| Describe the development of the present system of stellar classification, from the 1860s to the 1940s. | [5] |
| Explain why most astronomers in the 19th century believed that all nebulae were small systems within the Milky Way. Discuss the extent to which this belief was influenced by the classification of nebulae. | [3] |
| What evidence convinced early 20th century astronomers that some nebulae were not small systems within the Milky Way? Why did this information become available at that time? Would it have been possible to deduce this earlier? | [2] |
| [2007 Q4.] |