may happen towards the star core's center, where M(r) is small, if nuclear energy production is strongly peaked at the center, as in relatively massive stars. Thus such stars have a convective core. A smaller value of . For semi-ionized gas, where approximately half of the atoms are ionized, the effective value of drops to 6/5, giving . Therefore, all stars have shallow convection zones near their surfaces, at low enough temperatures where ionization is only partial. Main sequence stars For main sequence stars—those stars that are generating energy through the thermonuclear fusion of hydrogen at the core, the presence and location of radiative regions depends on the star's mass. Main sequence stars below about 0.3 solar masses are entirely convective, meaning they do not have a radiative zone. From 0.3 to 1.2 solar masses, the region around the stellar core is a radiation zone, separated from the overlying convection zone by the tachocline. The radius of the radiative zone increases monotonically with mass, with stars around 1.2 solar masses being almost entirely radiative. Above 1.2 solar masses, the core region becomes a convection zone and the overlying region is a radiation zone, with the amount of mass within the convective zone increasing with the mass of the star. The Sun In the Sun, the region between the solar core at 0.2 of the Sun's radius and the outer convection zone at 0.71 of the Sun's radius is referred to as the radiation zone, although the core is also a radiative region. The convection zone and the radiation zone are divided by the tachocline, another part of the Sun. Notes and references External links SOHO ... Solar and Heliospheric Observatory — official site of this NASA and ESA joint project. Animated explanation of the Radiation zone (University of South Wales). Animated explanation of the temperature and density of the Radiation zone (University of South Wales). Sun