If you have ever looked up at a moonless night sky, you have seen a galaxy. The stars you can see are all part of the Milky Way galaxy. The Milky Way gets its name because its central core looks like a milky stream over a dark sky. Earth is in the outer part of this galaxy, which is made up of at least 100 billion stars. (https://imagine.gsfc.nasa.gov/science/objects/milkyway1.html) I have always been fascinated by space, but have never been able to truly comprehend how big it is. Astronomers estimate that the observable universe has one hundred billion galaxies. The most distant galaxy found by the Hubble telescope is 30 billion light years away from earth. This does not seem possible, because the whole universe is just 13.8 billion years old. However, the astronomers are seeing the galaxy the way it was 13.4 billion years ago, and it is in fact 30 billion light years away from earth, due to the rapid expansion of the universe. (https://www.nasa.gov/feature/goddard/2016/hubble-team-breaks-cosmic-distance-record) Another way to think about the enormity of a galaxy is to think that if our solar system were the size of a quarter, the sun would be just a speck of dust and our galaxy, the Milky Way would be about the size of North America. A galaxy is a giant collection of gas, dust and stars and their solar systems all held together by gravity. Most galaxies have a black hole in their center, which is usually about 1/1000th of the mass of the galaxy. Galaxies range from dwarfs, which are as “small” as ten million stars to giants, with a hundred trillion stars. (https://science.nasa.gov/astrophysics/focus-areas/what-are-galaxies) Democritus, an ancient Greek philosopher, who lived from 460 to 370 BC, was the first person to hypothesize that the Milky Way is made of stars. Galileo Galilei in 1610, with his telescope, was the first person to identify and show that the Milky Way was made of many stars. In the 1920s, astronomer Edwin Hubble figured out a new way to measure the distance between other galaxies based on how bright a star is. He was able to prove that there were other galaxies outside the Milky Way and that the universe was millions of times bigger than our galaxy.(https://science.howstuffworks.com/dictionary/astronomy-terms/galaxy2.htm) Because other galaxies are so incredibly far away, we do not see them as they are, we see them the way they were. For example, the Andromeda galaxy is the farthest object the human eye can see. Two million years ago, light left the Andromeda galaxy, this light is now just coming to Earth. We are seeing Andromeda as it was in prehistoric times. Two million years from now, people on Earth will see Andromeda as it is today. To put this distance into perspective, we are seeing the moon as it was 1.25 seconds ago and the sun as it was 8.33 minutes ago.There are four main types of galaxies, spiral, barred spiral, elliptical and irregular. Spiral galaxies are spinning pinwheel shaped galaxies and examples include the Milky Way and the Andromeda. The oldest stars are located in the galactic center. The long swirling arms are made up of young stars, pink nebulas, gas, and dust, which all rotate around the galactic center. This movement causes the spiral shape. The disc spins hundreds of kilometers per second. Barred spiral galaxies are a subtype of spiral galaxies. They have a central bar and two swirling arms of stars trail, like water from a spinning garden sprinkler. Elliptical galaxies are huge egg shaped galaxies. They are the oldest of the galaxies, some are over 12 billion years old. Elliptical galaxies contain mostly old stars, but they don’t contain much dust or interstellar matter, which means they cannot make many new stars. Giant elliptical galaxies are the universe’s largest known galaxies and can be up to two million light years in diameter. There are also small elliptical galaxies which are called dwarf galaxies. They contain only a few hundred or a few thousand stars. Dwarf galaxies frequently lose their stars to larger close by galaxies because of gravity. Galaxies with no identifiable shape are called irregular. They are typically small with young stars and bright gas clouds. The large Magellanic cloud is a great example. It is one of the closest galaxies to the Milky Way. It is about 1/20th the size of the Milky Way, and is trapped by the Milky Way’s gravity. At some point, the Milky Way’s gravity will pull the Magellanic cloud into itself, and the two galaxies will combine.There are at least one hundred billion galaxies in the observable universe. The Hubble Space telescope looked at a small patch of space for 12 days, and found 10,000 galaxies of all different types. Astronomers then used this number to estimate the number of galaxies in the whole universe. However, in 2016 scientists from Nottingham published data suggesting that there may be more than 2 trillion galaxies, more than 10 times what was previously thought. The Hubble Space telescope has been orbiting 600 km above the earth since 1993, and has given astronomers huge amounts of new information about the universe. The James Webb Space Telescope (JWST), which is one hundred times more powerful than the Hubble Space Telescope, will be launched into space in the spring of 2019. The JWST will use infrared light rather than visible light to produce images. This will allow astronomers to see through clouds of dust and other space debris. The hope is that JWST will help astronomers see how the universe began, and how galaxies are formed. Starting in 2022, the Large Synoptic Survey Telescope, a ground based telescope being built in Chile will spend ten years photographing the sky every night. In 2024, the European Extremely Large Telescope, another ground based telescope in Chile will begin surveying the sky, looking for anything and everything. As astronomers develop better ways to see deeper into the universe, the number of galaxies in the universe will continue to increase, and astronomer’s knowledge of space will continue to grow. Most astronomers believe that galaxies began forming shortly after the “big bang” that began the universe 10 to 20 billion years ago. In the seconds after the explosion, huge amounts of gases were released. As these masses of gas began to shrink under the pressure of gravity, the gas at the center became dense enough to form stars, thus forming the beginnings of galaxies. Some of the galaxies started spinning, forming a spiral or barred spiral galaxy. The spinning mass of gas compresses under the pressure of gravity and the first stars are formed in the center. As the cloud of gas shrink, it starts to spin faster. This sucks in more gas clouds to the surrounding swirling disk because of gravity, and stars start to form in the arms. At this point, there is no more gas left in the center to make stars, but the arms have lots of material to create new stars.