Astronomy Picture of the Day
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October 2, 1996
Orion's Horsehead Nebula
Credit: Photograph made from plates taken with the
UK Schmidt Telescope.
Colour photography by David Malin.
Copyright: Anglo-Australian Telescope Board
Explanation: The Horsehead Nebula
is arguably the most famous nebula on the sky. It is visible
as the black indentation to the red emission nebula
seen just to the right of centre of the above photograph. The
bright star near the centre is located in the belt of the familiar
constellation of Orion.
The horse-head feature is dark because it is really an opaque
dust cloud which lies in
front of the bright red emission nebula.
Like clouds in Earth's atmosphere,
this cosmic cloud has assumed a recognizable shape by chance.
After many thousands of years, the internal motions of the cloud
will alter its appearance. The emission nebula's
red colour is caused by electrons recombining with protons to form
hydrogen atoms. Also
visible in the picture are blue reflection nebulae,
which preferentially reflect the blue light from nearby stars.
Tomorrow's picture: Three Views of Jupiter's Io
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