Newsletter 12                                        July 1999      

The Brightness Of Stars


If you looked up at the night sky in the Drakensberg and tried to separate out all the stars and grouped them according to their brightness - how many groups do you think you would you come up with?

 Only six.

 The average human eye can distinguish only six grades of brightness in the stars visible to the naked eye. The very first star catalogue (drawn up by Ptolemy in the 2nd century AD) classed the stars into these six grades of brightness or magnitude. The brightest stars were said to be 1st magnitude, those less bright 2nd magnitude, and so on. The faintest stars that could just be seen clearly with the naked eye were called 6th magnitude stars.

 
IMPROVED VISION
Following the invention of the telescope almost 400 years ago, this brightness scale was extended to include all the new fainter stars visible - ie. magnitudes 7, 8 and so on. The Astronomy Interest Group 8-inch telescope can see stars as faint as magnitude 13.

 With the invention of light-sensitive detectors in the 19th century, it became possible to measure the relative brightness of stars more accurately. Today's instruments can record magnitudes in tenths, hundreds or even thousandths of a magnitude!

 What though of celestial objects brighter than the brightest stars - like some planets or even large satellites such as the two space stations? Logically, if the brightest stars are around magnitude 1, then even brighter objects (like planet Venus) would extend the scale backwards. In fact, the magnitude of Venus is -4.4; the full moon is -12.7 and the Sun (the brightest object in the sky) is magnitude -28.8.

 
APPARENT MAGNITUDE
Of course, the magnitude scale we are discussing is based on how bright the stars appear to us and our instruments on earth, and is therefore only the apparent magnitude.

 
ABSOLUTE MAGNITUDE
If you went and stood up close to each of the stars you would notice that they vary in their actual brightness, which we call their absolute magnitude. Thus a very bright star very far away can appear dimmer to us than a less bright one closer by (see real example in diagram).

So, why not pop outside tonight and reflect for a moment on the six magnitudes of brightness visible to our unaided eyes. And think too of how fortunate we are to be living at a time when we understand the Universe so well...

Mario Di Maggio

Betelgeuse is a star 10 000 times brighter than the Sun, yet it looks fainter than Sirius, which is nearly 20 times brighter than the Sun, because Betelgeuse is 50 times further away.

                      Mario Di Maggio

All enquiries:                     Mario Di Maggio Tel: 300 6228 (w)    or   082 829 7645   or  Mario Di Maggio                                                                       

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