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According to Live Science (This article and its images were originally posted on Live Science September 4, 2018 at 07:28AM.)
Get away from city lights, look up, and you’ll see one of the wonders of the night sky — the bright cloudy bands of stars that make up the Milky Way galaxy. Our galactic home is vast and beautiful, but many mysteries about it remain.
For instance, despite decades of attempts, astronomers have yet to determine exactly how much our galaxy weighs, with estimates ranging from anywhere between 700 billion and 2 trillion times the mass of our sun.
Weighing a galaxy, especially while you’re living in it, is no easy task. “It’s like trying to take a census of the U.S. population but you can’t use the internet and you can’t leave the city you live in,” astronomer Ekta Patel of the University of Arizona in Tucson told Live Science. Nobody can stick the Milky Way on a cosmic scale and just read out the result. [Stunning Photos of Our Milky Way Galaxy]
Part of the problem is that most of a galaxy’s mass is invisible. Dark matter, an enigmatic substance that gives off no light of any kind, makes up around 85 percent of our Milky Way, Patel said. So simply counting the stars in our galaxy won’t get you very far.
Therefore, researchers usually look at the orbit of some celestial object, Patel said. The method is based on the equations of gravity derived by Isaac Newton more than 300 years ago, which tell you that the speed and distance at which a smaller body rotates around a larger one is related to the mass of the larger object.
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