<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> <P> Currently, it is unclear whether waves are an efficient heating mechanism . All waves except Alfvén waves have been found to dissipate or refract before reaching the corona . In addition, Alfvén waves do not easily dissipate in the corona . Current research focus has therefore shifted towards flare heating mechanisms . </P> <P> Theoretical models of the Sun's development suggest that 3.8 to 2.5 billion years ago, during the Archean eon, the Sun was only about 75% as bright as it is today . Such a weak star would not have been able to sustain liquid water on Earth's surface, and thus life should not have been able to develop . However, the geological record demonstrates that Earth has remained at a fairly constant temperature throughout its history, and that the young Earth was somewhat warmer than it is today . One theory among scientists is that the atmosphere of the young Earth contained much larger quantities of greenhouse gases (such as carbon dioxide, methane) than are present today, which trapped enough heat to compensate for the smaller amount of solar energy reaching it . </P>

How hot is the surface of the sun in celsius