Preliminary Remarks about Magnitude

Preliminary Remarks about Magnitude
A few historical and general remarks may be in order for those just beginning their studies in astronomy. While these remarks on the brightness of stars may seem obvious to the seasoned observer, they are not quite so obvious to the beginner.

    The larger a star's visual magnitude, the dimmer the star.
         This convention is a result of the work of Hipparchus (c130 BC) who classified stars into six magnitudes. The ‘first magnitude stars’ were the brightest in the heavens, which included Capella (alpha Aurigae), Sirius (alpha Canis Majoris), Vega (alpha Lyrae) and the like.
       Hipparchus distributed the other stars according to their relative brightness, down to the dimmest that the naked eye could see, which were called sixth magnitude.
       On an exceptional night in the blackest of skies, the human eye can just register a sixth magnitude star. Thus, it was the dimmest object in the skies for ancient astronomers, which is why you won't find any stars dimmer than sixth magnitude as part of the constellation's ‘asterism’, or imaginary figure.
       With the advent of telescopes and other instruments the visual magnitudes became more refined. In the mid-1850s a first magnitude star was defined as exactly one hundred times brighter than a sixth magnitude star. What this meant was that every increase of one magnitude (from first to second, or second to third, etc.) amounts to a 2.5 times increase in brightness (since the fifth root of one hundred is 2.5).
       With the development of sophisticated photometric devices, visual magnitudes soon became much more accurate. It quickly became apparent that a few stars were actually brighter than one hundred times a sixth magnitude star. By necessity, these stars had magnitudes of less than one (Betelgeuse has a visual magnitude of 0.45). And even brighter stars wound up with negative visual magnitudes, such as Sirius, the brightest star in the heavens with a visual magnitude of -1.44.

    Through experience you will become able to estimate the visual magnitude of a star. This experience comes from recalling the values of the stars as you learn them and comparing their values to other stars. Not that you'll be spending your time memorising statistics. But you'll soon surprise yourself as how much you remember as you begin to learn to find the various stars.

    I have been using the term visual magnitude. Its definition is what you have probably assumed: the brightness of a star as seen by the observer. This is also called ‘apparent magnitude’.

    This visual magnitude depends considerably on how close the star is as well as its intrinsic brightness. Obviously a very bright star at a considerable distance will appear dimmer to us than a less bright star which is quite a bit closer. In order to compare the intrinsic brightness of stars the concept of absolute magnitude was introduced.
       If all the stars were placed at the same distance from the observer, we could then compare their intrinsic brightnesses. Using a distance of ten parsecs (or 32.6 light years) for all stars, the resulting brightness is called the "absolute magnitude". Many apparently bright stars show they are really quite dim when placed at that distance. Our sun, which has a visual magnitude of -26.8, has an absolute magnitude of only 4.8. This is the same as 4 Ursae Minoris, a very so-so star. Betelgeuse, already a bright 0.45 visual magnitude, has an absolute magnitude of -5.1.

         While naked-eye viewing can bring out fifth- and perhaps sixth-magnitude stars (very dimly), binoculars understandably increase one's ability to see many more stars.
         Many factors influence the dimmest magnitude visible through binoculars, factors such as the amount of light pollution, the presence of the moon, the age of the observer, the size of one's binocular objectives, etc. In general, one should be able to see stars at least with a visual magnitude of 8 and perhaps even 9. Large objectives (50mm) might extend this to tenth magnitude stars.

    In any event, our trip around the constellations will primarily be designed to help the beginner learn the relationships between various constellations, and the principal stars and deep sky objects within those constellations. The individual observer can make this trip as involved or as casual as he or she wishes. The main point is to enjoy the trip!


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© 1999-2000 by Richard Dibon-Smith.