M42


              The Orion Nebula (M42, NGC 1976) 

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 four 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 upper left in this
picture) is a feature called the Dark Bay, which is a thick cloud of
neutral gas which has not yet been ionized.

Location: 05 hrs 35.4 min, -05 deg. 27 min (2000). 
Distance: nearly 500 parsecs (1600 light years). 
Size: about 66 by 60 arc minutes. 
Mass: about 300 solar masses. 
Magnitude: 4.0. 
Power source: O and B stars. 

Photograph: Bill Schoening, KPNO 4m telescope, October 1st
1973. Original Ektachrome color transparency.

Minimum credit line: Bill Schoening/AURA/NOAO/NSF (for details see
Copyright Statement)