<P> The Sun orbits the center of the Milky Way, and it is presently moving in the direction of the constellation of Cygnus . The Sun's orbit around the Milky Way is roughly elliptical with orbital perturbations due to the non-uniform mass distribution in Milky Way, such as that in and between the galactic spiral arms . In addition, the Sun oscillates up and down relative to the galactic plane approximately 2.7 times per orbit . It has been argued that the Sun's passage through the higher density spiral arms often coincides with mass extinctions on Earth, perhaps due to increased impact events . It takes the Solar System about 225--250 million years to complete one orbit through the Milky Way (a galactic year), so it is thought to have completed 20--25 orbits during the lifetime of the Sun . The orbital speed of the Solar System about the center of the Milky Way is approximately 251 km / s (156 mi / s). At this speed, it takes around 1,190 years for the Solar System to travel a distance of 1 light - year, or 7 days to travel 1 AU . </P> <P> The Milky Way is moving with respect to the cosmic microwave background radiation (CMB) in the direction of the constellation Hydra with a speed of 550 km / s, and the Sun's resultant velocity with respect to the CMB is about 370 km / s in the direction of Crater or Leo . </P> <P> The temperature of the photosphere is approximately 6,000 K, whereas the temperature of the corona reaches 1,000,000--2,000,000 K . The high temperature of the corona shows that it is heated by something other than direct heat conduction from the photosphere . </P> <P> It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating . The first is wave heating, in which sound, gravitational or magnetohydrodynamic waves are produced by turbulence in the convection zone . These waves travel upward and dissipate in the corona, depositing their energy in the ambient matter in the form of heat . The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of large solar flares and myriad similar but smaller events--nanoflares . </P>

The outer 20-25 of the interior of the sun makes up this layer