History Of the Observation of Galaxies

Galaxies were really only discovered recently in the history of astronomy. Up until the 1500s, the universe was thought to only compose a small sphere centered on the Earth, with all the stars on a fixed background at the outer edge of the sphere. With the dawning of the age of telescopes, things began (forgive the pun) to be seen more clearly.


Staring with Galileo, when he turned his telescope to the fuzzy patch of light that crossed the entire night sky, he noticed that the patch actually composed thousands of stars. He was able to conclude correctly that the Milky Way was actually part a large band of stars. The significance of his observation was not really known at the time.


Immanuel Kant was a German philosopher who also studied astronomy. He deduced that the Solar System was formed by the collapse of a rotating cloud (nebula). He also correctly concluded, using the knowledge that the Milky Way was actually a band of stars, that the Milky Way was a disk of stars. He also theorized that there could be many of the disk of stars which he called island universes.


The name "galaxy" comes from a Greek word, galaxias, which means "milky one", which is derived from the name of our galaxy, the Milky Way. Ancient astronomers though that the band of the Milky Way looked like milk that had been spilt across the sky, and the Greeks believed that it came from the breast of a sleeping Hera as she was nursing Heracles. When she woke up to discovered Heracles there, she pushed the baby away and what we see is the breast milk as it was sprayed into the night sky.


The first true catalogue of galaxies was not developed until the 1700s when Charles Messier started cataloguing objects in the night sky that were not stars or planets as he was searching for comets. In his catalogue, he discovered open and globular clusters, nebulae, and galaxies that were visible from the northern hemisphere (Messier was a French astronomer), and named them according to their order of discovery by him. A famous example of a galaxy that Messier catalogued was M31, which is commonly known as the Andromeda galaxy.


William Herschel and his son John also catalogued nebulae, clusters, and galaxies in the General Catalog (renamed New General Catalogue after updated by John Louis Emil Dreyer). The Andromeda Galaxy is NGC 224, so some objects in both the Messier Catalogue and the NGC overlap. The NGC does contain many more southern hemisphere objects than the Messier Catalogue.

Edwin Hubble took things even further when he began observing galaxies at Mount Wilson. He was provided significant evidence that these fuzzy patches of light in the Messier Catalogue and NGC were actually galaxies outside the Milky Way. Vesto Slipher was actually the first astronomer to provide evidence for extragalactic galaxies.

Hubble was able to differentiate between elliptical, spiral, and irregular galaxies, and in fact, thought that there might be an evolutionary sequence between the three types. He proposed something call the tuning fork diagram which shows what he believed to show the sequence. However, we now know that the sequence is not evolutionary, but is a good image to see the differences in galactic types.

Another thing Hubble used galaxies for was to determine the age of the Universe. He noticed that galaxies farther from the Milky Way seemed to be moving faster (recessional velocity). He knew the distances based on a standard candle, in this case Cepheid variable stars, and by using the Doppler shift in the spectrum, was able to formulate a simple equation to determine the age of the Universe:

v=H*d

• v is the recessional velocity in km/s
• d is the distance in Mpc = megaparsecs (a million parsecs)
• H is the Hubble constant in (km/s)/Mpc which today is approximately 67.80

The inverse of H gives the age of the Universe and converting everything, the age of the universe is approximately 13.8 billion years (give or take a couple of billion). This is also called the Hubble Time.