Among the commonly used [[luminosity]] classes are class Ia (highly luminous [[super giants]]), class III (giants) and class V ([main sequence](https://learn2.open.ac.uk/mod/glossary/showentry.php?eid=1147161&displayformat=dictionary "S284 Glossary: main sequence") [stars](https://learn2.open.ac.uk/mod/glossary/showentry.php?eid=1147181&displayformat=dictionary "S284 Glossary: stars")). Each luminosity class can be represented by a line on the [Hertzsprung–Russell diagram](https://learn2.open.ac.uk/mod/glossary/showentry.php?eid=1147176&displayformat=dictionary "S284 Glossary: Hertzsprung–Russell diagram").