How to Become a Star – ESO Telescopes Provide Most Detailed View Ever Into a Dark Cloud

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

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

Contacts

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