![[Orion]](/image_gallery/images/d2/02176b.jpg)
The great nebula in Orion, Messier object 42 (M42), NGC1976, as seen by the Kitt Peak 4-meter Mayall telescope in 1973. North is to the left.
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This picture shows the great nebula in the constellation of Orion the
Hunter. On a good clear night, from a dark site well away from the
lights of modern civilization, this glowing cloud of gas and dust
can be seen with the naked eye as a fuzzy patch surrounding the star
Theta Orionis in the Hunter's
Sword, below Orion's belt. It is probably the most spectacular of all the
objects cataloged by Charles Messier and now called by their `M' numbers.
M42 had been known since the beginnings of recorded astronomy as a star,
but it is so outstanding that it was first noted as an extended
nebula in 1610, only a year after Galileo's first use of the telescope.
Detailed descriptions started appearing later in the seventeenth
century, and it has been a popular target for anyone with a telescope
ever since. So many details are visible in even a small telescope that
M42 will more than repay the observer who makes it a frequent target,
and who will find that it is hard to make a realistic sketch that can
capture all of the finer features.
M42 is our closest example of an HII region, being composed
mainly of ionized hydrogen which gives off the red glow so dominant in
every picture of the nebula. Deep photographs such as this one show
that it is nearly a degree across, larger than the full Moon
(although the Moon is so bright that it looks much larger).
The energy to keep the nebula glowing comes from the very hot young
stars in a formation called the Trapezium, embedded in the brightest
part of the nebula and not visible in this photograph.
The nebula and the brighter stars are very young indeed by astronomical
standards, at about 30000 years. Compare this to our own Sun,
which is considered to be a middle-aged star at over 4 billion years!
M42 probably contains several hundred stars younger than a million
years, still bursting with the energy of youth.
Stars are still being born in a dense cloud behind the nebula, but they
are hidden from our view by a concentration of dust which reduces
their light to only a million-millionth of its original intensity.
Fortunately, astronomers have developed special cameras
and other detectors which are sensitive to infra-red radiation,
more popularly known as heat, which penetrates the dust and reveals to
us this stellar nursery.
Although M42 is mostly hydrogen, in both neutral and ionized states, with a fair
quantity of dust, it does contain significant amounts
of other elements, especially oxygen. The green glow of doubly-ionized
oxygen is strongest near the intense ultraviolet starlight at the
middle of the nebula. To the north-east (the lower left in this
picture, which is oriented with north to the left)
is a feature called the Dark Bay, which is a thick cloud of neutral
gas which has not yet been ionized.
M42, NGC 1976
Location: 05h 35.4m -05deg27m (2000).
Distance: nearly 500 parsecs (1600 light-years).
Size: 66x60 arc minutes
Mass: about 300 solar masses
Magnitude: 4.0.
Power source: O and B stars.
Photograph: Bill Schoening, KPNO 4m telescope, 1973.
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nebulae page,
reflection nebulae page,
emission nebulae page.
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