Mark Reid of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, and his colleagues made use of the Very Long Baseline Array (VLBA) to deduce the Milky Way's size and speed. Dr Reid was speaking at the 213th American Astronomical Society (AAS) meeting in Long Beach, California.

The VLBA is a system of 10 radio telescopes scattered across and around North America. Working together, they allow unprecedented resolution in astronomy measurements. According to the CfA, this resolution is equivalent to being able to read a newspaper in Cairo from an armchair in Edinburgh.

Using the VLBA to measure the apparent shift of far-flung star-forming regions when the Earth is on opposite sides of the Sun, researchers measured the distance to the regions using fewer assumptions than prior efforts. "These measurements use the traditional surveyor's method of triangulation and do not depend on assumptions based on other properties, such as brightness, unlike earlier studies," said team member Karl Menten of the Max Planck Institute for Radio Astronomy in Bonn, Germany. The results show that the Milky Way is about 15% wider than previously thought.

Tiny shifts in the frequency of radio emissions that arise because the regions are moving give an estimate of how quickly the Milky Way rotates around its centre. The estimate is about 914,000km per hour, significantly higher than the accepted value of 792,000km per hour. That speed allowed astronomers to calculate the total amount of dark matter in the Milky Way - the invisible component that makes up the majority of the galaxy's mass.

Researchers estimate the Milky Way contains about 50% more mass than earlier predictions - putting it on a par with the Andromeda galaxy, thought to be our much bigger neighbour and the largest in our Local Group of galaxies. "No longer will we think of the Milky Way as the little sister of the Andromeda Galaxy," Dr Reid said. That higher mass makes for a higher gravitational pull, suggesting that collisions with Andromeda and other nearby galaxies may happen much sooner than thought - but still billions of years in the future.

news.bbc.co.uk/2/hi/scien...7813635.stm

13:51 GMT, Tuesday, 6 January 2009