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Milky Way twice as thick as we think

Our home galaxy, the Milky Way, is twice as thick as we thought it was, Australian astrophysicists discover.

Professor Bryan Gaensler from the University of Sydney and his team found that the enormous spiral-shaped collection gas and stars is 12,000 light-years thick when seen edge-on, not 6000 as scientists previously thought.

"This was quite a stunning result," Gaensler says. "It was a bit of a shock to us. It's like walking out into your backyard and finding your tree is twice the size you remembered."

The researchers made their discovery without high-tech equipment or powerful telescopes.

Instead, they downloaded publicly available data from the internet and carefully analysed it.

"It took us just a few hours to calculate this for ourselves," Gaensler says. "We thought we had to be wrong, so we checked and rechecked and couldn't find any mistakes."

The surprising new result came about because the researchers were discerning about the data they used to make their calculations.

Choosing the right pulsars

To measure the size of the Milky Way, researchers study light coming from a pulsar, a type of star that sends beams of light through space like a searchlight.

"As light from these pulsars travels to us, it interacts with electrons scattered between the stars, which slows the light down," Gaensler says.

Scientists refer to those electrons as the Warm Ionised Medium, or WIM.

The WIM has a bigger effect on longer wavelengths of light, which are redder, than on the shorter, bluer, light.

"So by seeing how far the red lags behind the blue we can calculate how much of the WIM the pulse has travelled through," he says.

By comparing this effect on the light from stars different distances away from us, researchers can find where the WIM stops; in other words, the galaxy's edge.

The trick for getting a more accurate figure lay in choosing the right pulsars to include in that analysis, Gaensler says.

"What we did in terms of picking better data was picking pulsars that are either high above the galaxy or underneath it, and not the ones sitting inside the galaxy, which is what nobody had bothered to do before."

Doing the sums

The researchers presented their results recently at the world's biggest annual conference of stargazers, the 211th meeting of the American Astronomical Society in Austin, Texas.

"Overall, most people were accepting because they agreed that our analysis was a better approach than had been done in the past. What's more, it fixes a lot of things that hadn't made sense in our galaxy," he says.

"By making the galaxy twice as thick, dozens of other seemingly unrelated calculations ... all of a sudden work now."

For example, it helps make sense of how magnetic the galaxy is, he says.

The new figure could also lead to a rethink about the wider universe.

"Many of detailed calculations people do on galaxies across the universe - how gas is converted to stars, and stars are converted into gas, and how gravity and pressure balance to stop the galaxy flying apart - use that number," Gaensler says.