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We've explored many disasters Earth could face: from asteroids and aliens, to the Sun's death. Yet the Universe is perilous, and a new threat emerges—a blazar. Typically found at the galaxy's center, blazars are black holes of immense mass. For context, the Milky Way's black hole is four million times the mass of the Sun, but pales in comparison to others.

This episode investigates the oldest blazar, visibility differences among blazars, the Doppler effect's role in detecting blazars, active galactic nuclei, and the impact of radiation and gamma rays on Earth. We also discuss the distance of Markarian 421 from our planet.

For transcript and sources, visit What If Show.

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What If is a web series exploring hypothetical worlds and possibilities. Produced by Underknown in Toronto: Underknown.