The Milky Way - Essay Example

MILKY WAY Introduction Our galaxy consists of a highly evolved entity. It is in such a way that it is in perfect order yet of a high degree of complexity. It usually resembles the many grains of sand on a beach, but this time round they are in perfect arrangement in such a way that they form a spiral shape. This shape is common with the other galaxies you would think the universe delight in building them. However, how the universe formed the Milky Way is still a mystery, and the astronomers are still cracking their heads. Construction of models for the evolution of the Milky way based on its present appearance (Binney, 2010, P. 324). Appearance As one looks at the night sky, they observe a hazy band of white light that they always infer to be the Milky Way. Apparently, this is not true because every star and celestial body seen by the naked eye in the night sky all belong in the Milky Way. However, the band of white light comes about as a result of the accumulation of unresolved stars and other materials. A dark region can also be observed within the white band. The shaded area corresponds to blocked light from the distant star by interstellar dust. Anatomy of the Milky Way The Milky Way mainly consists of a bulging center and a spiral disk. The spiral disk can further be classified into a thin and thick disk. The flat disk rises about a 1000 light years below and above the galactic mid-plane, and the thick disk extends to about 3500 light years on both sides of the plane. When one considers the flatness of the galaxy then the galactic disk is said to be 120,000 light-years across. Our sun is about 28,000 light-years from the galactic center within the thin disk. There is also a halo that surrounds the entire disk, but it is not visible because of its dark matter composition. The Milky Way consists of many stars. These stars vary in lots of ways, from their individual size to their composition. The stars also vary depending on where they are found within the galactic disk, for example, those in the halo differ from those found on the disk. Consequently, they are said to belong to different stellar population. The mass of a star also translates to its life span; their weight is usually measured in terms of solar mass. The lighter the solar mass the longer the stars life span. The Milky Way is just one galaxy in a collection of a group of galaxies called the Local group. The Milky Way moves at 300 km/sec in the direction of the constellation Virgo. The Milky Way is in constant movement with the other galaxies in the Local Group. The galaxies found within the local group include the Milky Way Galaxy, the Andromeda Galaxy, Triangulum Galaxy and others, each containing their satellite system. Galactic rotation The stars and the gasses of the Milky Way rotate about its center differently meaning that there is a variation in the period of rotation depending on location. However, the orbital speed of the stars in a spiral galaxy is not entirely dependent on the distance from the center. The typical stellar orbital velocity of a star away from the central bulge is between 210 and 240 km/s. Hence, we can deduce that the orbital period of a typical star in the Milky Way is directly corresponds to the distance it travels. The other fact is that, towards the center of the Milky Way, the orbital speeds are too low but as one goes further away, the orbital speeds increase. It reaches the point that it defies the universal law of gravity. Formation of the Milky Way There are many models that try to explain the formation of the Milky Way, but we will put our focus on one particular model that is popularly accepted. The ELS model was conceived in the early 1960’s by three astronomers, namely Donald Lynden-Bell, Allan Sandage and Olin Eggen. This model was based on the relative velocities and chemical compositions of stars in population I (metal-rich) and II (metal-poor). The population I stars follow orbits in the plane of the galactic disk whereas the population II stars found in the halo follow elliptical orbits that cut across the plane of the Milky Way. The ELS Model According to the ELS, the Milky Way began as a spherical cloud of gas than was collapsing towards the center. The original gas had minimal levels of metal, and this consequently resulted in the stars being formed having low levels of metal. These stars that were formed maintained the kinematic properties of the gas from the collapsing cloud, thus making them follow eccentric orbits around the central part of the galaxy. Consequently, this led to the formation of population II stars (Noriega-Crespo, 2012, P. 7). The rotational speed of the cloud increased due to angular momentum and loss of energy by the contracting cloud led the cloud to collapse towards its rotational axis thus making it progressively flatter hence forming a disk. The gas in the flattened area would be abundant in metal that was produced by supernovae from the first-generation stars thus forming the population I stars. All this is said to have taken 300 million years according to the ELS model. A Two-In fall Model This second model seeks to explain the distribution of stars in the halo and the thin disk. It starts by saying that an initial collapse formed the halo. The efficiency of star formation in the halo led to a decrease in gas density up to an individual threshold. The gas lost due to star formation accumulated at the center, which resulted in the formation of the bulge. After the halo forms and the formation of the stars stops, a second in fall event, probably the merging up with a small galaxy forms the thin disk. The in fall event could also have probably been as a result of the large amount of time required for materials with high angular velocity to decrease (Kalberla & Kerp, 2009, P. 167). The Stars in the Milky Way The Milky Way currently contains approximately 200 billion stars and more than enough dust and gas to make billions more. More than half of the stars found in the Milky Way are older than our sun that is 4.5 billion years old. Common stars found in the Milky Way are the red dwarfs that are about a tenth of the suns mass. They were once thought to be unsuitable as life-bearing planets due to their proximity to each other, but after further study, they are currently thought off as potential suspects. Collision course As the Milky Way spins around its axis, it is also moving through the universe. As empty, as space may seem it is filled with dust and gas from other galaxies. As the galaxies spin, it is quite likely that the stars are always crashing into one other; the Milky Way is not left out on this. It is said that in about four billion years to come there will be a collision between the Milky Way and the Andromeda galaxy. It is said that the two galaxies are rushing towards each other at about 112 km per second. When this happens, it is predicted that this will provide a fresh influx of materials that will be able to bring about the new star formation. Conclusion It is clear that more observations are needed to refine the models in Milky Way’s evolution. When it comes to the formation time scale, we are still uncertain especially when we consider the thin disk outside the solar neighborhood. However, the future is quite promising considering the element deuterium that was recently reported to the galactic center by Donald Lubowich, of the American Physics Institute in New York. The significance of this element is that it is a very sensitive chemical marker of the gas consumption in a given locale. Through measuring the levels of this factor, will be able to have an insight on how fast the inner and outer disks have evolved. Structure of the Milky Way References Binney, J. (2010). Distribution functions for the Milky Way. Notices of the Royal Society, 401, 2318–2330. Kalberla, P. M. W., & Kerp, J. (2009). The Hi Distribution of the Milky Way. Annual Review of Astronomy, 47, 27. Noriega-Crespo, A. (2012). Star Formation in the Milky Way. The Infrared View. In "Cosmic-ray phenomenology in star-forming environments: Proceedings of the 2nd Session of the Sant Cugat Forum of Astrophysics (pp. 1–12). Read More