Galaxy

Astronomers did not recognize galaxies as separate from the Milky Way until the early part of the twentieth century. The Andromeda Galaxy, which is the nearest spiral galaxy to the Milky Way and the Large and Small Magellanic Clouds, which are the nearest irregular galaxies to the Milky Way, are visible to the naked eye, and have therefore been observed since antiquity. Their nature was, however, unknown.

With the development of the telescope, astronomers were able to discern the whorled shape of spiral galaxies, which were called spiral nebulae at the time. Until the 1920s, there was a controversy: Were these "spiral nebulae" part of our Milky Way galaxy, or were they external galaxies similar to our Milky Way? In April 1920, there was a debate on this topic between Harlow Shapley and Heber D. Curtis before the National Academy of Sciences. Curtis argued that spiral nebulae were external galaxies, Shapley that they were part of the Milky Way. Curtis did not win the debate, but astronomy has since proven him right—"spiral nebulae" are external galaxies similar to the Milky Way.

To settle the controvery, scientists needed an accurate method to gauge the distance to galaxies. Working at Harvard College Observatory in the early twentieth century, the American astronomer Henrietta Leavitt (1868-1921) found the required celestial yardstick. Leavitt was studying a type of star in the Magellanic Clouds known as a Cepheid variable, when she discovered a way to measure the distance to any Cepheid variable by comparing the star's apparent and absolute magnitudes. The distance to the variable star gave the distance to the galaxy or cluster of stars containing the Cepheid variable. Cepheid variables have since become a fundamental yardstick for measuring the distance scale of the universe.

In 1924, the American astronomer Edwin Hubble (1889-1953) used Leavitt's Cepheid variable technique to measure the distance to the Andromeda galaxy. Hubble's original distance estimates have since been refined; the modern distance to the Andromeda galaxy is about 2.2 million light years. (A light year is the distance light can travel in one year, about 6 trillion mi, or 9.654 trillion km). The Milky Way galaxy is however only a little over 100,000 light years in diameter. Hubble therefore conclusively proved that the Andromeda galaxy must be outside the Milky Way. Other galaxies are more distant.

With his work, Hubble launched the science of extragalactic astronomy—the study of galaxies outside the Milky Way. Hubble devised the classification scheme for galaxies that astronomers still use today. More importantly, Hubble found that more distant galaxies are moving away from us at a faster rate. From this observation, known as Hubble's law, he deduced that the universe is expanding. Hubble used his study of galaxies to uncover a fundamental fact about the nature of the universe. Fittingly, one of the scientific goals of the Hubble's namesake, the Hubble Space Telescope, is to continue this work.