The Wide-Field-Imager at La Silla Unveils Intricate Structures
Illuminated by Hot Stars

The full text of this Press Release, with five photos (ESO PR Photo
31a-e/03) and all related links, is available at:

The giant HII nebula N44

The two best known satellite galaxies of the Milky Way, the Magellanic Cl
ouds, are located in the southern sky at a distance of about 170,000 ligh
t-years. They host many giant nebular complexes with very hot and luminou
s stars whose intense ultraviolet radiation causes the surrounding inters
tellar gas to glow.

The intricate and colourful nebulae are produced by ionised gas [1] that
shines as electrons and positively charged atomic nuclei recombine, emitt
ing a cascade of photons at well defined wavelengths. Such nebulae are ca
lled “H II regions”, signifying ionised hydrogen, i.e. hydrogen atoms tha
t have lost one electron (protons). Their spectra are characterized by em
ission lines whose relative intensities carry useful information about th
e composition of the emitting gas, its temperature, as well as the mechan
isms that cause the ionisation. Since the wavelengths of these spectral l
ines correspond to different colours, these alone are already very inform
ative about the physical conditions of the gas.

N44 [2] in the Large Magellanic Cloud is a spectacular example of such a
giant H II region. Having observed it in 1999 (see ESO PR Photos 26a-d/99
), a team of European astronomers [3] again used the Wide-Field-Imager (W
FI) at the MPG/ESO 2.2-m telescope of the La Silla Observatory, pointing
this 67-million pixel digital camera to the same sky region in order to p
rovide another striking – and scientifically extremely rich – image of th
is complex of nebulae. With a size of roughly 1,000 light-years, the pecu
liar shape of N44 clearly outlines a ring that includes a bright stellar
association of about 40 very luminous and bluish stars.

These stars are the origin of powerful “stellar winds” that blow away the
surrounding gas, piling it up and creating gigantic interstellar bubbles
2E Such massive stars end their lives as exploding supernovae that expe
l their outer layers at high speeds, typically about 10,000 km/sec.

It is quite likely that some supernovae have already exploded in N44 duri
ng the past few million years, thereby “sweeping” away the surrounding ga
s. Smaller bubbles, filaments, bright knots, and other structures in the
gas together testify to the extremely complex structures in this region,
kept in continuous motion by the fast outflows from the most massive sta
rs in the area.

The new WFI image of N44

The colours reproduced in the new image of N44, shown in PR Photo 31a/03
(with smaller fields in more detail in PR Photos 31b-e/03) sample three s
trong spectral emission lines. The blue colour is mainly contributed by e
mission from singly-ionised oxygen atoms (shining at the ultraviolet wave
length 372.7 nm), while the green colour comes from doubly-ionised oxygen
atoms (wavelength 500.7 nm). The red colour is due to the H-alpha line o
f hydrogen (wavelength 656.2 nm), emitted when protons and electrons comb
ine to form hydrogen atoms. The red colour therefore traces the extremely
complex distribution of ionised hydrogen within the nebulae while the di
fference between the blue and the green colour indicates regions of diffe
rent temperatures: the hotter the gas, the more doubly-ionised oxygen it
contains and, hence, the greener the colour is.

The composite photo produced in this way approximates the real colours of
the nebula. Most of the region appears with a pinkish colour (a mixture
of blue and red) since, under the normal temperature conditions that char
acterize most of this H II region, the red light emitted in the H-alpha l
ine and the blue light emitted in the line of singly-ionised oxygen are m
ore intense than that emitted in the line of the doubly-ionised oxygen (g

However, some regions stand out because of their distinctly greener shade
and their high brightness. Each of these regions contains at least one e
xtremely hot star with a temperature somewhere between 30,000 and 70,000
degrees. Its intense ultraviolet radiation heats the surrounding gas to a
higher temperature, whereby more oxygen atoms are doubly ionised and the
emission of green light is correspondingly stronger, cf. PR Photo 31c/03.

A selection of fields in the N44 complex

By contrast, ESO PR Photo 26a/99 was a three-colour composite produced by
means of two broad-band filter exposures in blue and green light and one
H-alpha exposure, and therefore outlining the general appearance of the
ionised region. Says Fernando Comeron, leader of the team: “Thanks to the
use of a UV-filter and two narrow-band filters isolating the emission of
specific ions, the new picture provides much more information about the
complicated physics of the excited interstellar gas in the region.”

Nausicaa Delmotte, member of the team, adds:”With its colourful beauty, N
44 is a display of the violent phenomena that occur as the heaviest known
stars unfold their power on the parental gas out of which they were born”

Technical information

The colour picture of N44 in the LMC is based on three monochromatic imag
es taken on 6 and 7 December 2001 with the Wide-Field-Imager (WFI) at the
ESO/MPG 2.2-m telescope, using the U-band filter (containing the forbidd
en line of singly-ionised oxygen, [OII], at 372.7 nm) and two narrow-band
filters centred, respectively, on the wavelengths of the forbidden line
of doubly-ionised oxygen ([OIII], at 500.7 nm) and hydrogen (H-alpha line
, at 656.2 nm). Each single-colour image is in turn composed of four indi
vidual frames of 20 minutes of exposure time each. The WFI detector syste
m is composed of eight individual 2k x 4k CCDs with small gaps between th
em; for this reason, the individual frames in each filter were obtained w
ith the telescope pointing at slightly different positions in the sky, so
that the parts of the sky falling in the detector gaps in any given fram
e are recorded on the others. A problem with one of the detector chips ca
uses double stellar images to appear over a small, narrow strip near the
upper left edge of the full field image. The monochromatic images were pr
oduced by superimposing the individual frames, correcting for the telesco
pe offsets. Finally, the combined images in each filter were aligned and
colour-coded to produce the resulting colour picture. North is up and Ea
st is left.


[1]: A gas is said to be ionised when its atoms have lost one or more ele
ctrons – in this case by the action of energetic ultraviolet radiation em
itted by very hot and luminous stars close by.

[2]: The letter “N” (for “Nebula”) in the designation of these objects in
dicates that they were included in the “Catalogue of H-alpha emission sta
rs and nebulae in the Magellanic Clouds” compiled and published in 1956 b
y American astronomer-astronaut Karl Henize (1926 – 1993).

[3]: The team is composed of Fernando Comeron and Nausicaa Delmotte from
ESO, and Annie Laval from the Observatoire de Marseille (France).