Summary

How do stars like our Sun come into being? Which fundamental
processes are responsible for transforming a dark and diffuse
interstellar cloud of gas and dust into a much denser, shining
object?

Astronomers have just taken an important step towards answering
this fundamental question. Based on the most detailed study ever made
of the internal structure of a small interstellar cloud, three
scientists from ESO and the USA [1] have found
that it is apparently on the verge of becoming unstable – and thus in
the stage immediately preceding a dramatic collapse into a dense and
hot, low-mass star.

Interestingly, the current structure of this cloud, a “Bok
globule” known as Barnard 68 (B68) [2], is governed by the same basic physics as is
that of a star. The cloud is obviously in a temporary state of
near-equilibrium, where the inward force of gravity caused by its mass
more or less balances that of the outward pressure due to its
temperature. But this situation may not last long.

The astronomers believe that this particular cloud, together with
some others in the same galactic neighbourhood, constitute the few
resistent remains of a much larger cloud that has disappeared due to
the influence of strong stellar winds and ultraviolet radiation from
young and heavy stars as well as supernova explosions.

The new and unique insight into the pre-collapse phase of the
complicated process of stellar birth is based on observations made
with ESO telescopes at the La
Silla and Paranal observatories in
Chile.

PR Photo 02a/01: The Bok
Globule B68, as seen in visual light.
PR Photo 02b/01: Looking through the
Bok Globule B68.
PR Photo
02c/01: A comparison of the visual and infrared views of
the Bok Globule B68.

From Dark Clouds to Stars

Astronomers have known for some time that stars like our Sun are
formed from interstellar clouds of gas and dust. When they contract,
the interior temperature rises. If the cloud is sufficiently heavy, it
will become so hot at the centre that energy-producing nuclear
processes ignite. After a while, the central regions of the cloud
reach equilibrium and a new star is born.

Planets are formed from condensations in the surrounding material
as this collects in a circumstellar disk.

A good understanding of the origin of stars and planetary systems,
like our own solar system, is therefore intimately connected to a
detailed knowledge about the conditions in the cold interiors of dark
clouds in interstellar space. However, such clouds are highly opaque
and their physical structure has remained a mystery for as long as we
have known about their existence. The following phases of stellar
evolution are much better known and some scientists therefore refer to
these very earliest stages as the “missing link” in our current
picture of star formation.

Finely balanced equilibrium

The present results are changing this situation. By means of a new
and straightforward observational technique, it has now been possible
to explore the detailed structure of a nearby cloud. It is found to be
quite simple, with the mean density steadily increasing towards the
centre. In fact, the way this happens (referred to as the cloud’s
“density profile”) is exactly as expected in an isolated gas sphere at
a certain temperature in which the inward force of gravity is finely
balanced against the internal thermal pressure.

With this clear physical description it is now possible to
determine with unprecedented precision (approx. 3%) the fundamental
parameters of the cloud, such as its distance and gas-to-dust
ratio.

ESO astronomer João Alves from the team is content:
“These measurements constitute a major breakthrough in the
understanding of dark clouds. For the first time, the internal
structure of a dark cloud has been specified with a detail approaching
that which characterizes our knowledge of stellar interiors”.

Seeing light through the dark

The observational technique that has led to the new result is
straightforward but rather difficult to apply to dark clouds.

It is based on measurements of the light from stars that are
located behind the cloud. When this light passes through the cloud, it
is absorbed and scattered by the dust inside. The effect depends on
the colour (wavelength) and the background stars will appear redder
than they really are. It is also proportional to the amount of
obscuring material and is therefore largest for stars that are
situated behind the cloud’s centre.

By measuring the degree of this “reddening” experienced by stars
seen through different areas of the cloud, it is thus possible to
chart the distribution of dust in the cloud. The finer the net of
background stars is, the more detailed this map will be and the better
the information about the internal structure of the cloud.

And that is exactly the problem. Even small clouds are so opaque
that very few background stars can be seen through them. Only large
telescopes and extremely sensitive instruments are able to observe a
sufficient number of stars in order to produce significant results. In
particular, until now it has never been possible to map the densest,
central areas of a dark cloud.

The structure of Barnard 68

ESO PR Photo 02a/01

ESO
PR Photo 02a/01

[Preview – JPEG:400x x 482 pix –
92k]
[Normal – JPEG: 800 x 964 pix –
560k]
[Hires – JPEG: 2296 x 2768 pix –
7.9M]

ESO PR Photo 02b/01

ESO
PR Photo 02b/01

[Preview – JPEG: 400 x 480 pix –
89k]
[Normal – JPEG: 800 x 960 pix –
432k]
[Hires – JPEG: 2301 x 2762 pix –
7.3M]

 

ESO PR Photo 02c/01

ESO
PR Photo 02c/01

[Preview – JPEG: 624 x 400 pix –
88k]
[Normal – JPEG: 1247 x 800 pix –
496k]
[Hires – JPEG: 2828 x 1814 pix –
5.6M]

Caption: PR Photo 02a/01 shows a colour
composite of visible and near-infrared images of the dark cloud
Barnard 68. It was obtained with the 8.2-m VLT ANTU telescope
and the multimode FORS1 instrument in March 1999. At these
wavelengths, the small cloud is completely opaque because of the
obscuring effect of dust particles in its interior. PR Photo
02b/01 is a false-colour composite based on a visible (here
rendered as blue), a near-infrared (green) and an infrared (red)
image. Since the light from stars behind the cloud is only visible at
the longest (infrared) wavelengths, they appear red. In PR Photo
02c/01, the central area of these two photos may be directly
compared. Technical information about these
photos is available below.

At a distance of only 410 light-years, Barnard 68 is one of
the nearest dark clouds. Its size is about 12,500 AU (= 2 million
million km; 1 Astronomical Unit [AU] = 150 million km), or just about
the same as the so-called “Oort Cloud” of long-period comets
that surrounds the solar system. The temperature of Barnard 68
is 16 Kelvin (-257 °C) and the pressure at its boundary is 0.0025
nPa, or about 10 times higher than in the interstellar medium (but
still 40,000 million million times less than the atmospheric pressure
at the Earth’s surface!). The total mass of the cloud is about twice
that of the Sun.

A new investigation of Barnard 68 was carried out by means
of instruments at the 3.58-m New Technology Telescope (NTT) at La
Silla and the Very Large Telescope (VLT) at Paranal. Long exposures
revealed a total of about 3700 background stars (of which over 1000
can only be seen at infrared wavelengths), cf. PR Photos
02a-c/01.

Careful measurements of the colours of these stars and hence, the
degree of obscuration, allowed the most finely sampled (in more than
1000 individual areas) and most accurate mapping of the dust
distribution inside a dark cloud ever performed. In order to further
increase the accuracy, the mean dust density was measured in
concentric circles around the centre – this resulted in a very
accurate determination of the change in dust density with the distance
from the centre.

It was found that this dependance is almost exactly as that
predicted for a sphere in which the opposite forces of gravity and
internal pressure closely balance each other. Nevertheless, it is also
evident that Barnard 68 is only marginally stable and is on the
verge of collapse.

The origin of Barnard 68

This first-ever, detailed characterization of a dark interstellar
cloud that is currently in the stage immediately preceding collapse
and subsequent star formation constitutes a very important step
towards a better understanding of earliest phases of the stellar life
cycle.

The astronomers suggest that Barnard 68 (and its
neighbouring brethren, the dark clouds Barnard 69, 70 and 72) may be
the precursors of an isolated and sparsely populated association of
low-mass solar-like stars. However, where did these clouds come
from?

João Alves thinks he and his colleagues know the
answer: “It is most likely that they are the remnant cores of
particularly resistent parts of a larger cloud. By now, most of it has
been ‘eaten away’ because of strong attrition caused by ultraviolet
radiation and stellar winds from hot massive stars or ‘storms’ from
exploding supernovae”. He adds: “Our new observations show that
objects with just the right mass like Barnard 68 can reach a
temporary equilibrium and survive for some time before they begin to
collapse.”

The team is now eager to continue this type of investigation on
other dark clouds.

More information

The research described in this Press Release is reported in a
research article (“Seeing Light Through the Dark: Measuring the
Internal Structure of a Cold Dark Cloud”), that appears in the
international research jounal Nature on Thursday, January 11,
2001.

Notes

[1]: The team consists of João F.
Alves (ESO-Garching, Germany), Charles J. Lada
(Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass. USA)
and Elizabeth A. Lada (University of Florida, Gainsville, Fl.,
USA).

[2]: The Dutch astronomer Bart Bok
(1906-1983) studied the dark clouds in the Milky Way and described the
small, compact ones as “globules”. The early stages of the present
investigation of Barnard 68 were presented in ESO PR Photos 29a-c/99,
with more background information about this cloud.

Technical information about the photos

PR Photo 02a/01 of the sky area of Barnard 68
is based on three frames through B- (440 nm = 0.44 µm – here
rendered as blue), V- (0.55 µm – green) and I-band 0.90 µm
– red) optical filters, as obtained with FORS1 instrument at the VLT
ANTU telescope on March 27, 1999. The field measures 6.8 x 6.8
arcmin2 (2048 x 2048 pix2 a 0.20 arcsec. PR
Photo 02b/01 is a false-colour composite based on B- (wavelength
0.44 µm – 1.5 min; here rendered as blue), I- (wavelength 0.85
µm – 1.5 min; green), and Ks-filters (2.16 µm – 30 min;
red), respectively. The B and I images were obtained on March 1999,
with the FORS1 instrument at the 8.2-m VLT ANTU. The Ks image was
obtained in March 1999 with the SOFI instrument at the ESO 3.58-m New
Technology Telescope (NTT) at La Silla. The sky field measures about
4.9 x 4.9 arcmin2 (1024 x 1024 pixels2 a 0.29
arcsec). North is up and East is left. PR Photo 02c/01 allows a
direct comparison between the two views.

Contacts

João F. Alves
European Southern Observatory
Garching, Germany
Tel.: +49-89-32006503
email: jalves@eso.org