The Horizontal Branch

Evolution on the horizontal branch (HB) is similar to main sequence evolution since it is a period of stable core burning (but of helium, rather than hydrogen). For this reason, it is sometimes called the "helium main sequence". However, the time of stable helium core burning is much shorter than the time of stable hydrogen core burning on the main sequence. The calculated evolution of a 5 solar mass star on the HB branch is illustrated in the figure shown below (the HB evolution is between points 7 and 14).

Technically Speaking: Timescales for Advanced Burning

The timescales for various stages of nuclear burning in stellar evolution are set by three primary factors:

  • The amount of fuel available
  • The energy per fusion reaction derived from the fuel
  • The rate of energy loss from the star (luminosity), which governs how fast the fusion reactions must run to keep the star stable

    • All of these factors individually shorten the timescales for burning beyond hydrogen. Taken together, they make burning stages after the main sequence very short compared with main sequence lifetimes.

    Generally, in the HB phase the luminosity is lower than in the preceding RGB phase, but higher than on the main sequence.

    Horizontal Branch Distribution
    The distribution of stars horizontally on the HB branch of a cluster HR diagram is because of differences in detailed structure, in particular the mass of the envelope. The left end of the branch corresponds to stars with lower mass envelopes and the right end to stars with higher mass envelopes. Note that this mass refers to the mass while on the HB branch, not the initial mass. This is significant, because we shall see shortly that giant stars lose large amounts of mass as they evolve.

    Termination of HB Evolution
    The star remains on the HB branch as long as it has helium core fuel. When the helium is depleted in the center, we are left with an inert carbon-oxygen core and two burning shells: an inner helium-burning shell surrounding the inert core, and a hydrogen-burning shell outside of that. This signals the end of the horizontal branch evolution and the beginning of asymptotic giant branch evolution.