<P> As stars of at least 0.4 M exhaust their supply of hydrogen at their core, they start to fuse hydrogen in a shell outside the helium core . Their outer layers expand and cool greatly as they form a red giant . In about 5 billion years, when the Sun enters the helium burning phase, it will expand to a maximum radius of roughly 1 astronomical unit (150 million kilometres), 250 times its present size, and lose 30% of its current mass . </P> <P> As the hydrogen shell burning produces more helium, the core increases in mass and temperature . In a red giant of up to 2.25 M, the mass of the helium core becomes degenerate prior to helium fusion . Finally, when the temperature increases sufficiently, helium fusion begins explosively in what is called a helium flash, and the star rapidly shrinks in radius, increases its surface temperature, and moves to the horizontal branch of the HR diagram . For more massive stars, helium core fusion starts before the core becomes degenerate, and the star spends some time in the red clump, slowly burning helium, before the outer convective envelope collapses and the star then moves to the horizontal branch . </P> <P> After the star has fused the helium of its core, the carbon product fuses producing a hot core with an outer shell of fusing helium . The star then follows an evolutionary path called the asymptotic giant branch (AGB) that parallels the other described red giant phase, but with a higher luminosity . The more massive AGB stars may undergo a brief period of carbon fusion before the core becomes degenerate . </P> <P> During their helium - burning phase, a star of more than 9 solar masses expands to form first a blue and then a red supergiant . Particularly massive stars may evolve to a Wolf - Rayet star, characterised by spectra dominated by emission lines of elements heavier than hydrogen, which have reached the surface due to strong convection and intense mass loss . </P>

Describe what is known about star sizes and temperatures