<P> The Solar System travels alone through the Milky Way in a circular orbit approximately 30,000 light years from the Galactic Centre . Its speed is about 220 km / s . The period required for the Solar System to complete one revolution around the Galactic Centre, the galactic year, is in the range of 220--250 million years . Since its formation, the Solar System has completed at least 20 such revolutions . </P> <P> Various scientists have speculated that the Solar System's path through the galaxy is a factor in the periodicity of mass extinctions observed in the Earth's fossil record . One hypothesis supposes that vertical oscillations made by the Sun as it orbits the Galactic Centre cause it to regularly pass through the galactic plane . When the Sun's orbit takes it outside the galactic disc, the influence of the galactic tide is weaker; as it re-enters the galactic disc, as it does every 20--25 million years, it comes under the influence of the far stronger "disc tides", which, according to mathematical models, increase the flux of Oort cloud comets into the Solar System by a factor of 4, leading to a massive increase in the likelihood of a devastating impact . </P> <P> However, others argue that the Sun is currently close to the galactic plane, and yet the last great extinction event was 15 million years ago . Therefore, the Sun's vertical position cannot alone explain such periodic extinctions, and that extinctions instead occur when the Sun passes through the galaxy's spiral arms . Spiral arms are home not only to larger numbers of molecular clouds, whose gravity may distort the Oort cloud, but also to higher concentrations of bright blue giants, which live for relatively short periods and then explode violently as supernovae . </P> <P> Although the vast majority of galaxies in the Universe are moving away from the Milky Way, the Andromeda Galaxy, the largest member of the Local Group of galaxies, is heading toward it at about 120 km / s . In 4 billion years, Andromeda and the Milky Way will collide, causing both to deform as tidal forces distort their outer arms into vast tidal tails . If this initial disruption occurs, astronomers calculate a 12% chance that the Solar System will be pulled outward into the Milky Way's tidal tail and a 3% chance that it will become gravitationally bound to Andromeda and thus a part of that galaxy . After a further series of glancing blows, during which the likelihood of the Solar System's ejection rises to 30%, the galaxies' supermassive black holes will merge . Eventually, in roughly 6 billion years, the Milky Way and Andromeda will complete their merger into a giant elliptical galaxy . During the merger, if there is enough gas, the increased gravity will force the gas to the centre of the forming elliptical galaxy . This may lead to a short period of intensive star formation called a starburst . In addition, the infalling gas will feed the newly formed black hole, transforming it into an active galactic nucleus . The force of these interactions will likely push the Solar System into the new galaxy's outer halo, leaving it relatively unscathed by the radiation from these collisions . </P>

What were the three major components of the solar nebula