A Brief Overview of the Motions of Galaxies Within Our Universe
A galaxy is a huge collection of a few hundred million to over a trillion stars and other galactic matter bound by gravity. In the early 1800’s through the 1900’s, galaxies were undefined. They were seen as fuzzy patches of light in the sky, classified as nebulae. In 1917 Harlow Shapley suggested that the nebulae were actually distant star systems that were outside of our own star system (Marvel 153-154). Further research and advanced technology confirmed Shapley’s discovery and these galaxies are classified into three different types: Spiral, elliptical, and irregular. Our solar system is located in the Milky Way Galaxy, which is a spiral galaxy. Each type of galaxy has a special type of motion within itself.
Spiral galaxies make up about 77% of all the galaxies in the universe. They are large disks of stars. Inside the disk of stars is a smaller disk of gas and dust where many of the new stars are formed (Marvel 156). In the inner regions of the disk, objects move in solid-body rotation. This type of rotation means that the angular velocity is constant with the distance from the center of rotation. This kind of rotation can be seen if you were to twirl a baton. The outer parts of the baton rotate at the same angular speed as the inner. In reality, the outer portion has to move faster than the inner to keep the same angular velocity. After a certain distance, differential rotation begins within a spiral galaxy.
In this type of motion, objects further away from the center move at a slower rate and angular velocity than objects closer to the center. The spiral pattern that we can see is due to a spiral density wave. This wave is similar to sound wave in the air. The cause of the wave is unknown, but believed to be related to differential rotation in the galaxy (Marvel 156-157). As the stars orbit the galaxy, they also make a up and down bobbing motion within the disk. This motion is caused by gravity. Whenever a star is too far above the disk, gravity pulls it back down. Because the density of the interstellar gas is not high enough to stop the star in the disk, the star moves below the disk. When the star moves too far below the disk, the gravity pulls it back up (Bennett 612). Spiral galaxies have the most uniform motion of all other types of galaxies.
About 20% of all galaxies are elliptical. They are football shape formations of stars with very little gas and dust. There is very little evidence of star formation in these galaxies. Some ellipticals are formed by two or more galaxies merging together (Marvel 158). The stars within an elliptical galaxy have more random orbits than the stars within a spiral galaxy. The last 3% of galaxies in our universe are irregular galaxies. They have a decent amount of gas and dust, and active star formation. Irregular galaxies are subdivided into peculiar and normal. Normal irregulars are more symmetrical in shape. Peculiars just look odd. Most peculiars are a result of gravitational interaction with another galaxy. There has been a few recorded that is the remains of a spiral arm stuck through and elliptical.
There have been galaxies recorded with super massive black holes in the center. Some of these galaxies emit large amounts of light, x-rays, or radio waves. The black holes that emit this are feeding. When feeding, the black hole swirls gas, dust, and stars around them at high speeds. This motion causes intense heating of gas and leads to the strong x-rays and optical light. These galaxies are called active galaxies (Marvel 159).
Galaxies also interact with other galaxies. Galaxies are closer together relative to their size than stars are. This means that the chance of a galaxy colliding with another is much greater than two stars colliding. When galaxies collide, little damage is done, stars usually do not run into each other. When they collide, gas clouds and dust clouds interact and the enhanced gravity pulls the gas and dust to a higher density and forms new stars. During an extreme interaction, a tail of material can be thrown into space from the two galaxies. There has even been a few cases where an elliptical galaxy goes straight through the center of a spiral galaxy, forming an expanding ring of star formation spreading out from the impact point (Marvel 158-160). A collision between galaxies happens over a period of hundreds of millions of years. Two spiral galaxies colliding can also form an elliptical galaxy (Bennett 658). Collisions between galaxies are truly a spectacular event!
Galaxies also exist in clusters. When galaxies form, they tend to form in groups. Within the groups, galaxies seem to be orbiting each other. Galaxy clusters are also moving towards or away from other galaxy clusters. The cluster that the Milky Way exits within is called the Local Group. Our local group seems to be headed towards the next closest cluster called the Virgo Cluster. The Virgo Cluster along with our Local Group is also moving towards a super cluster called the Great Attractor (Marvel 159-160). The clusters are moving outward and expanding our universe!
Edwin Hubble began studying galaxies and found that the light from them were Doppler shifted. He also discovered that the further away it was, the more it was shifted. Hubble announced that the distance of a galaxy from Earth was directly related to the speed at which it is moving away from us. This is defined as Hubble’s law. This law leads us to the fact that the universe is expanding (Marvel 161). The further away a galaxy is from Earth, the faster it is moving away from Earth. For example, galaxies that are 100 million light years away are moving at about 5.5 million miles per hour. Galaxies that are 200 million light years away are moving twice as fast, at 11 million miles per hour. Galaxies that are 300 million light years away are moving three times as fast, at 16.5 million miles per hour. This trend continues the further away you look from Earth (Greene 229). Before this discovery, Einstein had included a constant in his theory of relativity to keep the universe from expanding. When Hubble discovered this, Einstein called the inclusion of his constant his greatest mistake (Marvel 161).
By observing our galaxies and their movements, scientists have discovered something very unusual. For example, in a galactic disk, the actual speed of rotation stays constant with distance from the center. Due to differential motion, we expect that at a greater distance from the center, the actual speed should slow down. In reality, the speed does not decrease. This means that there must be more matter there that we cannot detect. Since this matter is not visible, it is called Dark matter (Marvel 156).
By observing the way things move within our galaxies as well as how entire galaxies move within our universe, we can learn more about the universe as a whole. Not only do the different galactic motions teach us, the also are used to confirm certain theories. We can see that galaxies are constantly evolving and affecting other galaxies. We can also look at their paths to help us determine the fate of our universe. Galaxies and their movements are a vital link to the chain of our universe. Could our universe and it’s motion be another link to an even greater chain? Only time will tell!
