NOTES ON THE DISCOVERY OF GALAXIES

1750

Thomas Wright published "An Original Theory of the Universe" in which he explained the Milky Way by proposing that the stars were distributed in a thick disk -- the first step in the discovery that the Milky Way is a galaxy.

1755

The philosopher Immanuel Kant, inspired by Wright's ideas, proposed that the Milky Way was only one of many "Island Universes", rotating according to Newton's Laws, and scattered throughout an infinite universe.

1781

Charles Messier published his Messier Catalogue of 110 "nebulae", of which 40 are galaxies. Thus, the brightest and biggest galaxies are often labeled by their number in the Messier Catalogue - e.g., M31, the Andromeda Galaxy.

1800

William Herschel published his Comprehensive Catalogue of Images of 2500 deep sky objects.

1864

John Herschel (son of William Herschel) published the General Catalogue of Nebulae and Clusters of Stars, which evolved into the New General Catalogue of more than 10,000 galaxies. Therefore, you will often see a galaxy labeled by its number in the New General Catalogue. For example, M31 is also called NGC224.

1912

Henrietta Leavitt, one of several women working at the Harvard College Observatory, while investigating variable stars in the Small and Large Magellanic Clouds, two small galaxy companions of the Milky Way, discovered the Period-Luminosity relation for Cepheid variable stars. The relation provided the first reliable method for measuring distances to nearby galaxies, and it remains today one of the most precise methods.

1912

American astronomer Vesto Slipher, working at the Lowell Observatory in Flagstaff, Arizona, began to measure the Doppler shifts of spectral lines from spiral galaxies. He found that the majority of galaxies were moving away from the Milky Way (with two exceptions, the Andromeda galaxy M31 and the Triangulum galaxy M33, now recognized to be part of the Local Group). Some galaxies were moving away at more than 1,000 km/s, an unprecedently high velocity.

1920

Harlow Shapley and Heber Curtis held a famous debate about the nature of the Milky Way, and of the "spiral nebulae". Curtis held to the traditional view that the Solar System is at the center of the Milky Way, whereas Shapley argued that the Solar System lies some 15,000 parsecs from the center, located in Sagittarius. On the other hand, Shapley argued that the spiral nebulae, such as the Whirlpool galaxy M51, were gas clouds within the confines of the Milky Way, whereas Curtis advocated the view that they were "Island Universes" well outside the Milky Way. The debate did not resolve the controversy. It would turn out that in both cases the more radical idea was correct: the Milky Way is centered in Sagittarius, about 8,000 parsecs from the Solar System (Shapley was right, but he overestimated the distance); and the spiral nebulae are Island Universes, galaxies in their own right.

1923

Working at at the great new 2.4-m telescope at Mt. Wilson, California, Edwin Hubble discovered a Cepheid variable in the Andromeda galaxy M31, and used it to make the first reliable measurement of the distance to Andromeda. This was the start of a systematic survey to measure distances and Doppler shifts of spiral galaxies. Hubble's discovery was definitive: as of 1924, all astronomers knew that the spiral nebulae were galaxies, comparable in size to our own Galaxy, the Milky Way.

1929

Hubble published his compilation of distances and recession velocities of galaxies, the first "Hubble Diagram" as astronomers now call it. The Hubble diagram showed that the expansion of the universe obeyed the equation V = H₀D, now known as "Hubble's Law". Hubble underestimated the distances of the galaxies, because of problems in calibrating the Period-Luminosity relation, and thus derived a value of the "Hubble Constant", H₀ = 500 km/s/Mpc, that is about seven times the modern value.

1995

Thanks to the Hubble Space Telescope, we can now see galaxies so distant that their light has been traveling to us for more than 10 billion years -- 80% of the time since the universe was formed. You can see some of these galaxies in the Hubble Deep Field.

2000

Wendy Freedman, leading the Hubble Space Telescope Key Project to measure the Hubble constant, announces the team's final conclusion for its value: H₀ = 72 ± 8 km/s/Mpc.

2003

A concordance of measurements, including observations of the Cosmic Microwave Background by the Wilkinson Microwave Anistropy Probe, yield a Hubble constant of H₀ = 71 ± 4 km/s/Mpc.