The new, increased radiation pressure actually causes the outer layers of the star to expand to maintain the pressure gradient. This so-called shell-burning causes some interesting effects. Although fusion is no longer taking place in the core, the rise in temperature heats up the shell of hydrogen surrounding the core until it is hot enough to start hydrogen fusion, producing more energy than when it was a main sequence star. Inward gravitational attraction causes the helium core to contract, converting gravitational potential energy into thermal energy. This explains why the main sequence is a broad band rather than a narrow line - stars move up and to the right on this band as they age.Įventually the hydrogen fuel in the core runs out and fusion stops, shutting off the outward radiation pressure. In fact, as a main sequence star ages its luminosity increases slightly, resulting in it expanding and its outer layer cooling. So far we have assumed that a star on the main sequence maintains a constant energy output. Moving Off the Main Sequence - Red Giant BranchĪ star remains on the main sequence as long as there is hydrogen in its core that it can fuse into helium. Once shell temperature is sufficient, helium shell burning starts and the star moves up into the asymptotic giant branch (AGB). A very short helium flash sees the start of helium core fusion and the star moves along the horizontal branch (HB). Stars such as our Sun move off the main sequence and up the red giant branch (RGB), fusing hydrogen into helium in hydrogen shell burning. Evolution of Low-Mass Stars Post-Main Sequence.Evolution of High-Mass Stars Post-Main Sequence.The Helium Flash and Triple Alpha Process.Evolution of a Solar-Mass Star off the Main Sequence.An artist's impression of a red supergiant engulfing a Jupiter-like planet as it expands.
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