How thick is your Milky Way shake?

Bad Astronomy
The entire universe in blog form
Feb. 20 2008 11:32 AM

How thick is your Milky Way shake?

Hey, don't forget about the lunar eclipse tonight!



Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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Somehow I missed this one at the AAS meeting I attended in January: a team of astronomers found that our galaxy is twice as thick as previously thought!

The Milky Way is a spiral, meaning it's a flat disk. Face on, you can see the glorious spiral arms and the central bulge, with the disk spreading across an impressive 100,000 light years or so.

But edge-on, it's much thinner. The exact thickness depends on what you're looking at, though, making this a bit dicey. High-mass stars tend to stick right to the mid-plane, the line dividing the galaxy in half (like a layer of icing through a cake). Lower mass stars can reach farther above and below this plane (lighter weight stars get tossed around more easily). Other methods produce different results yet.

The astronomers in question here looked at warm gas in the galaxy... sorta. What they did was examine pulsars, dead, dense stars that send out short, sharp radio signals. As those signals pass through warm gas, they get spread out in wavelength, so that the longer wavelengths arrive a wee bit later than the shorter ones. The amount of dispersion of wavelength tells you how much gas the signal passed through.

The astronomers looked at pulsars where the distances were known, and the pulsars themselves were well away from the galactic mid-plane. When they did this, they could determine the density of the warm gas between us and the pulsars, and where that gas stops.

The results: the warm gas stretches 6000 light years above and 6000 light years below the mid-plane. Before, it was thought the galaxy was only half this thickness.

Like I said before, this doesn't mean the galaxy itself is twice as thick as we thought in general, because the thickness depends on what you measure. But this does throw a monkey wrench in some ideas about the size of the galaxy, and the distribution of gas within it. Why is the gas spread out so much more than previously thought? How did it get there?

Surprises like this are generally important. It means we get to revise theories, and that in turn means we're edging closer to the truth. That's what science is all about.

Tip o' the dew shield to BABloggee Rich for sending this along!

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