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Click to enlargeESO Horsehead Nebula Photo

January 25, 2002
Horsehead Nebula

This photo shows the famous "Horsehead Nebula", which 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).

This beautiful colour image was produced from three images obtained with the multi-mode FORS2 instrument at the second VLT Unit Telescope (KUEYEN), some months after it had "First Light". The image files were extracted from the VLT Science Archive Facility and the photo constitutes a fine example of the subsequent use of such valuable data.

Three predominant colours are seen in the image: red from the hydrogen (H-alpha) emission from the HII region; brown for the foreground obscuring dust; and blue-green for scattered starlight. The blue-green regions of the Horsehead Nebula correspond to regions not shadowed from the light from the stars in the H II region to the top of the picture and scatter stellar radiation towards the observer; these are thus `mountains' of dust. The Horse's `mane' is an area in which there is less dust along the line-of-sight and the background (H-alpha) emission from ionized hydrogen atoms can be seen through the foreground dust.

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. They can also be seen elsewhere in Orion - another well-known example is the pillars of M16 (the "Eagle Nebula") made famous by the fine HST image.

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.

To the east of the Horsehead (at the bottom of this image) there is ample evidence for star formation in the Lynds 1630 dark cloud. Here, the reflection nebula NGC 2023 surrounds the hot B-type star HD 37903 and some Herbig Haro objects are found which represent high-speed gas outflows from very young stars with masses of around a solar mass.

The HII region to the west (top of picture) is ionized by the strong radiation from the bright star Sigma Orionis, located just below the southernmost star in Orion's Belt. The chain of dust and molecular clouds are part of the Orion A and B regions (also known as Orion's `sword').

Credits: The European Southern Observatory (ESO).

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