What comes to your mind when you hear "green monster"? Hulk? Shrek? Probably. But definitely not a young supernova in the Milky Way galaxy, yet, it's real. Cassiopeia A (aka Cas A) is the remnants of a stellar explosion astronomers spotted in the sky 340 years ago. The image is full of bright colors – brilliant green, orange, and pink. But is it dangerous to us on Earth?

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