On a certain planet, an astronomer A observes the explosion of a star located 4 light years away. of his planet. Ten years later, he observes the explosion of another star located a distance away. distance of 10 light years in the opposite direction (see picture). Suppose an alien B is traveling in his spaceship with speed v in the direction parallel to the line joining the stars, and passes the planet at the instant light from star 1 reaches the planet.

a) What must the speed of the spacecraft be for the alien to observe the explosions simultaneously?

b) For this speed, what is the distance between the stars, measured by the alien, in light years?

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