Energy transfers result in a hotter outer layer of the star but a smaller overall size. It thus starts to behave again as an ideal gas so can expand and cool. The energy released by the helium flash raises the core temperature to the point where it is no longer degenerate. Post-main sequence evolutionary tracks for 1, 5 and 10 solar mass stars. In this state it no longer behaves as an ideal gas. Electrons within the core form a degenerate electron gas, they are packed tightly together in a volume governed only by the uncertainty principle. It is only about one-third its original size. Whilst the envelope is tenuous and cool, the contracted helium core is incredibly dense. Mass loss from a giant is typically about 10 -7 solar masses per year, compared with only 10 -17 solar masses per year currently for the Sun. It is only weakly held by gravitational force to the rest of the star and easily ejected. The outer envelope is grossly extended and thus at a density below that of a vacuum on Earth. A G (V)-class star may end up as a high-K or low-M luminosity class III giant.Ī red giant displays extremes of density. During this stage of expansion, the star will move up and to the right on the HR diagram along the Red Giant Branch (RGB).

The star's luminosity eventually increases by a factor of 1000 × or so. Convection transports the energy to the outer layers of the star from the shell-burning region. This expansion and cooling causes the effective temperature to drop. As the gas expands it cools, just as a spray can feels colder after use as the gas has been released. 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.

Fusion core shell star life cycle