The distance to the Orion Nebula is
approximately 450 parsecs, or 1500 light years.
The angular size of the Orion Nebula, as seen in a telescope, is
about 1 degree (twice the angular diameter of the full moon).
Thus, the diameter of the Orion Nebula can be computed to be 8
parsecs, or 26 light years.
At the distance of the Orion Nebula, an angular size of 0.05 arcseconds corresponds to a physical size of 22 astronomical units. Thus, the Hubble Space Telescope can detect fairly small details within the nebula.
At the center of the brightest part of the Orion Nebula is a quartet of hot stars. These four stars are called the ``Trapezium'', from their trapezoidal shape.
The hottest of the Trapezium stars:
The hottest Trapezium star is pouring out ultraviolet photons at a prodigious rate. These UV photons excite atoms and ions in the Orion Nebula, causing them to glow with their characteristic emission spectra.
The above picture is a mosaic of fifteen Hubble Space Telescope
snapshots; it is about 6 arcminutes across (about one-fifth the
width of the full moon). Click on the image for a really
spectacular enlarged view. The large-scale image shows the
Trapezium (the four young hot luminous stars in the center of the
Orion Nebula) as the four pinkish points of light to the left.
(Image credit: C. R. O'Dell [Rice University] and NASA)
The Trapezium stars must be young. The hottest has a mass 40 times that of the Sun, but is using up its hydrogen in fusion at > 300,000 times the Sun's rate. At most, the Trapezium stars can be 2 million years old. Star formation has RECENTLY taken place in the Orion Nebula; the Trapezium stars are evidence.
The density of atoms and ions within the Orion Nebula is about 600 per cubic centimeter. Without the Trapezium stars to excite them, the Orion Nebula would not be a nebula -- merely a boring HI cloud.
At visible wavelengths, as in the image above, the Orion nebula
is lit by the pinkish glow of excited hydrogen and the greenish
glow of ionized oxygen. The four apparently bright stars in the
center are the Trapezium. (This image was taken using the 1.8
meter Perkins Telescope, at Lowell Observatory, by R. Pogge of
the OSU astronomy department. Since the picture was taken from
the ground, it has a much lower resolution than the Hubble Space
Telescope mosaic, which covers roughly the same region.)
The optical images of the Orion Nebula contain glowing hydrogen (pink) and ionized oxygen (green). They also reveal the existence of dust. In order to see through the dust, you must look at near-infrared wavelengths (about 1500 nanometers). Just as red light is scattered less by dust than blue light, infrared light is scattered even less than red light. At near-infrared wavelengths, you can see straight through the dust.
At infrared wavelengths, as in the image below,
can we see the warm, contracting protostars hidden in the dusty
molecular cloud which lies behind the cloud of
ionized hydrogen. (This infrared image was also taken using the
1.8 meter Perkins Telescope, this time with an infrared-sensitive
camera attached. The observations were made by D. DePoy and B.
Ali of the OSU astronomy department.)
(This image was taken at a wavelength of 2200 nanometers. Most of
the stars seen in this image lie behind the
emission nebula seen in the visible image.)
It is also possible to look at the molecular
gas within which the protostars and young stars are embedded:
The above image is a picture of the central region of the Orion
Nebula (5 by 9 arcminutes), as seen in the emission of molecular
hydrogen. (Click on the image for an enlarged view.) The picture
shows the vicinity of the Becklin-Neugebauer object (a very young
star of spectral type B) and the Kleinman-Low object (a cluster
of protostars). The filamentary fingers which point away from the
center are jets of molecular material being blown away by
recently formed stars.
The molecular cloud which lies beyond the Orion Nebula is more dense (1 million molecules per cubic centimeter) and more massive (1000 solar masses or so) than the Orion Nebula itself, which is just a bright blister on the near side of the dark molecular cloud.
Just for amusement, here's another picture of
protostars in the vicinity of the Orion Nebula:
The above image is a near-infrared false-color image of OMC2 IRs2
(Orion Molecular Cloud 2, Infrared Source 2), a star formation
complex embedded deep within the molecular cloud which lies
behind the Orion Nebula. (Click on the image for an enlarged
view). The picture is a composite of 3 images taken at
wavelengths of 1200 nanometers, 1600 nanometers, and 2200
nanometers.
Star formation is propagating through the general vicinity of the Orion Nebula.
Star formation propagates by a chain reaction.
