Explanation:
Every element when heated, will show an emission spectrum. If it is surrounded by a colder element, the observer will see an absorption spectrum. The stars show us an absorption spectrum as the hotter core is surrounded by relatively cooler outer layers.
Here, to find which element is present inside which star, we just need to match the spectrum of elements with that of the stars. The missing lines in the spectrum of stars are actually that of the element present in the stars.
For star 1, the missing lines are roughly at the wavelength of 450, 500, 590, 670 nm. It implies that Helium is present in the star as its wavelength matches with that of the star 1.
Similarly, for star 2, the missing lines are roughly at the wavelength of 425, 560, 640 and 670 nm. It implies that Sulfur is present in this star as its wavelength matches with that of the the star 2.
Similarly, for star 3, the missing lines are roughly at the wavelength of 430, 510, 600 and 680 nm. It implies that Carbon is present in this star as its wavelength matches with that of the the star 3.
