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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If Earth were to be replaced by an Earth-mass black hole, almost nothing in the universe would change. It wouldn’t start sucking things up, there would be no major gravitational disturbances, nothing. All that would happen is that anything that passed through a region the size of a peanut would be absorbed into the black hole, and anything that passed nearby would be distorted. All objects have an event horizon (the surface of a black hole), but most objects are far larger than their event horizon (a black hole with Earth’s mass is about the size of a peanut). Black holes also have something called a photon sphere, where photons (light waves/particles) orbit the black hole. Finally, if you were to try and fall into a black hole (for science of course) you would experience a process known as spaghettification. Since, as you approach the black hole, the parts of your body closest to the black hole are being pulled harder by gravity than those far away, you will be stretched, slowly are first but then more quickly, until all of your body is within the event horizon. What happens then? Nobody knows. The current mathematical models used in physics cannot describe what happens within the event horizon of the black hole.