A new 3-D animated map of our cosmic hometown

For the first time, we now have a three-dimensional map of our closest cosmic neighbourhood which shows not only how our nearest neighbour stars are distributed today - it also shows precisely how fast each of them moves, and in which direction. Astronomers have measured the velocities of many thousands of stars and computed both where they were millions of years ago, and where they will be millions of years from now. These results are published today in the renowned European journal Astronomy & Astrophysics and represent a sort of "cosmic opinion poll", based on 63,000 precise observations of 14,000 Sun-like stars made over more than a decade. The results show that our "home turf" was a much livelier place than traditional models assume.

This task has been shouldered by a team of Swedish, Danish, and Swiss astronomers led by Dr. Birgitta Nordström from the Niels Bohr Institute in Copenhagen, who is currently also a Visiting Professor at the Astronomy Department of Lund University. The article has already stirred much interest from many colleagues, who have requested copies even before publication as a basis for a wide range of detailed studies of the origin and evolution of the Milky Way and other galaxies.

- For the first time we can study the history of the Milky Way in three dimensions and in time from a large and representative body of the most typical stars in our Galaxy - those just like the Sun, says Prof. Nordström. Astronomers have so far had to work with relatively small samples of stars, and the tendency has been to focus on unusual or exceptional objects, not the typical ones.

Sun-like stars - the so-called F and G dwarf stars - can live on to very large ages. Some are over 10,000 million years old - twice as old as our Sun and almost as old as the Universe itself. Much was already known about where the Sun-like stars in our neighbourhood are today, but now their velocities have also been measured so their orbits in the Galaxy can be computed. The ages of the stars and their content of heavier elements, like silicon or iron, have also been very carefully determined.

- The Sun orbits the centre of our Galaxy, once every 225 million years or so, and its orbit has been stable for a very long time. But we found that many of the stars closest to us are in reality "transit passengers" from afar who are just passing by, with the oldest stars having the highest speeds, notes Birgitta Nordström and adds,

- The reason is that the oldest stars have had their velocities disturbed by passing close to heavy objects in the Milky Way, e.g. giant molecular clouds, supernovae, spiral arms, or small captured satellite galaxies. Galaxy models have long assumed that the Milky Way disk near the Sun evolved quietly and essentially in isolation, but now we see that its entire life has been rather turbulent. Our new results will lead to much more realistic models of how our home galaxy formed and evolved. But one thing is clear already - the history of the Milky Way was much more dramatic than we believed.

- My Visiting Professorship in Lund, with primary support from the Swedish Research Council , was of critical importance for the completion of this marathon project. It is also a good example of a fruitful collaboration between the countries on both sides of Ôresund. A postdoc from Lund working in Copenhagen and a Danish Ph.D. student in Lund made decisive contributions to the completion of the paper concludes Prof. Nordström

The paper in Astronomy & Astrophysics (Vol. 418, no 3,2004) is entitled The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities and kinematic properties of ~14 000 F and G dwarfs. Folllowing Birgittta +other authors are J. Andersen, J. Holmberg, and E. H. Olsen from Copenhagen, B. R. Jørgensen from Lund, and M. Mayor, F. Pont, S. Udry, and N. Mowlavi from Geneva.