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How bright are the stars?


Every star is giving out vast amounts of light, and also radio waves and heat and lots of other forms of energy. Today’s professional astronomers have huge telescopes and other instruments which allow them to measure all of these, however far away the star is from the Earth.

But the only thing that mattered to the ancient astronomers, and today the only thing which matters to most people just looking at the Night Sky, and to Sam and his friends, is what today’s astronomers call their apparent visual magnitude, how bright they appear to be to people just looking at the sky without a telescope.


In the same way that football teams are put into Leagues and Divisions, stars are put into magnitudes: a magnitude 1 star is brighter than a magnitude 2 star, and so on. Without a telescope, on a clear moonless night most people can see stars down to magnitude 6. Most ancient astronomers had realised that there were probably many more stars that were not bright enough for them to see. With his telescope Sam can see many more stars.


Today’s astronomers also rank stars within each magnitude, for example magnitude 3.7 - but they are actually using what mathematicians call a logarithmic scale. Sam does not understand logarithms, as he has not yet started learning about them at school, he just accepts that a magnitude 5.7 star is brighter than a magnitude 6.3 star, how much brighter does not worry him. But a logarithmic scale means we can have stars and planets with magnitudes of zero, or even negative magnitudes. Sam can cope with this, he just remembers that on a number line -2 is smaller than -1 and the lower the magnitude the brighter the star.

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The brightest star in the sky is Sirius, with a magnitude of -1.46. Logarithmetic scales are discussed on another Page FTPO.

Some stars are variable, that is, they become brighter and dimmer in a regular cycle. Other stars have changed their magnitude since the earliest astronomers were looking at them. In Roman mythology Castor and Pollox were twins, and the two very bright stars in the constellation Gemini (The Twins), were named after them. Two thousand years ago Castor and Pollox were equally bright but today Pollux has a magnitude of 1.14 but Castor only 1.58. This difference may not seem much, but because it is a logarithmic scale it is actually quite a lot.

The planets are not always the same distance from the Earth so they change their magnitude. Venus at its brightest has a magnitude of -4.4, bright enough to cast a shadow, and is the brightest object in the sky, after the Sun and Moon of course.

Sam once looked up the magnitudes of the Sun and Moon in one of the books his Granddad had given him, and wished he hadn’t, the logarithmic figures were quite meaningless to him. But you do not need a book to tell you that the Sun is much brighter than the Moon, and that the Moon is much brighter than anything else in the sky.

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©Barry Gray December 2018