Page last updated at 13:51 GMT, Tuesday, 6 January 2009

Milky Way 'bigger than thought'

Milky Way (CfA)
Andromeda was previously thought to be larger than the Milky Way

Our galaxy is much bigger than once thought, according to research presented at a major astronomy meeting this week.

The results suggest the Milky Way is roughly the same size as Andromeda, the largest galaxy in our local group.

What is more, it is moving 15% faster than earlier predictions.

The greater mass means that future collisions with nearby galaxies could happen sooner than thought, according to the researchers.

Mark Reid of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, US, 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 that together allow unprecedented resolution in astronomy measurements.

This resolution, according to the CfA, is equivalent to being able to read a newspaper in Cairo from an armchair in Edinburgh.

No longer will we think of the Milky Way as the little sister of the Andromeda Galaxy
Mark Reid, CfA

By using the VLBA to measure the apparent shift of far-flung star-forming regions when the Earth is on opposite sides of the Sun, the researchers were able to measure the distance to those regions using fewer assumptions than prior efforts.

"These measurements use the traditional surveyor's method of triangulation and do not depend on any 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.

Spinning around

Tiny shifts in the frequency of the radio emission that arise because the regions are moving gave the researchers an estimate of how quickly the Milky Way rotates around its centre.

They estimate this to be about 914,000km per hour, significantly higher than the widely accepted value of 792,000km per hour.

That speed, in turn, allowed the 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.

The researchers estimate that the Milky Way contains about 50% more mass than earlier predictions - putting it on a par with the Andromeda galaxy, previously 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.

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