Galaxies are gravitationally bound systems of stars, dust, gas, stellar relics, and mysterious non-atomic dark matter. Galaxies come in different sizes, and can range from small dwarfs hosting only a few hundred million stars to giants that contain one hundred trillion stellar constituents, each in orbit around its galaxy’s center of mass.
In astronomical literature, the capitalized word “Galaxy” usually refers to our own Milky Way, thus distinguishing it from other galaxies. The English term Milky Way has been traced back to a story written by the English poet Geoffrey Chaucer (1340?-1400) in 1380:
“See yonder, Lo, the Galaxye
Which men clepeth the Milky Way,
For hit is whyt.”
Geoffrey Chaucer, The House of Fame
Galaxies are categorized according to their observed morphology, and are generally designated as spiral, elliptical and irregular–although the situation is somewhat more complex. Many, if not all, large galaxies are thought to host a supermassive black hole in their active hearts. Our own Milky Way’s supermassive heart of darkness is dubbed Sagittarius A* (pronounced Sagittarius-A-Star) –or Sgr A*, for short. It has a mass of about four million Suns, and it is dormant, only becoming active now and then when a buffet consisting of a star or cloud of gas floats too close to the gravitational snatching claws of this “frumious Bandersnatch” residing in the heart of our Galaxy.
Estimates of the number of galaxies that inhabit the observable Universe range from 200 billion to a trillion–or more. It has also been determined that there are more stars in the observable Universe than all of the grains of sand on Earth. Most galaxies are about 3000 to 300,000 light-years in diameter and are separated by distances on the order of megaparsecs (millions of parsecs). By comparison, our own Milky Way sports a diameter of at least 100,000 light-years and is separated from the Andromeda galaxy, its nearest large spiral neighbor, by 2.5 million light-years.
The space between galaxies is not empty–although it is close to it. Instead, intergalactic space is filled with a tenuous gas that is called the intergalactic medium, and it has an average density of less than one atom per cubic meter. Most galaxies are gravitationally organized into groups, clusters, and superclusters. Our own Milky Way is a member of the Local Group, which is dominated by it and the Andromeda galaxy, and is a part of the Virgo Supercluster. On the largest scale, these galactic associations are arranged into sheets and filaments surrounded by black, and almost empty, voids. The largest known structure of galaxies is a cluster of superclusters named Laniakea, which hosts the Virgo Supercluster. On this scale, the Universe resembles a honeycomb or a natural sponge, with filaments and voids intricately intertwined. Indeed, some astronomers think that the large-scale structure of the Universe is composed of only one filament wrapped around a single void.
Before the 20th century, astronomers generally thought that our Milky Way was the only galaxy in the Cosmos, and the existence of other galaxies was not well established. Indeed, the idea that other galaxies danced throughout the Universe was so controversial at that earlier era that it led to what is now called the Shapley-Curtis Great Debate, named after the two American astronomers Harlow Shapley (1885-1972) and Heber Doust Curtis (1872-1942). During this debate, the two scientists articulated their opposing views on the identity of “nebulae” and the size of our Galaxy. The debate was held at the National Academy of Sciences in Washington D.C. on April 26, 1920. Shapley contended that the Milky Way was the entire Universe, and that all of the observed nebulae (clouds)–which are now recognized as being galaxies in their own right–resided within the Milky Way. In dramatic contrast, Curtis argued correctly that the Milky Way was smaller than the entire Universe, and that the observed nebulae were really other galaxies similar to our own.
At last, in late 1923, the astronomer Edwin Hubble–referred to as the “father of modern observational astronomy”–measured the distance to the Andromeda galaxy (M31) using a type of variable star called Cepheid Variables to make his measurements. By measuring the period of these variable stars, Hubble was able to calculate their intrinsic luminosity and upon combining this with their measured apparent magnitude he arrived at a distance of 300 kiloparsecs, which is an order of magnitude greater than the estimated size of the Universe made by Shapley. This measurement verified that not only was the Universe much, much bigger than previously proposed, it also revealed that the observed nebulae were really distant galaxies with a wide variety of morphologies.
Despite Hubble’s discovery that the Universe played host to myriad galaxies, most satellite galaxies of our Milky Way and the entire Local Group remained undiscovered, and undiscoverable, until the advent of modern astronomical surveys–although the duo of small satellite galaxies, the Large and Small Magellanic Clouds, have been observable in the Southern Hemisphere with the unaided eye since ancient times. 720p mkv movie download