Before the invention of the telescope, the word nebula was applied to all astronomical objects without a definite shape.  As a result, many objects now called star clusters or galaxies were called nebulas.  In the middle of the 19^th century, astronomers still thought nebulas were distant planetary systems.  However, after the invention of the spectroscope, William Huggins demonstrated in 1860 that at least some nebulas are made of gasses.