Thursday, May 29, 2008

Weighing Our Galaxy

How do you measure a galaxy you're stuck in the middle of? You can't exactly, but you can get some pretty good estimates. Astrophysicists used a large sample of data from stars in the galactic halo to calculate that the Milky Way, earth's own galaxy, weighs slightly less than 1 trillion solar masses. Since a solar mass is equal to the mass of the sun, our galaxy weights about 1 trillion times as much as the sun.

Turns out that the Milky Way (despite being 1 trillion solar masses) is actually skinnier than previously thought. Less weight means our galaxy has less dark matter. It also means that the Milky Way is productive in converting its original hydrogen and helium into stars.

Our galaxy is made up of overlapping spiral disks, each of which contains a number of stars totaling 100 billion. What we know about the Milky Way is constantly changing; its size and shape can only be pieced together a little at a time as technology improves. Gaps in knowledge abound, but a close estimate of its weight is sure to be beneficial.

A bright halo of gas and stars, 100,000 light years in diameter and 1,000 light years thick, surrounds the outer edge of the Milky Way. Researchers used the speeds of stars in the halo to calculate the amount of gravity needed for the stars to remain in orbit, subsequent calculations were used to estimate the weight of the galaxy.

The sun, which orbits the Milky Way once every 225 million years, is perched on one of the spiral disks, about 26,000 light years away from the galactic center.

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