ESO Press Photo 24a/00

ESO Press Photo 24a/00
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PR Photo 24a/00 is a near-infrared, colour-coded
composite image of a sky field in the south-western part of the
galactic star-forming region Messier 17. It is based on
exposures obtained on August 15, 2000, with the SOFI
multi-mode instrument at the ESO 3.6-m
New Technology Telescope (NTT)
at La Silla. In this image,
young and heavily obscured stars are recognized by their red
colour. Bluer objects are either foreground stars or well-developed
massive stars whose intense light ionizes the hydrogen in this
region. The diffuse light that is visible nearly everywhere in the
photo is due to emission from hydrogen atoms that have (re-)combined
from protons and electrons. The dark areas are due to obscuration of
the light from background objects by large amounts of dust – this
effect also causes many of those stars to appear quite red. A cluster
of young stars in the upper-left part of the photo, so deeply embedded
in the nebula that it is invisible in optical light, is well visible
in this infrared image. Technical information: The exposures
were made through three filtres, J (at wavelength 1.25 µm;
exposure time 5 min; here rendered as blue), H (1.65 µm; 5 min;
green) and Ks (2.2 µm; 5 min; red); an additional 15 min was
spent on separate sky frames. The seeing was 0.5 – 0.6 arcsec. The
objects in the uppermost left corner area appear somewhat elongated
because of a colour-dependent aberration introduced at the edge by the
large-field optics. The sky field shown measures approx. 5 x 5
arcmin2 (corresponding to about 3% of the full moon). North
is up and East is left.

Beautiful images of astronomical objects often contain a lot of
scientifically interesting information – PR Photo 24a/00, shown
above, is a fine example of this old maxime.

It provides a deep and unsually wide look into a giant star forming
region in the Milky Way. It is known as Messier 17
(M 17)
, or the Omega, Swan, Horseshoe, or Lobster
, because of its characteristic shape when photographed in
visible light. It is located at a distance of approx. 5000 light-years
(1.6 kpc), and is seen in the southern constellation of Sagittarius
(The Archer), near the main plane of the Milky Way.

This impressive image was obtained by astronomers Leonardo
(Arcetri Astrophysical Observatory, Florence, Italy;
p.t. Visiting Scientist at ESO-Chile) and Leonardo Vanzi
(ESO-Chile) with the SOFI
multi-mode instrument at the ESO 3.6-m
New Technology Telescope (NTT)
at the La Silla observatory. The
observations were made in the course of a research project that is
aimed at the detection and study of the formation of massive stars, by
means of near-infrared direct and spectral exposures with this

The new data offer a unique combination of a wide field-of-view,
high sensitivity and excellent image quality. The goal of these
particular observations was to identify massive stars that are in the
act of formation in this area and to record their infrared spectra for
a detailed physical study of these rather rare objects.

The formation of massive stars

It is now well established that the formation of stars in our
galaxy, the Milky Way system, predominantly takes place in Giant
Molecular Clouds
. However, while low-mass stars are common and
relatively easy to find in such clouds, it is much more difficult to
find massive stars while they are in the very early stage of their

This is because massive stars are comparatively rare and pass
through the different evolutionary phases much faster than low-mass
and solar-like stars. They are usually found within dense stellar
clusters, located at large distances from the Sun.

The earliest evolutionary stages of both low- and high-mass stars
cannot be observed at visible wavelengths. This is due to the very
heavy obscuration in that wavelength region by the dust in the
parental molecular clouds. However, infrared and millimeter emission
from these objects is able to penetrate the dust and thus allow us to
investigate the complex processes that occur at the earliest phases of
stellar evolution.

A search for massive stars in the formation stage

The best possible comprehension of this crucial stage of the
stellar life cycle constitutes a key to the understanding of the
formation and evolution of galaxies. And in this context, it is
especially the massive stars with their powerful radiation fields,
strong stellar winds and dramatic final interaction with the
interstellar medium through supernova explosions that dominate the
energetics of normal galaxies. Massive stars live much shorter (about
1 million years) than lighter solar-type stars (10 – 12,000 million
years) and only they undergo such violent explosions at the end.

A most challenging question in current astrophysics is therefore
concerned with the nature of the various physical processes that take
place during the formation of massive stars. It is for instance still
unclear whether massive stars form like lower-mass stars of the solar
type do, i.e., by accretion of gas from the surrounding cloud during a
progressive contraction process, or if they are rather the results of
stellar collisions in dense clusters.

The only way to know is by finding the youngest massive stars and
then to study them in detail by means of spectroscopic
observations. At the same time, their immediate surroundings, e.g.,
the stellar population in the star-forming cluster of which they are
members, must also be investigated before the full picture will

Massive stars in Messier 17

ESO Press Photo 24b/00

ESO Press Photo 24b/00
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PR Photo
shows an enlargement of a small area of the field in PR
Photo 24a/00
(slightly above and to the right of the centre),
centred on the position of a massive star in the process of being
formed. Its colour is such that it can only be seen in the
longest-wavelength image (Ks). Below is the infrared spectrum of this
object, obtained with SOFI on August 16, 2000. As can be seen, it is
nearly featureless, with rapidly increasing intensity towards longer
wavelengths. Technical information: The spectrum was obtained
during 20 min exposure, with an additional 20 min on the background
sky. The spectral resolution is approx. 1000. The “absorption line” at
2.06 µm is an artefact from a helium line when the “background”
nebular spectrum was subtracted to cancel the hydrogen


It is in this context that new infrared observations have been made
of the inner regions of Messier 17. In the area to the
south-west of the centre of this nebula, shown in PR Photo
, several objects appear very red – this clearly identifies
them as very young stars.

An infrared spectrum of one of the reddest objects that was only
detected at the longest wavelength (the Ks band; cf. PR Photo
), confirmed it as a massive star in its earliest
evolutionary phase.

The spectrum shows a featureless “continuum” that rises steeply
towards long wavelengths, exactly as expected for a star in the
process of formation, deeply embedded in dust. It is easily
distinguishable from stars that have already formed, since their
spectra display several absorption features and rise towards shorter
wavelengths instead.

This is the caption to ESO PR Photos 24a-b/00. They may be
reproduced, if credit is given to the European Southern