Horsehead


About this image 

This color picture was made by combining several exposures taken on
the night of December 28th 1994 (UT of observation 29/12/94 around
04:00) with a 2048x2048 CCD detector at the 0.9m telescope of the Kitt
Peak National Observatory.  Observing conditions were not ideal
throughout, and so only a select few of the original observations were
used. The final tally used five frames in the B (blue) filter for a
total of 22 minutes, three frames with the V (green) filter, 15
minutes, and two with the R (red), total 10 minutes. Each frame was
carefully cleaned, a particularly difficult task for the blue filter
due to internal reflection problems in the telescope, and then aligned
and combined by computer to create this (approximately) true color
picture. The pixel size on the sky is 0.68 arc seconds; after
combination, the final size is 1480x1366, or about 17x15 arc minutes.
Orientation of thumbnail image: N to the left, E down.

About this object 

The Horsehead Nebula, a part of the optical nebula IC434 and also
known as Barnard 33, was first recorded in 1888 on a photographic
plate taken at the Harvard College Observatory. Its coincidental
appearance as the profile of a horse's head and neck has led to its
becoming one of the most familiar astronomical objects. It is, in
fact, an extremely dense cloud projecting in front of the ionized gas
that provides the pink glow so nicely revealed in this picture. We
know this not only because the underside of the `neck' is especially
dark, but because it actually casts a shadow on the field to its east
(below the `muzzle').  The marked change in the density of stars
visible on either side indicates that the strip of glowing hydrogen
marks the edge of a substantial dark cloud. As a cloud core emerging
from its parental cloud, and as an active site of low-mass star
formation, the Horsehead is a simple system of considerable use for
testing models of photodissociation regions, and revealing the
intricate interrelations between gas, dust, and the light from hot
stars. Polarization maps suggest that the entire region is illuminated
by the bright OB star Sigma Orionis, which is also responsible for
exciting the emission nebula.  (The much brighter Zeta Orionis is a
foreground star, not related to the nebulosity.)  The `streamers'
visible in the brighter region appear to be due to a magnetic field
which leaves the Horsehead cloud approximately radially, having been
entrained by outflowing matter. Small red spots in the base of the
Horsehead betray the presence of hidden protostars, and red streaks
near the yellowish nebula surrounding V615 Orionis (bottom left) are
Herbig-Haro objects, which are jets of material ejected from
protostars. The Horsehead is a fascinating, active, and complex
neighborhood.  Location: 05 38 27 -02 29 (1950.0), constellation of
Orion.  Distance: about 1500 light years.  Credit: Nigel
Sharp/NOAO. Image \copyright AURA Inc./NOAO.

Minimum credit line: N.A.Sharp/AURA/NOAO/NSF (for details see
Copyright Statement)