Star quality

Stars appear as dots of bright light in the night sky. As with our sun, every star represents an immense light source that is dazzling close up. So given the brightness of stars, why are the intervening spaces, viewed from Earth, black? ■  Stars are so far away that the amount of light reaching us is small. A relatively radiant star, such as Antares, is as bright as a single candle a kilometre away. This is not enough to dazzle you.  If your eyes are able to focus at infinity, the light of a star falls on just one rod or cone of your retina (or a few if there is dispersion). If your eyes are out of focus, each star will appear as a little disc, but dim stars will not really be visible, so you will still see black between the bright ones. Eric Kvaalen Les Essarts-le-Roi, France ■  The naked eye can see perhaps 9000 stars. Using medium-power telescopes this grows to millions, and modern observatories may detect billions. Estimates vary  as to the number of stars in the universe – or the observable universe, because what we see  is limited by the speed of light.  A minimum figure commonly cited is 1022.  So why then, are we not ablaze at night? This is what is known as “Olbers’ Paradox”, named after the 19th-century German astronomer Heinrich Olbers. At the time, the universe was thought to be static and set in an infinite sea of stars. Now we have considerably more knowledge about our cosmos. The simplest, most widely accepted hypothesis is that many of these stars are so far away that their light has not yet reached us, and maybe never will – at least in a form we can see. The expanding “Light does not reach us because of the distances involved and the speed at which galaxies recede” universe delays or prevents  light reaching us because of the distances involved, and maybe because galaxies recede from us  at speeds effectively greater than that of light. And the wavelength of the light radiation lengthens  as galaxies recede from our standpoint. This is known as redshift because the receding light source is shifted to the red end of the light spectrum. Interestingly, the essence of  this explanation is the same as that given by Edgar Allen Poe in Eureka. K. D. Perring Ashford, Kent, UK Thanks to all who sent in the following section from Poe’s essay, Eureka, written in 1848 – Ed ■  “Were the succession of stars endless, then the background  of the sky would present us an uniform luminosity, like that displayed by the Galaxy – since there could be absolutely no point, in all that background Wasps on the wing.