of the most
identifiable nebulae in the sky,
Nebula in Orion,
is part of a large, dark,
cloud. Also known as Barnard
33, the unusual shape was first
plate in the late 1800s. The red
glow originates from
gas predominantly behind the nebula, ionized by the nearby bright star
Orionis. The darkness of the
is caused mostly by thick
although the lower part of the
neck casts a shadow to the left. Streams of gas leaving the nebula are
funneled by a strong
field. Bright spots in the
Nebula's base are
young stars just in the process
Light takes about 1,500 years to reach us from the
image was taken with the
Peak National Observatory.
The "Horsehead Nebula" is situated in the Orion molecular cloud complex. Its official name is Barnard 33 and it is a dust protrusion in the southern region of the dense dust cloud Lynds 1630, on the edge of the HII region IC 434. The distance to the region is about 1400 light-years (430 pc).
At the high resolution of this image the Horsehead appears very chaotic with many wisps and filaments and diffuse dust. At the top of the figure there is a bright rim separating the dust from the HII region. This is an `ionization front' where the ionizing photons from the HII region are moving into the cloud, destroying the dust and the molecules and heating and ionizing the gas.
Dust and molecules can exist in cold regions of interstellar space which are shielded from starlight by very large layers of gas and dust. Astronomers refer to elongated structures, such as the Horsehead, as `elephant trunks' (never mind the zoological confusion!) which are common on the boundaries of HII regions.
Such structures are only temporary as they are being constantly eroded by the expanding region of ionized gas and are destroyed on timescales of typically a few thousand years. The Horsehead as we see it today will therefore not last forever and minute changes will become observable as the time passes.