The vacuum of space may be emptier than the best vacuum that we can make with current technology, but it is not completely empty; every cubic centimeter of the emptiest parts of space contains about one atom. this gas, which exists everywhere between the stars, is part of the interstellar medium (ism). based on what you have learned about light from previous chapters, how would we determine what this gas in the interstellar medium is made of?

We can use light absorption technique to determine which element this atom belongs to.

When light meets an atom it is absorbed. The particles of light (photons) are absorbed by electrons and electrons jump to a higher energy state. When electron jump to a lower energy state they emit photons. If we observe continuous spectrum of a light that comes from this atom we can see black stripes. These stripes are unique for each atom and represent these jumps that electron made. By comparing observed stripes with those that were found in a laboratory we can figure out which element this atom belongs to.