Tenerife. The four bright stars at each corner and the three stars of Orion’s belt range from magnitude 0.15 (Rigel) to magnitude 2.4 (Mintaka). Many fainter stars can be seen within and around Orion, with brightnesses ranging from third to sixth magnitude.

Figure 1 The constellation of Orion, as seen from the Teide observatory. The four bright stars at each corner and the three stars of ...

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Figure _unit4.2.2 Figure 1a An annotated diagram of the constellation of Orion

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The constellation of Orion as seen from Tenerife. The main stars are labelled in yellow and lines outlining the shape of the constellation join these bright stars. At the centre of the constellation are three stars (Alnitak, Alnilam and Mintaka) in a vertical line, forming the belt of Orion. To the left is the bright red star Betelgeuse, with a bluer star Bellatrix above and to the right. These two stars form Orion's shoulders. To the right of the belt is the bright star Rigel, with the slightly fainter star Saiph below – these two stars form Orion's knees. Between the belt and these two stars a small line of three stars form the sword of Orion. A faint nebulosity can be seen around the centre of these three stars – this is the great Orion nebula.

Figure 1a An annotated diagram of the constellation of Orion

The original scale of magnitudes devised by Hipparchus was a simple classification into six groups of bright, fainter and faintest visible stars. Today, telescopes and scientific instruments can be used to measure the actual brightness of each star very precisely and so the magnitude scale has been given a mathematical basis and extended to cover objects far fainter than can be seen with the naked eye. On this numerical scale magnitude 1 stars are very bright and magnitude 6 stars very faint, as before. With accurate measurement stars can be given values in between, such as magnitude 2.7 for a star that is fainter than magnitude 2 but brighter than magnitude 3. (This may at first seem the wrong way round, with brighter stars having smaller magnitude numbers than fainter ones but as you get familiar with the scale you will soon get used to it.)

How bright a star appears as seen from the Earth depends on two things: the intrinsic brightness of the star (in other words, its total light output) and how far away the star is. A star of a particular intrinsic brightness further away from the Earth will appear fainter than a star of the same intrinsic brightness closer to the Earth because of the extra distance.

This course is concerned mainly with brightness as seen from the Earth which is referred to as the apparent magnitude, because it includes the effect of distance.

As one of the first people to use a telescope good enough for astronomical observations, Galileo extended the scale to include stars too faint to see with the unaided eye. Faint stars visible only in telescopes have magnitudes of +7, +8, and so on, with large telescopes such as COAST able to detect very faint stars of magnitudes up to +16. Larger observatory telescopes such as the Gran Telescopio Canarias and the Hubble telescope are able to go even fainter.

The scale has also been extended in the other direction – when placed on an accurate numerical scale some very bright stars turn out to be brighter than magnitude +1, and have been given values of zero or even negative magnitude values! On this scale Sirius, the brightest star in the sky, has a magnitude of –1.45.