scienceillustrated.com.au | 25Carried out correctly, the result should be a
multicoloured line in which the light has been
refracted at different wavelengths. The line in
this image is straight and even because it was
taken using a camera with a motorised pan.IS IT REALLY TRUE THAT ...
... stars are twinkling?
From Earth, almost all stars look as if they twinkle or flash.
Is this due to solar eruptions on the surface of the stars?
ASTRONOMY Astronomers only know
of a few stars that actually flash: these are
called variable stars. The reason that
almost all stars seem to twinkle as we
observe them from Earth is to be found in
the atmosphere. The air above us is never
really at rest; it includes countless small
whirls and bubbles caused by slight
temperature differences, or disruption
from objects in motion including birds,
planes, and the wind itself. These all
combine to cause microscopic differences
in the air’s mass which function as tiny
lenses that refract light on its way through
the atmosphere. We see a more extreme
version of this phenomenon over a hot
sunlit asphalt road, where the light’s
refraction becomes more apparent.
Observed from Earth, stars are just
tiny, pinhead-sized light sources, so even
these tiny lenses in the atmosphere can
efficiently refract their light. The refraction
means that different colours of the stars’
light reach us at different times, and this
continuously changing colour from
individual stars is what we register as
flashes or twinkling. The brighter the
star, the less the refraction affects its
light, so the less it flashes. The same is
true for planets such as Venus and Mars,
which deliver light beams so wide that
the lenses do not interrupt them at all.Capture the colour
change of starlight
By means of a mirror reflex
camera with a powerful zoom
lens, you can prove that the star’s
flashing is due to colour change.Thick air interrupts starlight
The ‘flashing’ of a star becomes more marked
the lower on the horizon it is located, because of
the light’s longer passage through the atmosphere.2
Use maximum zoom and a
shutter speed of 10 to 20 seconds.
Press the release button.3
Turn the camera very slowly, so
the star moves from one side of
the image field to the other during
the period in which the shutter is open.Light from a star close to
the horizon travels further
through the atmosphere, so
is more likely to twinkle.1
Place the camera on a tripod
mount or a flat smooth surface.
Point it at a star close to the horizon.Light from a star right above you
travels the minimum possible
distance through the atmosphere,
so the star ‘twinkles’ only slightly.JES
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ATMOSPHERE