Formation of Galaxies - Mrs. Yu's Website



Earth Science Unit 7 Lesson 14 GALAXIESThe Sun in our solar system is very impressive. It is an enormous nuclear reactor, generating enough power every second to easily destroy any of the planets. As impressive as the Sun is, it is only one of the billions of stars in the Milky Way Galaxy. The Milky Way, in turn, is only one of an estimated 200 billion galaxies in the observable universe.OBJECTIVESIdentify the characteristics of a galaxy.Distinguish between spiral, irregular, and elliptical galaxies.Describe the development of galaxies over time.VOCABULARYdark mattera hypothesized form of matter that does not emit enough radiation to be detected directly but whose presence is inferred from its gravitational effects and may account for the unexpected velocities of stellar orbits in galaxieselliptical galaxya galaxy whose stars are symmetrically dispersed in a spherical or elongated shapegalaxya large-scale collection of stars, dust, and gas held together by gravityirregular galaxya galaxy formed by the collision of other galaxiesspiral galaxya galaxy with a central bulge, flattened disk, and curving armsGalaxy CharacteristicsThe Characteristics of GalaxiesA galaxy is stars, planets, interstellar gas and dust, and dark matter that are gravitationally connected. You can think of a galaxy as a collection of solar systems that revolve together around a gravitational center. The gravitational centers of many galaxies are super massive black holes that contain the masses of tens or hundreds of billions of stars. Just as planets orbit stars, stars orbit the centers of their galaxies.The number of stars in galaxies can range from as few as 10 million stars or as many as one trillion. This means that the largest galaxy has 10,000 times as many stars as the smallest galaxy. Our Milky Way galaxy is estimated to contain between 200 and 400 billion stars.[Messier 101 Galaxy] Galaxies are characterized by their size and shape. As with stars, galaxies can be described in terms of the mass they contain and the space they occupy. The Milky Way is estimated to contain the mass of about 580 billion Suns. There are many stars similar to ours that are found in the Milky Way; in fact, our sun is a rather ordinary star.The Milky Way is a large galaxy and most of the stars of the Milky Way are within a 100,000-light-year disk. Remember that a light year is the distance that light will travel in one earth year.Most of the nearby galaxies are only about one-tenth the size of the Milky Way. The smallest galaxies may be only about 5,000 light years across. However, there are galaxies that are more than 30 times the size of the Milky Way, and the largest galaxy may be three million light years across.It wasn't until the 1920s, when Edwin Hubble provided evidence that there were groups of stars far outside of our Milky Way, that astronomers accepted that there were other galaxies in the universe. One of the first things that Hubble noticed about the galaxies was that they had different shapes. Hubble began to classify galaxies into three basic shapes—spirals, ellipses, and irregular.Elliptical galaxiesElliptical galaxies range in shape from spherical to flattened ellipses. Although their shapes vary, all elliptical galaxies appear fairly symmetrical and uniform in their distribution of stars. The stars in elliptical galaxies move in all directions around the galaxy. Approximately 15% of galaxies are elliptical.Spiral galaxiesA spiral galaxy is a flat, rotating disk of stars, planets, gas, and dust with a bulge in its center. Spiral galaxies also have "arms" of stars that curve outward from the center like the arms of a pinwheel. New stars form in the arms, which make them brighter than the areas in the disk around them. Some spiral galaxies also have a "bar" of stars that cuts across the center of the galaxy. Hubble called these barred spiral galaxies. About half of all galaxies are spiral.The Milky Way is a barred spiral galaxy with two major spiral arms and four or five minor arms. Astronomers estimate that the Milky Way contains between 200 and 400 billion stars. The disk of the Milky Way is estimated to be about 100,000 light years in diameter and about 1,000 light years thick. This means it would take a photon of light 100,000 years to go from one side of the Milky Way to the other. It is very hard to imagine distances this great.Irregular galaxiesWhen Hubble began classifying galaxies, he found that some had some features of elliptical or spiral galaxies but the features were distorted. Other galaxies had no recognizable shape. They appeared to be random collections of stars, gas, and dust. Hubble named these irregular galaxies. Approximately 25% of galaxies are irregular.Astronomers now think that most irregular galaxies form from collisions between galaxies. When galaxies "collide," there may be few actual collisions between stars. Since galaxies are mostly empty space, the stars may largely just pass by one another. However, the gravitational attraction between the stars of the different galaxies will change their orbits, causing the shapes of the galaxies to change. The Hubble Space Telescope has photographed many examples of galaxies colliding.Astronomers have found that the Milky Way and Andromeda galaxies are approaching one another and, if they proceed on their present courses, will likely collide. If they do, they may merge and form a large elliptical galaxy. Again, if this occurs, other stars are unlikely to strike the Sun and Solar System. However, we don't know if interstellar dust or other particles would affect Earth as the galaxies merge. Fortunately, we don't have to be concerned now because a collision should not occur for at least 3 billion years.Formation of GalaxiesAstronomers think that galaxies began to form about a billion years after the big bang from huge clouds of gas. If the stars in a galaxy develop slowly, the galaxy has more time to spin and flatten into a disk. This would be how spiral galaxies form. If a protogalactic cloud is very dense, more stars can form quickly, before the galaxy has time to flatten. This is how astronomers think some elliptical galaxies formed. As mentioned above, astronomers think that some elliptical galaxies have formed from collisions between two or more spiral galaxies.Grouping of GalaxiesJust as stars group to form galaxies, galaxies also form groups. There are three types of galaxy aggregation: groups, clusters, and superclusters. At each level of grouping, the galaxies orbit around the center of the structure.A group of galaxies will typically contain fewer than 50 galaxies within an area about 4 million light years across. The Milky Way is contained within the Local Group, a group of about 40 galaxies. The second largest galaxy within the Local Group is Andromeda, and this galaxy can only be seen in the southern hemisphere. The center of the group is located between the Milky Way and Andromeda.A cluster of galaxies can contain from 50 to 1,000 galaxies. The galaxies in a cluster appear to move together, but the force of gravity produced by their masses cannot explain their speeds of motion.In seeking an explanation for the discrepancy between the mass in galaxy clusters and their motion, astronomers discovered the existence of hot gases between galaxies. The mass of the hot gases is twice the mass of the stars and gases within galaxies, but even with the additional mass of the hot gas, the movements in galaxy clusters require either additional, undiscovered matter or new, unexplained forces.[Superclusters] Groups and clusters of galaxies form even larger structures called superclusters. The Milky Way is contained within the Local Group along with some nearby clusters, including the Virgo Cluster; the Milky Way is in the Local or Virgo Supercluster. The nearest galaxy group to the Local Group of the Milky Way is the M81 group and it is located 11 million light years away.Astronomers estimate that there may be ten million Supercluster in the universe. ................
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