Weight of the Milky Way

Discovery of the Week
What the Milky Way Weighs



How much does our galaxy weigh? About half as much as astronomers once thought. Or so says a recent study by an international team of stellar scientists. According to their calculations, the Milky Way's mass is equal to that of about 1 trillion suns. Previous studies had put our galaxy's bulk at roughly 2 trillion suns.


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Why the wide discrepancy? Well, you can't just plop a galaxy onto a scale. To estimate the Milky Way's mass, researchers measure the speeds of stars and dwarf galaxies that orbit its main disk in a "galactic halo." But they don't always agree about which stars' speeds to measure--or about other factors that could affect the speeds they see.

As one researcher notes, "the total mass of the galaxy is hard to measure because we're stuck in the middle of it." Still, the new study has at least one big advantage over previous ones. Thanks to better technology, it started from a much larger sample of stars.

Of course, even on the newer, slimmer estimate, our little corner of the universe is mind-bogglingly big. To try to get a sense for just how big it is, let's survey the Milky Way by some other astronomical numbers.

The Milky Way's Astronomical Numbers

100,000 - Diameter of the Milky Way, in light-years. It's also around 1,000 light-years thick, and that's just the main stellar disk. One light-year is a little less than 6 trillion miles (10 trillion km), so we're talking about a very big disk. In fact, if our solar system was just the size of a coin in your pocket, the Milky Way would still be the size of the United States.

26,000 - Distance from our solar system to the Milky Way's center, in light-years. Astronomers think our solar system completes an orbit around the galaxy's center once every 225 to 250 million years. That means it was just one "galactic year" ago when the dinosaurs began to appear on Earth.

2.6 million - Low-end estimate of the number of suns it would take to equal the mass of the supermassive black hole that lurks at our galaxy's center. The sun is roughly 333,000 times more massive than Earth. So, it would take at least 858 billion Earths to equal that black hole's mass. Of course, no one can see the black hole. But astronomers theorize that such massive monsters lurk at the hearts of most galaxies.

200-400 billion - Number of stars in the Milky Way. That number is staggering enough. But consider this. Most astronomers say there are at least 100 billion other galaxies out beyond ours. Big as it is, our little corner of the universe really is just that. The Milky Way is but a drop in the bucket.

--Steve Sampson