Orion the Hunter is perhaps the best known constellation in
the sky, well placed in the evening at this time of the year for
observers in both the northern and southern hemispheres, and instantly
recognisable. And 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.
Just below Orion's belt, 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, a small telescope or even binoculars show the
nebula to be a few tens of light-years' wide complex of gas and dust,
illuminated by several massive and hot stars at its core, the famous
Trapezium stars.
However, the heart of this nebula also conceals a secret from the
casual observer. There are in fact about one thousand very young
stars about one million years old within the so-called
Trapezium Cluster, crowded into a space less than the distance
between the Sun and its nearest neighbour stars. At visible wavelengths,
the dense cluster of stars at the
centre is drowned out by the light from the nebula and obscured by
remnants of the dust in the gas from which they were formed.
However, at longer wavelengths, these obscuring effects are reduced, and
the cluster is revealed, as seen in this
spectacular image of this area, obtained in December 1999 with the
the infrared multi-mode ISAAC
instrument on the ESO Very Large Telescope (VLT) at Paranal (Chile).
The large collecting area of the VLT and the excellent seeing of
the Paranal site makes it possible to see extremely faint objects,
including very-low mass stars, brown dwarfs, and even objects with
masses only a few times that of Jupiter. In addition, powerful explosions
and winds from the most massive stars in the region are evident, as well
as the contours of gas sculpted by these stars, and more finely focused
jets of gas flowing from the smaller stars.
The beautiful infrared image shown here is first
"finding chart" made at the beginning of a long-term research project
into the nature of the very lowest-mass objects in the region,
but it already carries plenty of new astrophysical information. For
the astronomers, images like these and the follow-up studies will help
to solve some of the fascinating and perplexing questions about the
birth and early lives of stars and their planetary systems.
The new VLT data covering the Orion Nebula
and Trapezium Cluster were obtained as part of a long-term project by
Mark McCaughrean (Principal Investigator, Astrophysical
Institute Potsdam [AIP], Germany), João Alves (ESO,
Garching, Germany), Hans Zinnecker (AIP) and Francesco
Palla (Arcetri Observatory, Florence, Italy). The data also form
part of the collaborative research being undertaken by the European
Commission-sponsored Research Training Network on "The Formation and
Evolution of Young Star Clusters" (RTN1-1999-00436), led by the
Astrophysical Institute Potsdam, and including the Arcetri Observatory
in Florence (Italy), the University of Cambridge (UK), the University
of Cardiff (UK), the University of Grenoble (France), the University
of Lisbon (Portugal) and the CEA Saclay (France).
The image was made using the near-infrared camera ISAAC
on the ESO 8.2-m VLT ANTU telescope on December 20 - 21, 1999. The
full field measures approx. 7 x 7 arcmin, covering roughly 3 x 3
light-years (0.9 x 0.9 pc) at the distance of the nebula (about 1500
light-years, or 450 pc). This required a 9-position mosaic (3 x 3
grid) of ISAAC pointings; at each pointing, a series of images were
taken in each of the near-infrared Js- (centred at 1.24
µm wavelength), H- (1.65 µm), and Ks- (2.16
µm) bands. North is up and East left.
The total integration time for each pixel in the mosaic was 4.5 min
in each band. The seeing FWHM (full width at half maximum) was
excellent, between 0.35 and 0.50 arcsec throughout. Point sources are
detected at the 3-sigma level (central pixel above background noise)
of 20.5, 19.2, and 18.8 magnitude in the Js-, H-, and
Ks-bands, respectively, mainly limited by the bright
background emission of the nebula.
After removal of instrumental signatures and the bright infrared
sky background, all frames in a given band were carefully aligned and
adjusted to form a seamless mosaic. The three monochromatic mosaics
were then unsharp-masked and scaled logarithmically to reduce the
enormous dynamic range and enhance the faint features of the outer
nebula. The mosaics were then combined to create this colour-coded
image, with the Js-band being rendered as blue, the H-band
as green, and the Ks-band as red. A total of 81 individual
ISAAC images were merged to form this mosaic.
( credit: McCaugrean, Alves, Zinnecker, Palla )
