Mapping the universePrint
In 1920, two leading astronomers - Heber Curtis and Harlow Shapley - disagreed on whether there was one galaxy [galaxy: A cluster of billions of stars, held together by gravity.] or many galaxies in the universe [universe: All the energy and matter that exists.] .
Observations with telescopes had identified that the Sun was one of many stars in our galaxy (the Milky Way). Astronomers had also seen lots of fuzzy objects in the sky at night. Astronomers called these objects nebulae.
Curtis argued that there were many galaxies in the universe and that these nebulae were distant galaxies. Shapley believed that the universe contained only one galaxy and that the nebulae were gas clouds within the Milky Way.
A few years later, Edwin Hubble recorded observations of Cepheid variable stars [Cepheid variable star: A very bright star with a strong relationship between its brightness and how frequently it pulses.] in one nebula and realised that they were much further away than any star in the Milky Way. This evidence supported Curtis’s proposal that there were many galaxies in the universe.
Observations of other Cepheid variable stars have clearly shown that most nebulae are distant galaxies. By measuring the distance to these galaxies, astronomers have been able to estimate the scale of the universe.