main sequence stars accumulate helium in their cores as a result of hydrogen fusion, but the core does not become hot enough to initiate helium fusion. helium fusion first begins when a star leaves the red giant branch after accumulating sufficient helium in its core to ignite it. in stars around the mass of the sun, this begins at the tip of the red giant branch with a helium flash from a degenerate helium core and the star moves to the horizontal branch where it burns helium in its core. more massive stars ignite helium in their cores without a flash and execute a blue loop before reaching the asymptotic giant branch. despite the name, stars on a blue loop from the red giant branch are typically not blue in color, but are rather yellow giants, possibly cepheid variables. they fuse helium until the core is largely carbon and oxygen. the most massive stars become supergiants when they leave the main sequence and quickly start helium fusion as they become red supergiants. after helium is exhausted in the core of a star, it will continue in a shell around the carbon-oxygen core.