A Blinding Light

One principal characteristic of any star seen in a normal optical telescope is its brightness (or luminosity). It is clear that the Sun is far brighter than any other star in the sky, yet we stated above that as stars go it is rather average. This tells us that when considering how bright an object is we must take into account how close it is to us. A car with its headlights on full beam is not particularly annoying until it gets to within about 300m of us. Similarly, the distance a luminous star is from us has an important bearing on how bright we perceive it to be. The Sun being so close appears to be far brighter than any star seen in the night sky.

Astronomers define two types of brightness when referring to the light being emitted from a star or other celestial object. The apparent brightness is how bright we perceive the star in relation to other stars. In the 2nd century BC the Greek astronomer, Hipparchus, categorized all naked eye stars into six brightness classes, numbered 1 through 6. A star of brightness 1 corresponds to the brightest visible and a star of brightness 6 is just discernible to the naked eye. Despite the subjectivity of this system it has remained in use to the present day. In terms of apparent brightness, the Sun would be roughly four trillion times brighter than an identical star 30 light years away.

A more objective measure of a star's brightness is obtained when we correct for distance and define the intrinsic brightness of a star. This is done by allowing for the different distances of the stars from the earth. This is possible because we know exactly how the light from a given object is diluted as it spreads out in space. When each star is assigned an absolute brightness, a direct and fair comparison between different stars is possible. Under this system of measurement the Sun has an absolute brightness equivalent to 4.6 on Hipparchus' naked eye scale. This is not particularly impressive in stellar terms but does constitute 4 x 10^26 Watts of energy continually bathing the earth. At this rate of energy generation the Sun is expected to last for 10,000 million years.

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