The beauty of the stars have always held a fascination for humankind. Before the glare from urban lights dimmed the view, people were regularly afforded a clear glimpse of the galaxy. As the tools became available to further investigate and explore what was seen in the night sky, scientists began to learn more about the galaxy and what lies beyond it. Even today the vastness of space, including the Milky Way, still holds many mysteries. Although space travel is a reality and astronomers better understand what they see, they are still making new discoveries about the stars and galaxies beyond the Milky Way. For people who are interested in learning about the galaxy and the universe, it is best to start with the basics.
What is a Galaxy?
Throughout the universe there are systems in which gravity holds together anywhere from millions to billions of stars, plus gas, dust, and dark matter. These systems are what are known as galaxies and researchers believe that there are over one hundred billion of them in the universe. The name "galaxy" comes from the word galactos, which is the Greek word for milk or milky. The galaxy that houses Earth is called the Milky Way. It was given this name because of the milky appearance of the band of light in the sky.
The first person to determine that the Milky Way was a collection of countless stars was Galileo Galilei who used his first telescope in 1610 to view the night sky. In 1785 the astronomer William Herschel used his telescope to map the Milky Way. Much later, in the 1920s, Edwin Hubble, an astronomer, discovered that the Milky Way was not the only galaxy in the universe when he used the Hooker telescope in Mount Wilson, California to take pictures of what is now known as the Andromeda galaxy. Up until the point many believed that the Milky Way was not just a galaxy but the universe.
Types of Galaxies
Not all galaxies look or are shaped alike. There are three general types or categories of galaxies in the universe - spiral, elliptical, and irregular. The galaxy's bulge and disc help determine the type. A bulge in a galaxy has a round shape and consists of old stars that are tightly packed. The flattened area around the bulge contains younger stars, dust, and gas, and is referred to as the disk. An elliptical galaxy is a galaxy that does not have a disc, and is primarily bulges. They vary in shape from elongated to spherical. Spiral galaxies are the most common type of galaxy and are some of the most visually appealing. This type of galaxy has a central bulge that is surrounded by a disk. The disk of the spiral galaxy forms spiral shaped arms. The Milky Way is a spiral galaxy. The final category of galaxies are irregulars. These galaxies cannot be defined as having any defined shape or structure. They are made primarily of gas and dust. Dwarf galaxies are another type of galaxy that is small in comparison to other galaxies in terms of the number of stars. They may be found orbiting around larger galaxies.
What are Stars?
The universe is filled with stars. In the Milky Way alone there are hundreds of billions of stars. To the casual observer, stars appear to be little more than twinkling lights. In reality they are gas balls that consist primarily of helium and hydrogen. The Earth's sun is an example of a star. The color of a star is representative of its heat. Stars that are blue are the hottest while red stars are the coolest.
Formation and Evolution of our Galaxy
Astronomers do not fully understand how the Milky Way was formed. It is believed that the Milky Way's planetary system, as well as the sun, formed an estimated 4.5 billion years in the past. Prior to that, some 11 to 12 billion years ago, astronomers believe that a number of protogalactic clouds, globular clusters and individual stars merged into the protogalactic cloud that is the Milky Way. A protogalactic cloud is a glob of hydrogen and helium gas. As the cloud collapsed it flattened and began to form the galaxy's disk and it defined the direction of its rotation. As this occurred halo stars also began to form. As the galaxy aged, stars continued to form in the disk. It is believed that in the future, several billion years from now, a larger galaxy will be formed by the collision of the Milky Way and Andromeda.