ASTROPHOTOGRAPHY – How to tell a planet from a star

Astronomers studying variable stars have compiled catalogues listing about 10’000 stars visible with the naked eye (Mag 6.5 and less). This includes both hemispheres, so an observer at mid-Northern latitudes would technically be able to see about 6’800 stars on any given night (this takes into account the Southern stars visible in the North). Out of those 6’800 dots that one can see, five of them are planets. So how to tell the difference?

Stars emit their own light, whereas planets reflect the light of the Sun. Stars are point sources of light, meaning they have no apparent diameter – no matter how high the magnification, it’s impossible to see the actual sphere that makes up the star. Planets are significantly closer and one can see their actual shape, be it small (arc-seconds of a degree). As a result, star light is more prone to distorsions from the atmosphere and this is why stars twinkle, whereas planets do not.

How to photograph the twinkling of stars:

Expose for 2-3s and gently tap on your camera to induce movement and get a line on the photo, rather than a dot. Light from planets will be monochrome (Mars is a good target, because it’s nice and red), whereas light from a star will pass through all the colours of the rainbow as it’s distorted by the atmosphere.

Mars shows a constant red colour whereas a star will vary both in intensity and colour.

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