<P> The apex of the Sun's way, or the solar apex, is the direction that the Sun travels through space in the Milky Way . The general direction of the Sun's Galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 60 sky degrees to the direction of the Galactic Center . The Sun's orbit about the Milky Way is expected to be roughly elliptical with the addition of perturbations due to the Galactic spiral arms and non-uniform mass distributions . In addition, the Sun passes through the Galactic plane approximately 2.7 times per orbit . This is very similar to how a simple harmonic oscillator works with no drag force (damping) term . These oscillations were until recently thought to coincide with mass lifeform extinction periods on Earth . However, a reanalysis of the effects of the Sun's transit through the spiral structure based on CO data has failed to find a correlation . </P> <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>

What are the arms of the milky way