The Milky Way is a large spiral galaxy 130 000 light-years across, which began to form about 10 000 or 15 000 million years ago - shortly after the origin of the Universe. It is structured in a thin disk with spiral arms and a great bulge in the centre, which as seen from the Earth lies towards the constellation of Sagittarius. Our Solar System is in the edge of one of the arms, about 25 000 light-years from the centre: a very quiet area compared to the inner central bulge.

"The inner bulge of the Milky Way is like the core of a very busy metropolis. The density of stars is 500 times larger than elsewhere in the galaxy - stars can even bump into each other!. These populations of stars give us a lot of information about the whole galaxy. For example, their relative motions might reveal traces of other galaxies devoured by our own in the past", says Alain Omont, at the Institut d'Astrophysique de Paris.

Despite its interest, current knowledge about the centre of the Milky Way is far from complete because the dust enshrouding it has blocked the view of most telescopes so far. Only ESA's ISO, the first space observatory working at infrared wavelengths - and hence able to see through the dust - has performed a very deep exploration of its stellar populations. One of ISO's longest observing programme, ISOGAL, has devoted 255 hours to this aim, focusing especially on the inner central bulge. The first results from this programme, a joint effort by astronomers from France, the UK, Holland, Italy, Germany, Spain, Sweden, India, South Africa, Chile and the US are already being published in the scientific literature.

100 000 red giants newly identified

In a region of the galactic centre that as seen from Earth is only about four times the angular size of the full moon, ISO has identified a population of more than 100 000 stars of the 'red giant' type. Most of them are the so-called AGB (Asymptotic Giant Branch) stars, which for astronomers adds value to the finding.

AGB stars are very evolved stars that provide key clues to unveil the star-formation history of the Milky Way, because their masses vary according to their age; it is therefore easy to determine how long ago a certain population of AGB stars was born. Also, AGB stars are special because they are one of the main dust-producing factories of the galaxy. They expel huge amounts of dust to the environment during a brief stage of their lives - dust in which many chemical elements, including those essential for life - are present.

"These stars enrich the galaxy with the chemical compounds present in their gas and dust. Even some of the carbon out of which we are made comes from them", Omont says. "And they give us clues about the star-formation history of our galaxy, because we know their age. For instance, knowing that some of them formed a few thousand million years ago will allow us to infer the star-formation efficiency at that time".

The coordinates of the newly identified stars will be published soon in a new catalogue.

To Martin Kessler, ISO project scientist, "these results are an excellent example of ISO's pioneering role in unveiling a Universe that is hidden by dust and can only be seen in infrared light. ISO's continuing discoveries are laying the foundations on which all future infrared space missions will build."

Traces of 'galactic cannibalism'

Studying the stars in the galactic centre gives also information about how the galaxy formed. Astronomers would like to know, for example, the origin of the extremely dense group of stars in the central inner bulge: is it a remnant of the original core around which the galaxy grew, or is it the result of a past collision with other small galaxies? The way to find out is by measuring the stellar motions, which are especially frantic in the centre of the galaxy due to the strong pull of the gravitation. If a star comes from a galaxy devoured by our own in the past, it will probably move differently than most of its companions.

As Omont explains, "We still don't know the details about the formation of the Milky Way. For instance we cannot yet fully confirm that our galaxy grew by 'swallowing' dwarf galaxies attracted by its gravitational force. We expect that stars from these smaller galaxies can still be found as they will move in different directions from the others".

Galactic 'cannibalism' and collisions have been identified as one of the most common processes driving galaxy evolution. Observations during the past years have in fact shown that the Milky Way is at present strongly attracting smaller neighbour galaxies, such as the Magellanic Clouds, and that it could collide with the closest large galaxy, Andromeda,in a time as short as 3 billion years.

ISOGAL observations were complemented with those made within the international DENIS project, a near infrared survey of the Southern sky using a 1-m telescope at La Silla (Chile), of the European Southern Observatory.

Footnote on ISO

The European Space Agency's infrared space telescope, ISO, operated from November 1995 till May 1998, almost a year longer than expected. As an unprecedented observatory for infrared astronomy, able to examine cool and hidden places in the Universe, ISO successfully made nearly 30 000 scientific observations. /td>