The Milky Way is twice the size we thought it was

University of Sydney

20 February 2008

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We were tossing around ideas about the size of the Galaxy, and thought we had better check the standard numbers that everyone uses," Professor Gaensler said. Image credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

It took just a couple of hours using data available on the internet for University of Sydney scientists to discover that the Milky Way is twice as wide as previously thought.

Astrophysicist Professor Bryan Gaensler led a team that has found that our galaxy - a flattened spiral about 100,000 light years across - is 12,000 light years thick, not the 6,000 light years that had been previously thought.

Proving not all science requires big, expensive apparatus, Professor Gaensler and colleagues, Dr Greg Madsen, Dr Shami Chatterjee and PhD student Ann Mao, downloaded data from the internet and analysed it in a spreadsheet.

"We were tossing around ideas about the size of the Galaxy, and thought we had better check the standard numbers that everyone uses. It took us just a few hours to calculate this for ourselves. We thought we had to be wrong, so we checked and rechecked and couldn't find any mistakes."

The University of Sydney team's analysis differs from previous calculations because they were more discerning with their data selection. "We used data from pulsars: stars that flash with a regular pulse," Professor Gaensler explains. "As light from these pulsars travels to us, it interacts with electrons scattered between the stars (the Warm Ionised Medium, or WIM), which slows the light down.

"In particular, the longer (redder) wavelengths of the pulse slow down more than the shorter (bluer) wavelengths, so by seeing how far the red lags behind the blue we can calculate how much WIM the pulse has travelled through.

"If you know the distance to the pulsar accurately, then you can work out how dense the WIM is and where it stops - in other words where the Galaxy's edge is.

"Of the thousands of pulsars known in and around our Galaxy, only about 60 have really well known distances. But to measure the thickness of the Milky Way we need to focus only on those that are sitting above or below the main part of the Galaxy; it turns out that pulsars embedded in the main disk of the Milky Way don't give us useful information."

Choosing only the pulsars well above or below us cuts the number of measurements by a factor of three, but it is precisely this rejection of data points that makes The University of Sydney's analysis different from previous work.

"Some colleagues have come up to me and have said 'That wrecks everything!'" says Professor Gaensler. "And others have said 'Ah! Now everything fits together!'"

The team's results were presented in January this year at the 211th meeting of the American Astronomical Society in Austin, Texas.

About Professor Bryan Gaensler:

Professor Gaensler is a graduate of the University of Sydney and a former Young Australian of the Year. After working at the Department of Astronomy at Harvard University he was lured back to Australia on prestigious Federation Fellowship. One of the world's leading astronomers, his research interests include studying the essential role that magnetic fields play in the generation of turbulence and large-scale structures, the production of high-energy cosmic ray particles, and the formation of the first stars and galaxies.