Space Week Day 7: The Milky Way

If you live in or near or have access to a dark place at night, preferably when the moon is new or below the horizon, you may be able to see twin bands of stars surrounding a band of relative darkness. The band of darkness is the galactic plane, where most of our galaxy’s dust, debris and non-stellar gas is found, and the twin bands of light are stars that are slightly outside of the galactic plane. The estimated diameter of the Milky Way Galaxy is about 100,000 light-years. For scale comparison, if the galaxy and everything in it were scaled down to be one kilometer across, the solar system including the hypothetical Oort comet cloud, would be less than a centimeter across. The Milky Way Galaxy is made of two main arms, two minor arms and at least two smaller spurs. The two main arms are the Scutum-Centaurus and Perseus arms, the two minor arms are the Carina-Sagittarius and the Norma-Cygnus/Outer arms. The Solar System resides in a smaller spur between the Carina-Sagittarius and Perseus arms called the Orion spur. The entire Milky Way galaxy moves around a radio emitting object at the center of the galaxy called Sagittarius A* (the * is pronounced “star”), the current explanation for the object is that Sagittarius A* is a supermassive black hole containing more than 4 million solar masses.

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