Milky Way Past Was More Turbulent Than Previously Known (Forwarded)

ESO Education and Public Relations Dept.

Contacts:

Birgitta Nordström
Niels Bohr Institute for Astronomy, Physics and Geophysics
Juliane Maries Vej 30
2100 Copenhagen, Denmark
Phone: +45 353 25420 (Copenhagen) or +46 46 222 1572 (Lund)
Email: (Copenhagen) or (Lund)

Johannes Andersen
Astronomical Observatory
Niels Bohr Institute for Astronomy, Physics and Geophysics
Juliane Maries Vej 30
2100 Copenhagen, Denmark
Phone: +45 353 25934
Email:

Michel Mayor
Geneva Observatory
51 Ch. Des Maillettes
1290 Sauverny, Switzerland
Phone: +41 22 7 55 26 11
Email:

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For immediate release: 6 April 2004

ESO Press Release 08/04

Milky Way Past Was More Turbulent Than Previously Known

Results of 1001 observing nights shed new light on our Galaxy [1]

Summary

A team of astronomers from Denmark, Switzerland and Sweden [2] has
achieved a major breakthrough in our understanding of the Milky
Way, the galaxy in which we live.

After more than 1,000 nights of observations spread over 15 years,
they have determined the spatial motions of more than 14,000
solar-like stars residing in the neighbourhood of the Sun.

For the first time, the changing dynamics of the Milky Way since
its birth can now be studied in detail and with a stellar sample
sufficiently large to allow a sound analysis. The astronomers find
that our home galaxy has led a much more turbulent and chaotic
life than previously assumed.

PR Photo 10a/04: Distribution on the sky of the observed stars.
PR Photo 10b/04: Stars in the solar neigbourhood and the Milky Way
galaxy (artist's view).
PR Video Clip 04/04: The motions of the observed stars during the
past 250 million years.

Unknown history

Home is the place we know best. But not so in the Milky Way -- the galaxy in
which we live. Our knowledge of our nearest stellar neighbours has long been
seriously incomplete and -- worse -- skewed by prejudice concerning their
behaviour. Stars were generally selected for observation because they were
thought to be "interesting" in some sense, not because they were typical. This
has resulted in a biased view of the evolution of our Galaxy.

The Milky Way started out just after the Big Bang as one or more diffuse blobs
of gas of almost pure hydrogen and helium. With time, it assembled into the
flattened spiral galaxy which we inhabit today. Meanwhile, generation after
generation of stars were formed, including our Sun some 4,700 million years ago.

But how did all this really happen? Was it a rapid process? Was it violent or
calm? When were all the heavier elements formed? How did the Milky Way change
its composition and shape with time? Answers to these and many other questions
are 'hot' topics for the astronomers who study the birth and evolution of the
Milky Way and other galaxies.

Now the rich results of a 15 year-long marathon survey by a Danish-Swiss-Swedish
research team [2] are providing some of the answers.

1,001 nights at the telescopes

ESO PR Photo 10a/04
Sky distribution of the observed stars

Caption: ESO PR Photo 10a/04 shows the distribution on the sky of the
approx. 14,000 observed stars. The region on the left that is denser
than its surroundings is the nearby Hyades star cluster.

The team spent more than 1,000 observing nights over 15 years at the Danish
1.5-m telescope of the European Southern Observatory at La Silla (Chile) and at
the Swiss 1-m telescope of the Observatoire de Haute-Provence (France).
Additional observations were made at the Harvard-Smithsonian Center for
Astrophysics in the USA. A total of more than 14,000 solar-like stars (so-called
F- and G-type stars) were observed at an average of four times each -- a total
of no less than 63,000 individual spectroscopic observations!

This now complete census of neighbourhood stars provides distances, ages,
chemical analysis, space velocities and orbits in the general rotation of the
Milky Way. It also identifies those stars (about 1/3 of them all) which the
astronomers found to be double or multiple.

This very complete data set for the stars in the solar neighbourhood will
provide food for thought by astronomers for years to come.
A dream come true

ESO PR Photo 10b/04
Stars in the solar neighbourhood

Caption: ESO PR Photo 10b/04 provides an artist's view of the observed
group of stars orbiting the Milky Way together with the Sun, as seen
by an imaginary observer outside the Galaxy. The orbit of the Sun is
shown. For clarity, the stars surrounding the local volume have been
removed here.

These observations provide the long-sought missing pieces of the puzzle to get a
clear overview of the solar neighbourhood. They effectively mark the conclusion
of a project started more than twenty years ago..

In fact, this work marks the fulfilment of an old dream by Danish astronomer
Bengt Strömgren (1908-1987), who pioneered the study of the history of the Milky
Way through systematic studies of its stars. Already in the 1950's he designed a
special system of colour measurements to determine the chemical composition and
ages of many stars very efficiently. And the Danish 50-cm and 1.5-m telescopes
at the ESO La Silla Observatory (Chile) were constructed to make such projects
possible.

Another Danish astronomer, Erik Heyn Olsen made the first step in the 1980's by
measuring the flux (light intensity) in several wavebands (in the "Strömgren
photometric system") of 30,000 A, F and G stars over the whole sky to a fixed
brightness limit. Next, ESA's Hipparcos satellite determined precise distances
and velocities in the plane of the sky for these and many other stars.

The missing link was the motions along the line of sight (the so-called radial
velocities). They were then measured by the present team from the Doppler shift
of spectral lines of the stars (the same technique that is used to detect
planets around other stars), using the specialized CORAVEL instrument.

Stellar orbits in the Milky Way

ESO Video Clip 04/04
Motions of the observed stars in the Milky Way

Caption: ESO PR Video Clip 04/04 shows the stars studied during the
present programme making their most recent orbital revolution around
the Galactic centre before converging into the small volume where
they were observed by the team. The duration of the video corresponds
to about 250 million years. The yellow dot and white curve show how
the Sun moved during this last of its about 20 laps around our Galaxy.

With the velocity information completed, the astronomers can now compute how the
stars have wandered around in the Galaxy in the past, and where they will go in
the future, cf. PR Video Clip 04/04.

Birgitta Nordström, leader of the team, explains: "For the first time we have a
complete set of observed stars that is a fair representation of the stellar
population in the Milky Way disc in general. It is large enough for a proper
statistical analysis and also has complete velocity and binary star information.
We have just started the analysis of this dataset ourselves, but we know that
our colleagues worldwide will rush to join in the interpretation of this
treasure trove of information."

The team's initial analysis indicates that objects like molecular clouds, spiral
arms, black holes, or maybe a central bar in the Galaxy, have stirred up the
motion of the stars throughout the entire history of the Milky Way disc.

This in turn reveals that the evolution of the Milky Way was far more complex
and chaotic than traditional, simplified models have long so far assumed.
Supernova explosions, galaxy collisions, and infall of huge gas clouds have made
the Milky Way a very lively place indeed!

More information

The research presented in the Press Release will be published in the European
research journal Astronomy & Astrophysics: "The Geneva-Copenhagen survey of the
Solar neighbourhood: Ages, metallicities and kinematic properties of ~14,000 F
and G dwarfs" by B. Nordström et al. The full article is available in PDF format
here.

Notes

[1]: This Press Release is coordinated between ESO and the European research
journal "Astronomy & Astrophysics" (A&A Press Release). Read also the
information from Lund University (Sweden), with onwards links to more
animations, etc..

[2]: The team behind this large work consists of Birgitta Nordström (Niels Bohr
Institute for Astronomy, Physics and Geophysics, Denmark and Lund Observatory,
Sweden), Michel Mayor, F. Pont, Stéphane Udry and Nami Mowlavi (Geneva
Observatory, Switzerland), Johannes Andersen and J. Holmberg (Astronomical
Observatory, Copenhagen, Denmark and Nordic Optical Telescope Scientific
Association, Spain), B. Rosenkilde Jörgensen (Lund Observatory, Sweden), and
Erik Olsen (Astronomical Observatory, Copenhagen, Denmark).

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