Wednesday, May 28, 2008

Sizing up the Milky Way

An artist's impression of the Milky Way as seen from above
Image Credit: NASA/JPL-Caltech/R. Hurt

It surprises most people to learn that we astronomers don't have a good idea how massive our own Milky Way galaxy is. We have a pretty good idea about the masses of other galaxies, and we have a pretty good idea of the physical size of our galaxy. But we have a hard time measuring how hefty the galaxy is. Today, a new report on the mass of the Milky Way gives a new estimate of the mass of the Milky Way. It's down a bit from some earlier estimates, at a trim 1 trillion times the mass of the sun, a number that includes stars, dust, gas, and dark matter -- everything.

(If I use standard formulae for Body Mass Index, I estimate the BMI of the Milky Way to be 2.2, which is severely underweight. But most galaxies have lived for 13 billion years and don't have problems with their cholesterol, so I think the Milky Way is just fine.)

Measuring the mass of the Milky Way is hard. For other galaxies, we simply look at how fast stars and gas are orbiting around the galaxy. But we are inside the Milky Way (see the picture above), and we are moving around it ourselves. So, we have to figure out how fast we are moving around the Milky Way, take that motion out of the speeds of other stars, and then try and estimate how fast they are moving around the Milky Way. (You have to average things out, because a single measurement, including that of our own sun, likely suffers from many types of errors or oddities.)

It's a little like living in a dense forest, being tied to one tree, and trying to figure out how many trees are in the forest. You can estimate, but maybe you live in a dense part of the forest, and there are holes in other parts of the forest that you can't see. Maybe other parts of the forest are denser yet than where you are. You aren't even certain how big the forest is, although you can make some clever guesses. And imagine that most of the trees are invisible (like dark matter). You may see other forests, and you can guess how dense and how big they are, but you'll still struggle to get a good handle on your own. And that is the situation we find ourselves in here.

The new measurements use about five times as many points as earlier measurements, and that means the estimate of the Milky Way's mass is more likely to be correct. But there are still many uncertainties that are hard to quantify, or even to explain. So, it would not surprise me that, in a few years, a new measurement comes out that is different yet again from the 1 trillion times the mass of the sun.

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