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The galaxy with the dual explosions is a run-of-the-mill cluster of stars, not too close and not too far from the Milky Way in cosmic terms, Ms. Soderberg said. The galaxy, NGC2770, is about 100 million light-years away. One light-year is 5.88 trillion miles.

The star that exploded was only about 10 million years old. It was the same size in diameter as the sun, but about 10 to 20 times more dense.

The death of this star went through stages, with the core getting heavier in successive nuclear reactions and atomic particles being shed out toward the cosmos, Mr. Filippenko said. It started out in its normal life with hydrogen being converted to helium, which is what is happening in our sun. The helium then converts to oxygen and carbon, and into heavier and heavier elements until it turns into iron.

That is when the star core becomes so heavy that it collapses in on itself, and the supernova starts with a shock wave of particles piercing through the shell of the star, which is what the Soderberg team captured on X-rays.

People at home can simulate how this shock wave works, Mr. Filippenko said.

Take a basketball and a tennis ball, and approximately 5 feet above the ground, rest the tennis ball atop the basketball. Drop them together and the tennis ball will soar on the bounce. The basketball is the collapsing core and the tennis ball is the shock wave that was seen by astronomers, he said.