How a Telescope Revealed a Star City

On a very dark night, the book says, one or two of these star clusters may appear to the naked eye as faint spots against the black sky; others cannot be seen without a telescope^[1]. Through a powerful telescope, the little round patch takes on a startlingly different character: it is not a single light, but a gathering of minute stars crowded together^[2].

To help readers picture it, the book suggests shaking pepper onto a small circle of paper. The grains heap up at the centre, thin out towards the edge, and finally stand apart; imagine each grain shining as a star against a dark background^[3]. The book attributes to Herschel a calculation that more than 2,000 stars were visible in some such clusters. That is its historical estimate, not a present-day count^[4]. The astonishment lay in the change of scale: what the unaided eye could barely register as a smudge became, under the telescope, a densely patterned host of individual lights^[5].