Complex sentence: A model spectrum will usually be of a specific spectral type of star and may include other properties of the star, such as a surrounding nebula, presence or lack of an extended atmosphere or a circumstellar dust disc.
Split into simpler sentences: A model spectrum will usually be of a specific spectral type of star. It may include other properties of the star, such as a surrounding nebula, presence or lack of an extended atmosphere, or a circumstellar dust disc.