Southern Cross, short defocused star trails
AAO image reference MISC 23.     « Previous || Next »

Southern Cross, short defocused star trails
Image and text © 1985-2002, Australian Astronomical Observatory, Photograph by David Malin.

Star images are point sources of light of widely varying intensity and subtle colour. The colour is a direct indicator of the surface temperature of a star -- blue indicates temperatures much hotter than the 6000K (~5700C) of the Sun. The blue stars here have temperatures of 20,000K or more, while the yellow-orange stars are cooler, with temperatures of 3500K or lower.

Normally, no single, in-focus photographic exposure on colour film or with a digital camera can capture their subtle colours. Those stars whose intensity is just right for their colour to be recorded appear insignificantly small on the photograph, while anything brighter is overexposed and washed out. By changing the focus of the lens during a star trail exposure, star images are spread out over the film or detector surface. This effectively changes the amount of light in each star image as well as making it bigger. If this is done in a series of steps, as here, sooner or later all the star images will be recorded with an exposure that will reveal the true colours of the stars.

Here we see the stars of the Southern Cross and Pointers (alpha and beta Cen) recorded in an exposure of about 30 minutes, during which the lens focus was moved from infinity to about 1 m in about 15 equally spaced steps. For a fuller explanation see a similar image of the stars in Orion, or another picture of the Southern Cross, where a smaller number of focus steps were used.

Related images
MISC 11. Orion star colours, step-focus technique
MISC 16. Southern Cross and Pointers, star colours - step-focus technique, long trails
MISC 19. Sunset 'star' trail, the track of the setting sun
MISC 32. Antares and Jupiter, defocused star trails

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Updated by David Malin, 2012, March 17