Cepheid variable stars
The next crucial step on the distance ladder, still of prime importance today, was the work by Henrietta Leavitt on Cepheid variable stars. In 1912, while working at the Harvard Observatory, Leavitt discovered that these stars vary in luminosity in regular cycles, with the period of variation increasing with luminosity. This means that measuring the duration of the period of a Cepheid allows astronomers to deduce its luminosity. By comparing this with the observed brightness, the distance to the star can be calculated.
In 1924 Edwin Hubble used Leavitt’s discovery to estimate the distance of Messier 31, the Andromeda Nebula (now known as the Andromeda Galaxy). It clearly lay far outside our own Milky Way Galaxy, thus resolving the long-standing controversy about spiral nebulae and opening up the universe of galaxies.
Hubble classified galaxies as spiral, elliptical or irregular (see figure). Modern spectroscopic studies showed that many galaxies, especially the spirals, are rotating systems, but the orbital speeds of the stars were found to be too large for the amount of visible matter they contain. Some other form of dark matter must be attracting the stars gravitationally.
Dark matter is also implied by the velocities of galaxies seen in clusters of galaxies. The final line of evidence for dark matter is provided by an effect known as gravitational lensing. When we see a distant background galaxy behind a foreground galaxy or cluster of galaxies, the gravitational bending of light predicted by Albert Einstein’s General Theory of Relativity distorts the image of the galaxy, acting like a lens. This lensing effect allows us to map the dark matter in clusters of galaxies.