Press Release

VLT Images the Horsehead Nebula

By SpaceRef Editor
January 25, 2002
Filed under , ,

Images online at http://www.eso.org/outreach/press-rel/pr-2002/phot-02-02.html


Summary

A new, high-resolution colour image of one of the most photographed
celestial objects, the famous “Horsehead Nebula” (IC 434) in Orion, has
been produced from data stored in the VLT Science Archive. The original
CCD frames were obtained in February 2000 with the FORS2 multi-mode
instrument at the 8.2-m VLT KUEYEN telescope on Paranal (Chile). The
comparatively large field-of-view of the FORS2 camera is optimally suited
to show this extended object and its immediate surroundings in impressive
detail.

PR Photo 02a/02: View of the full field around the Horsehead Nebula.

PR Photo 02b/02: Enlargement of a smaller area around the Horse’s “mouth”

A spectacular object

Caption: PR Photo 02a/02 is a reproduction of a composite colour image of
the Horsehead Nebula and its immediate surroundings. It is based on three
exposures in the visual part of the spectrum with the FORS2 multi-mode
instrument at the 8.2-m KUEYEN telescope at Paranal. PR Photo 02b/02 is an
enlargement of a smaller area. Technical information about these photos is
available below.

PR Photo 02a/02 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”, cf. PR 17/99. 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. Details about how
the photo was made and some weblinks to other pictures are available below.

The comparatively large field-of-view of the FORS2 camera (nearly 7 x 7
arcmin^2) and the detector resolution (0.2 arcsec/pixel) make this instrument
optimally suited for imaging of this extended object and its immediate
surroundings. There is obviously a wealth of detail, and scientific
information can be derived from the colours shown in this photo. 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.

A chaotic area

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 – a new infrared view by VLT and ISAAC of this area was published
last month, cf. PR 25/01.

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.

The surroundings

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’).

Other images of the Horsehead Nebula

The Horsehead Nebula is a favourite object for amateur astrophotographers
and large numbers of images are available on the WWW. Due to its significant
extension and the limited field-of-view of some professional telescopes,
fewer photographs are available from today’s front-line facilities, except
from specialized wide-field instruments like Schmidt telescopes, etc.

The links below point to a number of prominent photos obtained elsewhere and
some contain further useful links to other sites with more information about
this splendid sky area.

“Astronomy Picture of the Day”:
http://antwrp.gsfc.nasa.gov/apod/ap971025.html

Hubble Heritage image:

http://hubble.stsci.edu/news_.and._views/pr.cgi?2001%2B12

INT Wide-Field image: http://www.ing.iac.es/PR/science/horsehead.htm

NOT image: http://www.not.iac.es/new/general/photos/astronomical/

NOAO Wide-Field image: http://www.noao.edu/outreach/press/pr01/ir0101.html

Bill Arnett’s site: http://www.seds.org/billa/twn/b33x.html

Technical information about the photos

PR Photo 02a/02 was produced from three images, obtained on February 1,
2000, with the FORS2 multi-mode instrument at the 8.2-m KUEYEN Unit
Telescope and extracted from the VLT Science Archive Facility. The frames
were obtained in the B-band (600 sec exposure; wavelength 429 nm; FWHM 88
nm; here rendered as blue), V-band (300 sec; 554 nm; 112 nm; green) and
R-band (120 sec; 655 nm; 165 nm; red) The original pixel size is 0.2 arcsec.
The photo shows the full field recorded in all three colours, approximately
6.5 x 6.7 arcmin^2. The seeing was about 0.75 arcsec. PR Photo 02b/02 is an
enlargement of a smaller area, measuring 3.8 x 4.1 arcmin^2. North is to the
left and east is down (the usual orientation for showing this object). The
frames were recorded with a TK2048 SITe CCD and the ESO-FIERA Controller,
built by the Optical Detector Team (ODT). The images were prepared by Cyril
Cavadore (ESO-ODT), by means of Prism software.

SpaceRef staff editor.