Dark clouds, young stars, and a dash of Hollywood
Astronomers have constructed the most realistic 3-D model of the star-forming dark cloud Barnard 68.
November 1, 2012
Stars are born in hiding when dense regions within clouds of gas and dust collapse under their own gravity. These clouds not only provide the raw material for star formation, but they also absorb most of the light from their interior, hiding from view the crucial details of stellar birth — one of the key astronomical processes if we want to understand cosmic origins.
False-color image of the dark cloud Barnard 68, prepared using data from the Herschel Space Telescope at different far-infrared wavelengths. The way in which the cloud appears to change shape depending on wavelength is a sign of uneven external illumination. In the bottom left corner, there are traces of an isolated object. This could be a cloud fragment in collision with Barnard 68. The wavelengths for the images are 100, 160, 250, and 350 micrometers, respectively. The image colors show the intensity of the radiation received at that wavelength, from purple and blue at low intensity to high intensities in red and white. // Credit: MPIA/Markus Nielbock
Now, two groups in the Earliest Phases of Star formation (EPoS) project led by Oliver Krause from the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, using the European Space Agency’s (ESA) Herschel Space Telescope, report new results in understanding the earliest stages of star formation.
On the trail of the origin of low-mass stars, with less than about twice the mass of our Sun, a team led by Markus Nielbock from MPIA has completed a detailed investigation of one of the best-known potential stellar birthplaces — the dark cloud (globule) Barnard 68 in the constellation Ophiuchus. Combining the Herschel Space Telescope’s unrivaled sharpness and sensitivity in the far-infrared range with a method more often encountered in visual effects companies than in astronomy, the researchers were able to construct the most realistic 3-D model of the cloud to date.
The method, adapted for this particular use by Ralf Launhardt from MPIA, uses what is known as ray tracing: For each minute portion of the object that we can see, the line of sight is traced back into the object itself. The contribution by each portion of the light’s path — Is light being absorbed at this particular point? Is it being emitted? If yes, at which wavelengths? — are added up. Ray tracing is routinely used to produce realistic-looking computer-generated creatures, objects, or whole scenes. Here, it helped match light emitted within Barnard 68 at different wavelengths with simplified models of the cloud’s 3-D shape, density, and temperature distribution.
The results have shaken up some of what astronomers thought they knew about this cloud. The emerging picture is one of Barnard 68 condensing from a drawn-out filament, heated by unevenly distributed external radiation from the direction of the central plane of our home galaxy. The astronomers also found some signs pointing to a cloud fragment in collision with Barnard 68, which might lead to the cloud’s collapse and the formation of one or more low-mass stars within the next hundreds of thousands of years.
As cosmic clouds go, Barnard 68 is rather small. Clouds of this size will give birth to a few low-mass stars at most. To find out how massive stars are born — those greater than about twice the mass of the Sun — a team led by Sarah Ragan from MPIA turned Herschel’s PACS camera to 45 significantly more massive dark clouds. The clouds contain numerous stars about to be born — so-called protostars. While previous missions, such as NASA’s Spitzer Space Telescope, also have searched for protostars, Herschel enables astronomers to probe deeper into the clouds than ever before. Younger protostars are hidden much more effectively within their clouds than older ones. Herschel managed to find the youngest and most primitive protostars known.
The new observations swelled the ranks of known protostars from 330 to nearly 500 and led to the discovery of a new type of not-quite-a-star — dense regions at a mere 15° above absolute zero with no sign of a protostar. These regions are likely to be in an early precursor stage of star formation. In astronomy, where timescales of hundreds of millions or of billions of years are the norm, the fact that this precursor stage is expected to last less than 1,000 years makes it extremely short-lived. Studying these elusive, pristine objects lays a necessary foundation for all subsequent studies of star formation.