The stars are not alone. In the disk of our Milky Way Galaxy about 10 percent of visible matter is in the form of gas,
called the interstellar medium (ISM). The ISM is not uniform, and shows patchiness even near our Sun. It can be quite
difficult to detect the local ISM because it is so tenuous and emits so little light. This mostly hydrogen gas, however,
absorbs some very specific colors that can be detected in the light of the nearest stars.