Lesson 4 — What Is a Black Hole?

Understanding the Complex: Black Holes — What Really Happens When Space Ends

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Anchor: Sagittarius A — the black hole at the center of the Milky Way.*

In 1974, two astronomers at Caltech — Robert Brown and Bruce Balick — detected an unusually bright radio source at the center of the galaxy. They called it Sagittarius A*. Nobody knew what it was. For the next three decades, the question hung open.

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