<P> It takes the Solar System about 240 million years to complete one orbit of the Milky Way (a galactic year), so the Sun is thought to have completed 18--20 orbits during its lifetime and 1 / 1250 of a revolution since the origin of humans . The orbital speed of the Solar System about the center of the Milky Way is approximately 220 km / s (490,000 mph) or 0.073% of the speed of light . The Sun moves through the heliosphere at 84,000 km / h (52,000 mph). At this speed, it takes around 1,400 years for the Solar System to travel a distance of 1 light - year, or 8 days to travel 1 AU (astronomical unit). The Solar System is headed in the direction of the zodiacal constellation Scorpius, which follows the ecliptic . </P> <P> The stars and gas in the Milky Way rotate about its center differentially, meaning that the rotation period varies with location . As is typical for spiral galaxies, the orbital speed of most stars in the Milky Way does not depend strongly on their distance from the center . Away from the central bulge or outer rim, the typical stellar orbital speed is between 210 and 240 km / s (470,000 and 540,000 mph). Hence the orbital period of the typical star is directly proportional only to the length of the path traveled . This is unlike the situation within the Solar System, where two - body gravitational dynamics dominate, and different orbits have significantly different velocities associated with them . The rotation curve (shown in the figure) describes this rotation . Toward the center of the Milky Way the orbit speeds are too low, whereas beyond 7 kpcs the speeds are too high to match what would be expected from the universal law of gravitation . </P> <P> If the Milky Way contained only the mass observed in stars, gas, and other baryonic (ordinary) matter, the rotation speed would decrease with distance from the center . However, the observed curve is relatively flat, indicating that there is additional mass that cannot be detected directly with electromagnetic radiation . This inconsistency is attributed to dark matter . The rotation curve of the Milky Way agrees with the universal rotation curve of spiral galaxies, the best evidence for the existence of dark matter in galaxies . Alternatively, a minority of astronomers propose that a modification of the law of gravity may explain the observed rotation curve . </P> <P> The Milky Way began as one or several small overdensities in the mass distribution in the Universe shortly after the Big Bang . Some of these overdensities were the seeds of globular clusters in which the oldest remaining stars in what is now the Milky Way formed . Nearly half the matter in the Milky Way may have come from other distant galaxies . Nonetheless, these stars and clusters now comprise the stellar halo of the Milky Way . Within a few billion years of the birth of the first stars, the mass of the Milky Way was large enough so that it was spinning relatively quickly . Due to conservation of angular momentum, this led the gaseous interstellar medium to collapse from a roughly spheroidal shape to a disk . Therefore, later generations of stars formed in this spiral disk . Most younger stars, including the Sun, are observed to be in the disk . </P>

Where is the milky way in our solar system