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A galactic map of our way to the stars

ASTRONOMERS HAVE unveiled a map of the cosmos - or at least part of it - which future space explorers may look back on as the equivalent of the first "mappa mundi" drawn by early cartographers. "Some day it will be hanging from every kid's wall," said one of the team that produced it.

The map charts the position of 15,500 galaxies within a sphere 1,400 million light years across (with each light year spanning about six million million miles), far bigger than our galaxy, the Milky Way, which is just 200,000 light years in diameter.

It has taken 10 years to compile what Professor Carlos Frenk calls "the biggest map of anything".

Yet despite its enormous size, the map covers only perhaps one ten-thousandth of the volume of the universe, which is expanding every moment, from the energy unleashed in the Big Bang 15 billion years ago, when it began.

Professor Frenk, of the physics and astronomy department at Durham University, said that the purpose of drawing the map was twofold: "One is the obvious one - we want to explore, in the same way as the pioneers in the 15th and 16th centuries wanted to find out what was on the Earth."

The second reason is more subtle: "The pattern of galaxies that we see around us is like encoded information about the phenomena that happened in the very early universe, close to the time of the Big Bang.

"By decoding that pattern, we can understand the fundamental processes of that instant."

The first cartographers found it easiest to map shorelines and large islands, and avoided excursions into the emptiness of oceans and "terra incognita".

Similar areas exist in space: the cosmic map shows that galaxies, each containing billions of stars, often form clusters containing tens or thousands. These in turn group into "superclusters", which stand out from the celestial voids on the map like islands in a vast ocean.

"They really are voids," said Professor Frenk. "They are not completely free of galaxies but they are far rarer than in the superclusters."

The map is an interpretation of data gathered by the now- defunct Infrared Astronomical Satellite (IRAS), which orbited the Earth, relaying data about the temperature and apparent distance of millions of celestial objects until its fuel ran out in 1993.

That dataset has now been analysed by a team from the UK and Germany, to produce a three-dimensional map of the view from inside or outside our location in the universe.

"When we first got this map from the computer we were thrilled," Professor Frenk said. "We knew it was one of the great maps of history."

IRAS could detect only matter that gave out heat, but these results could also help to solve the mystery of "dark matter".

By measuring the apparent movement of galaxies, astronomers have deduced that the universe contains more matter than we can see with astronomical instruments.

But what form it takes - whether as subatomic particles, atom-sized black holes or simply as cold lumps of rock - is not yet known.

Understanding the processes of the Big Bang could in turn make clear what happened to all the matter that we cannot now see.