Astronomers recently detected light emitted by a supernova that was originally observed in 1572, just reaching earth now. this light was reflected off a dust cloud; astronomers call such a reflected light a “light echo” (just like reflected sound is called an echo). how would you expect the spectrum of the light echo to compare to that of the original supernova?

At 20∘c, the hole in an aluminum ring is 2.800 cm in diameter. you need to slip this ring over a steel shaft that has a room-temperature diameter of 2.804 cm . 1. to what common temperature should the ring and the shaft be heated so that the ring will just fit onto the shaft? coefficients of linear thermal expansion of steel and aluminum are 12×10−6 k−1 and23×10−6 k−1 respectively.