SETI Secrets of a Dark Cloud

Larry Klaes (
Fri, 02 Jul 1999 15:13:21 -0400

>Date: Fri, 2 Jul 1999 13:34:02 +0200 (MET DST)
>From: ESO Education and Public Relations Dept.
>Date: Friday, July 2, 1999
>Subject: Secrets of a Dark Cloud (ESO PR PHotos 29a-c/99 and Video Clip
>Text with all links is available on the IAU Website at URL:
>Dear subscribers,
>you will find below the text (with reference to images and video clip
>on the website above) of today's ESO communication about unique
>observations of a nearby dark cloud with the infrared multi-mode SOFI
>instrument at the New Technology Telescope at La Silla. The original
>URL is:
>Please be informed that the June 1999 issue of the ESO Messenger is
>now available on the web at:
>It contains information about the Paranal Inauguration and the
>associated scientific Symposium in Antofagasta, the ALMA project
>etc. The printed version will be mailed shortly.
>Kind regards,
>ESO EPR Dept.
> Information from the European Southern Observatory
> ESO Press Photos 29a-c/99 and Video Clip 05/99
> 2 July 1999 [ESO]
> For immediate release
> ------------------------------------------------------------------------
>Secrets of a Dark Cloud
>Unique Infrared SOFI Images of Barnard 68 Probe the Very First Stages of
>Star Formation
>Astronomers at ESO have recently been "Seeing the Light Through the Dark"!
>Some months ago, the ESO Very Large Telescope (VLT) observed a classical
>dark globule, Barnard 68 (B68), in front of a dense star field in the Milky
>Way band. CCD images were obtained in various visual wavebands with the
>FORS1 multi-mode instrument at the 8.2-m VLT ANTU (UT1). They were combined
>into a colour photo, cf. ESO PR Photo 20a/99.
>This dark cloud is situated at a distance of about 500 light-years (160 pc)
>towards the southern constellation Ophiuchus (The Serpent-holder). The VLT
>photo shows it as a compact, opaque and rather sharply defined object, the
>central parts of which are so dense that they completely block out the light
>from the stars behind.
>It is known that clouds like B68 at some moment begin to contract and
>subsequently transform themselves into normal, hydrogen-burning stars. But
>how exactly does this happen? And what is going on just now inside B68? Is
>it currently at the beginning of the contraction phase or have stars already
>been formed? How dense and heavy is it really?
>Answers to some of these basic questions are now being provided by new and
>unique observations in the infrared part of the spectrum with the SOFI
>multi-mode instrument at the ESO 3.5-m New Technology Telescope (NTT) at La
>Silla. For the first time, it has been possible to look right through even
>the most opaque regions of such an object and learn what is inside in
>unsurpassed detail.
>The SOFI observations
> [ESO PR Photo 29a/99] ESO PR Photo Caption to ESO PR Photo 29a/99:
> 29a/99 False-colour, infrared composite
> photo of the dark cloud Barnard 68,
> [Preview - JPEG: 400 x 467 pix - obtained on March 8-9, 1999, with
> 312k] the SOFI instrument at the ESO 3.5-m
> New Technology Telescope (NTT) at La
> [Normal - JPEG: 800 x 934 pix - Silla. Three exposures were made
> 980k] through J- (wavelength 1.25 micron - 3
> min; here colour-coded as "blue"),
> [Full-Resolution - JPEG: 3000 x 3503 H- (1.65 micron - 30 min; "green") and
> pix - 5.6M] Ks-filters (2.16 micron - 30 min;
> "red"), respectively. The
> "reddening" of the background stars
> that are seen through the cloud is
> very obvious. It is due to the
> larger obscuration in shorter
> wavebands. The sky field measures
> about 4.9 x 4.9 arcmin2 (1024x1024
> pixels). A slight mismatch between
> the individual frames is present in
> the upper corners; this is caused by
> atmospheric effects. The pixel size
> is 0.29/arcsec. North is up and East
> is left.
>The new near-infrared imaging observations were taken with the SOFI
>multi-mode instrument at the NTT on La Silla during a spell of excellent
>observing conditions in March 1999. The measured seeing was about 0.6 arcsec
>during several hours while these exposures were being made.
>SOFI (Son OF ISAAC) is a scaled-down copy of ISAAC, the major VLT instrument
>that has already produced spectacular observations. SOFI is a unique
>instrument for the study of extended objects like B68 because of its very
>sensitive infrared detector and unrivalled large field-of-view.
>About 200 exposures (each lasting about 10 sec) were made in each of the H-
>and Ks-bands to reach as faint objects as possible; less time was spent in
>the shorter wavelength J-band. They were then added to produce three frames
>that form the basis for the subsequent study and which were used to produce
>the images shown here.
>Looking into the centre of a dark cloud
> [ESO PR Photo 29b/99] ESO [ESO PR Video Clip 05/99 [MPEG-version] ESO
> Photo Video
> 29b/99 Clip 05/99
> (1685 frames/1:07 min)
> [Preview - JPEG: 512 x 400 [MPEG Video; 160x120 pix; 1.5Mb]
> pix - 184k] [MPEG Video; 320x240 pix; 3.8Mb]
> [RealMedia; streaming; 56kps]
> [Normal - JPEG: 1023 x 800 [RealMedia; streaming; 200kps]
> pix - 580k]
> [Full-Resolution - JPEG:
> 3837 x 3000 pix - 4.6M]
> Caption to ESO PR Photo 29b/99: The sky area of the globule ("dark cloud")
> Barnard 68, imaged in six different wavebands, clockwise from the blue to
> the near-infrared spectral region. Three of these frames ("blue" B-band at
> wavelength 440 nm = 0.44 micron; "green-yellow" V-band at 0.55 micron;
> near-infrared I-band at 0.90 micron) were obtained with the FORS1 instrument
> at the VLT ANTU telescope and three with SOFI at the NTT through
> near-infrared filters (J-band at 1.25 micron; H-band at 1.65 micron;
Ks-band at
> 2.16 micron). It is evident that the obscuration caused by the cloud
> diminishes dramatically with increasing wavelength. Since the outer
> regions of the cloud are less dense than the inner ones, the apparent size
> of the cloud also decreases, as more background stars shine through the
> outer parts. Each frame covers an area of 4.9 x 4.9 arcmin2. North is up
> and East is left.
> ESO Video Clip 05/99 provides a direct demonstration of the wavelength
> dependance of the obscuration in Barnard 68 by means of "cut" and "fade"
> of the six exposures in different bands, shown individually in PR Photo
> 29b/99.
>Dark clouds are dark because they contain myriads of submicron-sized solid
>particles - the interstellar dust grains. They also harbour many different
>species of molecules. They are responsible for the obscuration of light at
>visible wavelengths. ESO PR Photo 29b/99 and ESO Video Clip 05/99 provide a
>very direct illustration of the dependence of this obscuration on the
>wavelength (astronomers speak about "dust extinction"): it is higher at
>shorter wavelengths than at longer ones.
>The new data are unique in the sense that it allows astronomers, for the
>first time, to see through the very centre of a dense molecular cloud, into
>the cold regions where stars like our Sun will form. We know this because a
>large number of background stars, not related to the cloud, are seen through
>the central, most dense regions of B68 in the Ks-image at 2.16 micron (to the
>lower left in Photo 29b/99).
>About molecular clouds
>Dark clouds are the coolest objects in the known Universe with temperatures
>around -263 oC, just ten degrees above the absolute zero. They are the
>nurseries of stars and planets. To understand them is to understand the
>processes that took place when the Solar System was formed about 4,500
>million years ago.
>Unfortunately, because they are mostly composed of molecular hydrogen (H2)
>and also because they are so cold, 99% of a molecular cloud's mass is
>virtually undetectable by means of direct observations.
>A traditional way to study such clouds is by means of observations with
>radiotelescopes of rare molecules (such as CO, CS and NH3) that "trace" the
>molecular hydrogen. However, the analysis of such data is rarely
>straightforward and a clear and unambiguous interpretation is frequently
>Thanks to the recent advent of improved infrared technology, incorporated
>into SOFI, it will now become possible to study molecular clouds in a more
>direct way, as illustrated here. By means of careful measurements of the
>change of colour of background stars seen through a molecular cloud (cf. the
>"reddening" of the stars near the center of PR Photo 29a/99), astronomers
>can chart the distribution of matter inside these clouds.
>The new SOFI observations of B68 allow such measurements to be done for the
>first time through the central, densest regions of a molecular cloud. These
>unique data provide astronomers with important clues on how a dark cloud
>transforms itself into stars.
>Structure and current state of B68
> [ESO PR Photo 29c/99] ESO PR Photo Caption to ESO PR Photo 29c/99: This
> 29c/99 photo shows the degree of
> obscuration in the area around B68,
> [Preview - JPEG: 400 x 446 pix - as an iso-extinction map. It is
> 56k] based on measurements of background
> stars. The angular resolution is 10
> [Normal - JPEG: 800 x 891 pix - arcsec. The outermost contour
> 140k] corresponds to a visual extinction
> of 4 magnitudes (this means that
> visible light is dimmed/obscured by
> a factor of 40) and increases in
> steps of 2 magnitudes (a factor of
> 6.3) up to 35 magnitudes at the
> centre (an obscuration factor of
> 1014 or 100 million million times)!
> Because the dust extinction is
> tracing the main mass component of
> this cloud, molecular hydrogen (H2),
> this map is also a very good
> representation of the distribution
> of mass inside B68.
>The new SOFI images of B68 are now being studied by ESO astronomer Joao
>Alves and his collaborators, in particular Charles Lada (Harvard-Smithsonian
>Center for Astrophysics, Mass., USA) and Elizabeth Lada (University of
>Florida, USA). Several interesting conclusions can be drawn already.
>Through careful measurements of the colour of the background stars that are
>seen through the cloud, it is now possible to determine the total amount of
>obscuration at the center of the cloud. It turns out to be no less than 35
>magnitudes in the V-band at wavelength 0.55 micron. This number
corresponds to a
>dimming of the starlight of a factor of no less than 1014!
>If, in a thought experiment, a sheet of dust with this high degree of
>obscuration were placed in front of the Sun, there would be eternal darkness
>on the Earth. Our central star would then shine with magnitude 9 only, i.e.
>it would be about 15 times too faint to be observable with the naked eye!
>An analysis of the map of obscuration (PR Photo 29c/99) shows the detailed
>distribution of dust within the cloud. The densest part is somewhat to the
>west (right) of the geometrical center of B68. It looks as if two smaller
>areas (to the lower left, i.e. southeast of the center) are detaching
>themselves from the rest of the cloud.
>The small-scale structure of B68 seems to be very smooth and homogeneous.
>The SOFI observations rule out the presence of "clumpy" structures inside
>the cloud, on nearly all scales.
>The new data clearly show that B68 is now in the very early phase of
>collapse, on its way towards star formation. The duration of such a stage is
>relatively short, of the order of 100,000 years, and to catch a cloud in
>this phase is likely to be a rare occurrence. If the collapse had been going
>on for a little longer, it would not have been possible to see through this
>cloud today, since the obscuration would then have been much higher, of the
>order of hundreds of magnitudes.
>Moreover, the observed distribution of matter inside B68 provides us a first
>glimpse of how nature begins to form stars. These outstanding observations
>will now be used to test current theories of protostellar collapse.
>The total mass of the dust in B68 can be determined quite accurately from
>the obscuration map by adding over the entire area of the cloud. It comes to
>about 0.03 solar mass. If the gas-to-dust ratio in B68 is what is normally
>assumed, about 100, then the total mass of this cloud is about 3 solar
>masses. Accordingly, only a few stars will eventually form in this cloud.
>Future work
>After this first, impressive demonstration of what is now possible in this
>exciting research field with top-class astronomical instruments, other
>clouds will be studied in the near future, with SOFI and ISAAC. With more
>data from more clouds, it will soon be possible to comprehend their elusive
>nature in much greater detail and to characterize the fundamental mechanisms
>that trigger star formation.
> ------------------------------------------------------------------------
>This is the caption to ESO PR Photos 29a-c/99 and ESO Video Clip 05/99. They
>may be reproduced, if credit is given to the European Southern Observatory.
> ------------------------------------------------------------------------
> Copyright ESO Education & Public Relations Department
> Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany
> ------------------------------------------------------------------------
> ESO Press Information is made available on the World-Wide Web
> (URL:
> ------------------------------------------------------------------------

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