Orion the Hunter is perhaps the best-known constellation in the sky,
well placed in the winter for observers in both the northern and
southern hemispheres, and instantly recognisable. Just below Orion’s
belt (three distinctive stars in a row), the hilt of his sword holds a
great jewel in the sky, the beautiful Orion Nebula. Bright enough to be
seen with the naked eye, the nebula, also known as Messier 42, is a
wide complex of gas and dust, illuminated by several massive and hot
stars at its core, the famous Trapezium stars.

For astronomers, Orion is surely one of the most important
constellations, as it contains one of the nearest and most active
stellar nurseries in the Milky Way, the galaxy in which we live. Here
tens of thousands of new stars have formed within the past ten million
years or so – a very short span of time in astronomical terms. For
comparison: our own Sun is now 4,600 million years old and has not yet
reached half-age. Reduced to a human time-scale, star formation in
Orion would have been going on for just one month as compared to the
Sun’s 40 years.

In fact, located at a distance of 1500 light years, the Orion Nebula
plays such an important role in astrophysics that it can be argued that
our understanding of star formation is for a large part based on the
Orion Nebula.

It is thus no surprise that the Orion Nebula is one of the most studied
objects in the night sky (see for example the various related ESO Press
Photos and Releases: ESO Press Photo 03a/98, ESO Press Photos
03a-d/01, ESO Press Photos 12a-e/01, ESO Press Release 14/01, …).

The richness of the stellar cluster inside the Orion Nebula makes it an
ideal, and unique, target for high resolution and wide-field imaging.
Following some pioneering work made a few years ago, an international
team of astronomers [1], led by Massimo Robberto (European Space Agency
and Space Telescope Science Institute), used the Wide Field Imager
(WFI), a 67-million pixel digital camera that is installed at the
ESO/MPG 2.2m telescope at La Silla, to obtain very deep images of this
region. ESO PR Photo 20/04 shows a false-colour composite of images
obtained in four different wavebands (see technical information below).

Among others, these observations allow the astronomers to measure the
rates of mass that falls onto the young stars (the mass accretion
rates) and to determine if it depends on the position of the stars in
the cluster. If this were the case, it would indicate that the final
stages of star formation are affected by the onset of ionising
radiation from the most massive stars. From a preliminary study with
the Hubble Space Telescope, the astronomers found that indeed the mass
accretion rates are lower in the Orion Nebula Cluster than in other,
more diffuse star-forming regions. The analysis of these new WFI images
should allow confirmation of this hypothesis.

The astronomers also obtained images of the Orion Nebula in several
narrow-band filters corresponding to emission lines – hydrogen
(Halpha), oxygen ([OIII]), and sulphur ([SII]) – enabling them to probe
the morphology of the nebula in these prominent lines. It is rather
obvious from the image that for example some regions are redder than
others, providing the astronomers with important clues on the
conditions prevailing in the nebula.

In the next months, a large international collaboration also led by M.
Robberto will use the Hubble Space Telescope to survey with
unprecedented sensitivity (23-25 mag) and spatial resolution
approximately 50% of the field imaged by the present WFI observations.
The astronomers expect to discover and classify an unknown but
substantial population of young double stars, low mass stars and brown
dwarfs.

ESO Press Photo 20/04 and all the relevant weblinks can be found on
http://www.eso.org/outreach/press-rel/pr-2004/phot-20-04.html

Note

[1] The team is composed of M. Robberto, S. V. W. Beckwith, R. Makidon,
G. Mora Carrillo, N. Panagia, N. Reisner, and J. Song.

Technical information: The individual images of the Orion Nebula where
obtained with WFI at the ESO/MPG 2.2m telescope on the night of
Dec.10-11, 2001. Several images were obtained in the following filters:
U (363.690 nm; 15 images for a total exposure time of 1.25 hour); B
(456.252 nm; 14 images; total exp.: 21 min); Oxygen [OIII] (502.393 nm;
15 images; total exp.: 32 min); Halpha (658.827 nm; 15 images; total
exp.: 32 min); Sulphur [SII] (676.340 nm; 10 images; total exp.: 30
min). ESO PR Photo 20/04 is a false-colour composite of all of the
images obtained in B, Halpha, [OIII], and [SII] were each waveband was
associated to a given colour: B to blue, [OIII] to green; Halpha to
orange, and [SII] to red. The field of view covers 34 x 33 arcmin2.
North is up and East is to the left. The images were first processed by
Massimo Robberto (STSci) then further combined by Haennes Heyer and
Henri Boffin (ESO).

Contact

Massimo Robberto
Space Telescope Science Institute
Baltimore, USA
Phone: (410)-338-4382
Email: robberto@stsci.edu