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Red giants are evolved stars that have exhausted the supply of hydrogen intheir cores and instead burn hydrogen in a surrounding shell. Once a red giantis sufficiently evolved, the helium in the core also undergoes fusion.Outstanding issues in our understanding of red giants include uncertainties inthe amount of mass lost at the surface before helium ignition and the amount ofinternal mixing from rotation and other processes. Progress is hampered by ourinability to distinguish between red giants burning helium in the core andthose still only burning hydrogen in a shell. Asteroseismology offers a wayforward, being a powerful tool for probing the internal structures of starsusing their natural oscillation frequencies. Here we report observations ofgravity-mode period spacings in red giants that permit a distinction betweenevolutionary stages to be made. We use high-precision photometry obtained withthe Kepler spacecraft over more than a year to measure oscillations in severalhundred red giants. We find many stars whose dipole modes show sequences withapproximately regular period spacings. These stars fall into two clear groups,allowing us to distinguish unambiguously between hydrogen-shell-burning stars(period spacing mostly about 50 seconds) and those that are also burning helium(period spacing about 100 to 300 seconds).

Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars