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RTD info logoMagazine on European Research N° 45 - May 2005   
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ASTRONOMY
Title  When distant worlds meet

Every year since 1991, European astronomers have gathered for a JENAM (Joint European and National Astronomy Meeting) to discuss the latest progress in fields where Europe is at the leading edge of research. RTD info looks at the five topics on the agenda for the next meeting, in Liège (BE) in July 2005. In addition to the sessions for specialist scientists, there will be public sessions designed to attract ‘stargazers’ from all walks of life who are interested in the latest advances in astronomy, especially in the field of space exploration.

The Einstein Cross, a gravitational mirage: the points that make up the cross are four images of the same distant quasar, formed by the gravitational action of a closer galaxy.  © HST
The Einstein Cross, a gravitational mirage: the points that make up the cross are four images of the same distant quasar, formed by the gravitational action of a closer galaxy.
© HST
1. Asteroseismology – Who could possibly claim to understand the workings of human biology simply by observing people’s skin? Yet this is precisely the paradox facing astronomers who observe the stars through telescopes. What they are seeing is no more than a fine light-emitting layer – the photosphere – that forms the surface of stars. The difficulty lies in penetrating this surface layer to understand the phenomena at work at the heart of these stellar masses.

"As difficult as it may seem, astronomy has started to gain access to the interior of stars by studying their oscillations, a discipline known as asteroseismology,” explains Conny Aerts (Katholiek Universiteit Leuven, BE). Some important discoveries are being made by this relatively recent field of research which makes it possible to model the internal structure by recording the pulsations emitted by seismic shocks caused by stellar oscillations. The SOHO satellite, for example, was able to demonstrate the complex internal rotation of the Sun and the existence of plasma rivers beneath its surface. The pulsations of other stars have revealed their fundamental properties, such as mass, chemical composition, rotation, etc. Today, the use of space observatories is revolutionising asteroseismology. “Satellites do not only permit a continuous monitoring of pulsating stars, but they also enable us to increase the recording precision by a factor of 1 000.” 

A hot star (over 120 000 degrees on the surface, compared with the 6 000 degrees of our Sun) heats the centre of the nebula. The mauve colour indicates the hottest areas where temperatures can reach several thousand degrees.  © ESO
A hot star (over 120 000 degrees on the surface, compared with the 6 000 degrees of our Sun) heats the centre of the nebula. The mauve colour indicates the hottest areas where temperatures can reach several thousand degrees.
© ESO
2. Solar system and astrobiology – Europe is a key player in the exploration of our solar system, whether it be Mars (Mars Express), Titan (Huygens), Mercury (Bepi Colombo), Venus (Venus Express) or the comets (Giotto, Rosetta). In addition to gaining precise knowledge of our neighbouring planets, astronomers make no secret of an ambition on an altogether different scale: the search for the constituents of extra-terrestrial life. 

Astrobiology – the name given to this new kind of quest – is also interested in detecting ‘biosignatures’ outside the solar system. These are exoplanets that gravitate around other stars. A Swiss team discovered the first of these in 1995; today over 120 have been found and the potential for new discoveries remains vast. 

3. Quasars: host galaxies and gravitational lenses – Quasars are distant galaxies (more than 2 000 light years away) with a huge black hole at their core. This giant ogre is continuously swallowing phenomenal quantities of the gas present in the host galaxy. Before dying, these gaseous masses engage in a magnificent swan song that makes them one of the brightest bodies in the universe.

“Due to this brilliance, quasars are the most distant objects we are able to study, giving us access to a primordial universe,” explains Frédéric Courbin (Ecole Polytechnique Fédérale de Lausanne, CH). The light emitted by these distant beacons illuminates the intergalactic environment that places its signature on their spectrum, making it possible to determine the moment when the first stars illuminated the universe. Also, due to general relativity, the light of quasars can be deflected by closer galaxies – which have a lens effect – and give rise to gravitational mirages. “The study of these cosmic mirages is an instrument of analysis by the Hubble Constant, the cosmological key to the theory of the expanding universe and thus to the time elapsed since the Big Bang.” 

4. Roadmap for interferometry   A fairly recent tool for astronomers, interferometry is based on a simple principle. Instead of using a very large telescope, a number of smaller devices are used and their signals are combined, resulting in a comparable observation quality at a significantly reduced cost. This technique, nevertheless, requires a degree of precision in combining the beams that is much smaller than the wavelength, which is why for a long time it was limited to long radio waves. European astronomers succeeded in transposing this technique to the field of the visible in which wavelengths do not exceed the micron. They are currently developing the most powerful interferometric instrument in the world, the VLTI (Very Large Telescope Interferometer). Installed in Chile, the VLTI uses siderostats(1) measuring 40 cm across. With this modest equipment, scientists have already obtained a number of important results, including the determination of the real size of Cepheids and the thickness or elongation of certain stars, etc. 

5. Massive stars and high-energy emission – "Very massive and very hot stars have a major effect on their environment, sculpting the interstellar environment and sowing the universe with the heavy chemical elements that are now all around us. Sometimes they are even able to modify the structure of galaxies,” explains Ian Stevens (University of Birmingham, UK).

European space observatories, such as Integral and XMM-Newton II, have made it possible to observe these objects in the high energies of the OB Associations. Here they show their true nature, revealing violent phenomena such as the mysterious collisions of stellar winds, of which we were unaware just a few decades ago.

There remain many unanswered questions. “We have a general idea of their evolution,” stresses Ian Stevens, “but the details remain vague, especially the way in which these stars die.” 

(1) Fixed direction observation device, compensating for the Earth’s rotation. 


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  For the stargazers

Light seems a familiar concept to us, but it holds some unsuspected secrets. It was not until about 200 years ago and the discovery of infrared rays that the immensity of the electromagnetic spectrum first became apparent. Over the past 50 years, the discovery of these new ‘colours’ that ...
 


   
  Top
  For the stargazers

Light seems a familiar concept to us, but it holds some unsuspected secrets. It was not until about 200 years ago and the discovery of infrared rays that the immensity of the electromagnetic spectrum first became apparent. Over the past 50 years, the discovery of these new ‘colours’ that are invisible to the naked eye – in the gamma, X, ultraviolet, infrared and radio fields – has caused a revolution in astronomy, revealing a mysterious background radiation, strange pulsating stars and very active galaxies. This ‘Rainbow Universe’ will be presented by Yaël Nazé, of the Institut d'Astrophysique et de Géophysique de Liège, on the occasion of the JENAM 20O5. Other planned activities include a nocturnal sky observation session entitled ‘Travel in Space’, and a ‘science café’ on the question "Is there life in the universe?". All these will take place in the company of astrobiologists and officials from the European Space Agency (ESA) space exploration programmes.



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