Tales Of A Thousand And One Nights: Past And Future Of The Milky Way

Home is the place we know best. The Milky Way, our Galaxy, is the exception to the rule, however: Our knowledge of our nearest stellar neighbours has long been seriously incomplete and – worse – skewed by prejudice concerning their behaviour. In the past, stars were selected for observation because they were thought to be ’interesting’ in some sense, not because they were typical of stars in general. Naturally, this gave a biased view of the evolution of our Galaxy – that serves as the prototype of all galaxies.

The best way to understand our Milky Way as a whole is to study Sun-like stars (so-called F- and G-type stars), which are numerous and some of which have existed since the Galaxy’s formation. The Danish- Swiss- Swedish team, led by Dr. Birgitta Nordström, has now determined the motions of about 14,000 F- and G-type stars – nearly all such stars in our neighbourhood. The figure below shows all of the observed stars projected on the sky.

To obtain this huge amount of data, Birgitta Nordström and her colleagues spent 1,000 observing nights over 15 years working with the Danish 1.5-m telescope at ESO La Silla Observatory, Chile, and the Swiss 1-m telescope at the Observatoire de Haute-Provence, France. In all, 63,000 single observations were made of the 14,000 stars – four times per star on average. Additional observations were made at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, USA.

Most of the stars are located within about 500 light-years from the Earth and were already observed by the ESA satellite Hipparcos to measure their precise distances and motions in the plane of the sky. But a key piece was missing in our knowledge of their space motions in the Galaxy: The radial velocities[1] of the stars were still not measured, so only their 2D motions were known. The team of astronomers has now filled this gap: For the first time, we now know the 3D space motion of a complete sample of typical stars in the solar neighbourhood. From their space motions, the team was able to compute the positions of these stars at different points in the Milky Way’s history. For example, the movie below shows the motions of the observed stars in their latest orbit around the Galactic Center.

Using the new data, the team has accurately determined the characteristics of all these stars, including their ages, their heavy-element content, and the orbits that they follow in their motion around the Galaxy. The team also identified the binary stars in the sample (about 1/3 of all stars!). If the point of light seen in the telescope is in reality two stars orbiting each other, their velocities vary and the motion observed at any given time is not that of any single star. By identifying these cases, misleading data can be eliminated or corrected.

A first analysis reveals that objects in the galactic disk, such as molecular clouds, spiral arms, black holes, or possibly a central bar-shaped feature, stirred up the stellar velocities throughout the history of the Galaxy. This implies that the evolution of the Milky Way was far more complex and chaotic than traditional, simplified models have long assumed. Supernova explosions, galaxy collisions, and infall of huge gas clouds made the Milky Way a turbulent place indeed!

The velocities now published by the team are a veritable treasure trove for astronomers throughout the world who study the evolution of galaxies – one of the ’hot’ topics in astrophysics today. This catalogue represents the most important improvement in our knowledge of our “home base” in the Milky Way since the results from Hipparcos appeared seven years ago. It will provide unique material for astronomers for years to come, until the vast database from ESA’s satellite GAIA will become available around 2015.