How bright would the night sky be if Earth was in a globular star cluster?


The view of the night sky from within a globular cluster would depend on your position within the cluster. This is because stars are packed together more tightly in the core of the cluster compared to the periphery. In the core of a typical cluster, the average distance between stars is about one light-year, compared to about five light-years in the solar neighbourhood.

Since the brightness of stars is inversely proportional to the square of distance, this means the brightness of stars in the night sky would be about 25 times what is seen from Earth. Now, on Earth, on a clear night, starlight has a brightness of about 0.001 lux (a unit of ‘illuminance’). So, on a planet in the core of a globular cluster, the starlight would be about 0.025 lux. By comparison, moonlight at a full Moon is about 0.2 lux, about eight times brighter.

So, although the night sky on our imaginary planet will be bright, and undoubtedly crowded with stars, it will be much fainter than Earth’s night sky during a full Moon. If our planet is similar to Earth, with a similar atmosphere, and is orbiting a star similar to the Sun, the daytime sky would be about 100,000 lux. So, our crowded night sky would still be drowned out by the daytime sky.

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Asked by: James Crannell, via email

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