Four hundred years ago, in 1609, the Italian astronomer Galileo Galilei turned his homemade telescope skyward and began a series of astronomical observations that would redefine human horizons. Among his discoveries was the realization that the Milky Way is a horde of stars too numerous and faint to be resolved with the naked eye. Before that telescopic revelation, the nature of the Milky Way--the name given to the hazy band of light stretching across the sky--was not known.
Over a century later, the German philosopher Immanuel Kant postulated that the Milky Way was a vast, disk-shaped collection of stars. He further suggested that the faint, fuzzy celestial clouds seen through telescopes and known as “nebulas” were large, distant collections of stars similar to the Milky Way. He called these nebulas island universes.
Although proof of the distance and separate nature of these island universes would not be acquired until Edwin Hubble’s work in the 1920s, Kant was correct. We merely had to wait another 150-plus years for our horizons to be again redefined: to learn that the Milky Way, our home galaxy, was not home to many of the so-called nebulas. The Milky Way was not the whole enchilada. There were other galaxies, other island universes, out there.
When you look at the sky on a clear night from a dark location, at any given time you can see about 2500 stars with the naked eye. A telescope can reveal millions more. Regardless of whether you’re observing with the naked eye or the aided eye, every star you can see is in the Milky Way Galaxy. Essentially, you look out at the cosmos through the vast starfield of your home galaxy, your own island universe.
But you can also see beyond. In fact, there’s an island universe visible to the naked eye, if you observe from a dark site. Originally called the Great Andromeda Nebula, Hubble’s work confirmed its galactic nature, and it became known as the Andromeda Galaxy. At an estimated 2.5 million light years away, it is the most distant object that the average person can see naked-eye. It is the nearest spiral galaxy to the Milky Way, and it’s comparable to our home galaxy in size and mass. Looking at the Andromeda Galaxy is a bit like looking in the mirror, that is, on a galactic scale.
1) About an hour after your local sunset time, face south. If you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your right shoulder to the west, and you’ll be facing approximately south.
Looking south to the Great Square and the Chains of Andromeda
Star maps created with Your Sky
2) First, locate the Great Square asterism (star pattern) in Pegasus, high in the southern sky, on or near the meridian.
3) Next, find the Chains of Andromeda, the two strands of stars that arc upward and to the left of the star that marks the Square’s upper left corner. Now look above the middle star of the upper chain for a faint, fuzzy patch. A Persian astronomer of the 10th century called it the “little cloud,” an apt description.
If you can’t see it, you may need to try again at a darker site with less light pollution. Additionally, your sky transparency, or atmospheric clarity, can be negatively impacted by the presence of clouds, haze, dust, or humidity. This could impact your ability to see a faint object like the Andromeda Galaxy. If this is the case, you should try again when conditions are improved.
It’s worth a little extra effort for the thrill of seeing, with your own eyes, a world beyond your galactic neighborhood: an island universe where perhaps other curious skywatchers are turning their eyes toward you, and wondering.
Astronomy Essential: The universe has a dark side.
By studying gravity’s influence on the gas that exists between galaxies in galaxy clusters--gas that can only be seen in X-ray wavelengths of light--astronomers can determine how much matter there is in the cluster. Interestingly, these measurements show that there is far more mass present than can be accounted for by the ordinary matter in the cluster.
From these results, astronomers have deduced the presence of another type of matter that can’t be observed directly (at least not yet). They call this dark matter.
Discoveries in the 1990s that the expansion of the universe is speeding up and in 2003 that the universe is flat suggest to astronomers the presence of yet another substance in the universe, in the form of energy. They call this dark energy. Like dark matter, dark energy is not directly observable, but dark energy would account for the density needed to maintain both a flat universe and a universe whose expansion is accelerating.
Current estimates for the structure of the cosmos indicate that the ordinary matter with which we‘re familiar--matter made of atoms, such as trees, rocks, people, air, planets, and stars--constitutes less than 5% of the universe! The rest is mysterious, unseen dark matter and dark energy. Understanding their nature is one of the key areas of exploration in astronomy today.