ESA Science & Technology - Star formation processes highlighted by Planck

Our Galaxy, the Milky Way, is home to a vast agglomeration of billions of stars, laced through with clouds of gas and dust, known as the interstellar medium (ISM). In visible light most of the newly-born stars are hidden by clouds of tiny dust particles dispersed throughout the gas between the stars. Moving from visible light to longer wavelengths, where the Cosmic Microwave Background can be probed, the picture is very different as clearly demonstrated in new images from ESA's Planck mission. The dust no longer blocks the emission from the innermost regions, and new aspects of our Galaxy are revealed.

Within the frequency range of Planck, the ISM dominates the emission. (Above) a region of low star formation in the Perseus constellation as seen with Planck (left) and in visible light with the Digitized Sky Survey (right); (below) an active star formation region in the Orion Nebula, as seen with Planck (left) and the Digitized Sky Survey (right). Credits: (for Planck images) ESA, LFI and HFI Consortia; (for optical image) STScI DSS (Click on each image for a larger version of the image.)

Probing the processes at play

At frequencies where the Planck instruments are particularly sensitive, the Milky Way emits strongly over a large part of the sky. This foreground emission arises primarily from a number of processes that can be isolated using Planck: At the lowest frequencies Planck maps the distribution of the synchrotron emission due to relativistic electrons interacting with the Galactic magnetic fields,and that of free-free emission arising from electrons interacting with hot gas. An additional diffuse component, the 'anomalous microwave emission', attributed to spinning dust particles, is also present here. At intermediate frequencies (corresponding to wavelengths of a few millimetres), the emission is dominated by thermal emission of ionized gas heated by newly-formed hot stars. At still higher frequencies, Planck maps the distribution of interstellar dust, including the coldest compact cores in the final stages of collapse towards the formation of new stars.

The stars, once formed, disperse the surrounding clouds, gently or violently depending on their age and character. A delicate balance between cloud collapse and dispersion regulates the number of stars that any given galaxy makes. Many physical processes influence this balance, including gravity, heating and cooling, turbulence, magnetic fields and more. As a result of this interplay, the ISM rearranges itself into 'phases' which coexist side-by-side, some (known as 'molecular clouds') containing dense and cool gas and dust, some (referred to as 'cirrus') containing more diffuse and warmer material. Hot gas, ionized by stars, is also present.

Multi-frequency data are necessary to reconstruct the properties of the various constituents and phases of the ISM. Planck will advance this effort hugely, because it provides, for the first time, data on all the main emission mechanisms in one go. Its wide frequency coverage, a requirement to study the Cosmic Microwave Background, proves also to be crucial for the study of the interstellar medium.

Mapping the sites of star formation

The power of multi-frequency observations to discern the processes at play in star-forming regions is beautifully demonstrated by these new Planck images. This first sequence shows the interstellar medium in a region of the Orion Nebula where stars are actively forming.

Several physical processes involved in star formation in this region of the Orion Nebula (M42) are revealed through images taken by Planck. The optical image at top left shows two bright 'blobs' of light, arising from two clusters of massive young stars and their reflected light from the surrounding clouds. At top right, a Planck image at 30 GHz shows how gas near the hottest stars dominates the emission; a faint semi-circular feature (called Barnard’s Loop) indicates the edge of the bubble blown in the gas by a supernova explosion. At 857 GHz (bottom right) most of the emission arises from dust, at a range of temperatures, which traces the wispy structure of the cloud from which the stars formed. At an intermediate frequency of 143 GHz (bottom left), the Planck image shows a mix of radiation sources. The combination of information from all these images provides a very complete picture of all the phases of the interstellar medium surrounding this very active star formation site. Credit: (for Planck images) ESA, LFI and HFI Consortia; (for optical image) STScI DSS. (Click on each image for a larger version of the image.)

A comparable sequence of images of a region of low star formation activity, near the constellation of Perseus, shows how the structure and distribution of the ISM can be distilled from the images obtained with Planck.

These images of a region of low star formation in the Perseus constellation make a striking contrast to the images from the Orion region. The optical image (top left) appears very dark due to the low level of star formation activity. However, the large amount of interstellar material present in the region becomes evident when imaged at higher frequencies with Planck (clockwise from top right: 30GHz, 857 GHz, 143 GHz). These images show the wealth of information that can be gleaned with Planck even from dark regions of the sky such as this one. Credit: (for Planck images) ESA, LFI and HFI Consortia; (for optical image) STScI DSS. (Click on each image for a larger version of the image.)

A lucrative by-product for a ground-breaking mission

Precise measurements of the Cosmic Microwave Background are crucial to cosmology, and to understanding how our Universe formed and evolved. Attaining the highest-sensitivity (a few parts per million), highest-angular resolution (5 arcminutes) maps of the CMB – the goal of the Planck mission – requires the removal of the 'foreground' emission arising from the Milky Way. The information gleaned during this process is providing, as a by-product, a unique view of the processes that led to the formation of the stars in the galaxies that populate our Universe.

Notes for editors

Planck maps the sky in nine frequencies using two state-of-the-art instruments, designed to produce high-sensitivity, multi-frequency measurements of the diffuse sky radiation: the High Frequency Instrument (HFI) includes the frequency bands 100 – 857 GHz, and the Low Frequency Instrument (LFI) includes the frequency bands 30-70 GHz.

The first Planck all-sky survey began in August 2009 and is 98% complete (as of mid-March 2010). Because of the way Planck surveys the sky, the last bit of the first scan will be completed by late-May 2010. Planck will gather data until the end of 2012, during which time it will complete four sky scans. A first batch of astronomy data, called the Early Release Compact Source Catalogue, is scheduled for release in January 2011. To arrive at the main cosmology results will require about two years of data processing and analysis. The first set of processed data will be made available to the worldwide scientific community towards the end of 2012.

For further details please contact:

Jan Tauber, Planck Project Scientist
Research and Scientific Support Department
Directorate of Science and Robotic Exploration
European Space Agency
Email: Jan.Tauberesa.int