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.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.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.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
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