Red giants are evolved stars that have exhausted the supply of hydrogen in their cores and instead burn hydrogen in a surrounding shell 1,2 . Once a red giant is sufficiently evolved, the helium in the core also undergoes fusion 3 . Outstanding issues in our understanding of red giants include uncertainties in the amount of mass lost at the surface before helium ignition and the amount of internal mixing from rotation and other processes 4 . Progress is hampered by our inability to distinguish between red giants burning helium in the core and those still only burning hydrogen in a shell. Asteroseismology offers a way forward, being a powerful tool for probing the internal structures of stars using their natural oscillation frequencies 5 . Here we report observations of gravity-mode period spacings in red giants 6 that permit a distinction between evolutionary stages to be made. We use high-precision photometry obtained by the Kepler spacecraft over more than a year to measure oscillations in several hundred red giants. We find many stars whose dipole modes show sequences with approximately regular period spacings. These stars fall into two clear groups, allowing us to distinguish unambiguously between hydrogen-shell-burning stars (period spacing mostly 50 seconds) and those that are also burning helium (period spacing 100 to 300 seconds).Oscillations in red giants, like those in the Sun, are thought to be excited by near-surface convection. The observed oscillation spectra are indeed remarkably Sun-like, with a broad range of radial and nonradial modes in a characteristic comb pattern [7][8][9][10][11]

Mass‐loss of red giant branch (RGB) stars is still poorly determined, despite its crucial role in the chemical enrichment of galaxies. Thanks to the recent detection of solar‐like oscillations in G–K giants in open clusters with Kepler, we can now directly determine stellar masses for a statistically significant sample of stars in the old open clusters NGC 6791 and 6819. The aim of this work is to constrain the integrated RGB mass‐loss by comparing the average mass of stars in the red clump (RC) with that of stars in the low‐luminosity portion of the RGB [i.e. stars with L≲L(RC)]. Stellar masses were determined by combining the available seismic parameters νmax and Δν with additional photometric constraints and with independent distance estimates. We measured the masses of 40 stars on the RGB and 19 in the RC of the old metal‐rich cluster NGC 6791. We find that the difference between the average mass of RGB and RC stars is small, but significant [ (random) ±0.04 (systematic) M⊙]. Interestingly, such a small does not support scenarios of an extreme mass‐loss for this metal‐rich cluster. If we describe the mass‐loss rate with Reimers prescription, a first comparison with isochrones suggests that the observed is compatible with a mass‐loss efficiency parameter in the range 0.1 ≲η≲ 0.3. Less stringent constraints on the RGB mass‐loss rate are set by the analysis of the ∼2 Gyr old NGC 6819, largely due to the lower mass‐loss expected for this cluster, and to the lack of an independent and accurate distance determination. In the near future, additional constraints from frequencies of individual pulsation modes and spectroscopic effective temperatures will allow further stringent tests of the Δν and νmax scaling relations, which provide a novel, and potentially very accurate, means of determining stellar radii and masses.

Context. Solar-like oscillations have been observed in numerous red giants from ground and from space. An important question arises: could we expect to detect non-radial modes probing the internal structure of these stars? Aims. We investigate under what physical circumstances non-radial modes could be observable in red giants; what would be their amplitudes, lifetimes and heights in the power spectrum (PS)? Methods. Using a non-radial non-adiabatic pulsation code including a non-local time-dependent treatment of convection, we compute the theoretical lifetimes of radial and non-radial modes in several red giant models. Next, using a stochastic excitation model, we compute the amplitudes of these modes and their heights in the PS. Results. Distinct cases appear. Case A corresponds to subgiants and stars at the bottom of the ascending giant branch. Our results show that the lifetimes of the modes are mainly proportional to the inertia I, which is modulated by the mode trapping. The predicted amplitudes are lower for non-radial modes. But the height of the peaks in the PS are of the same order for radial and non-radial modes as long as they can be resolved. The resulting frequency spectrum is complex. Case B corresponds to intermediate models in the red giant branch. In these models, the radiative damping becomes high enough to destroy the non-radial modes trapped in the core. Hence, only modes trapped in the envelope have significant heights in the PS and could be observed. The resulting frequency spectrum of detectable modes is regular for = 0 and 2, but a little more complex for = 1 modes because of less efficient trapping. Case C corresponds to models of even higher luminosity. In these models the radiative damping of non-radial modes is even larger than in the previous case and only radial and non-radial modes completely trapped in the envelope could be observed. The frequency pattern is very regular for these stars. The comparison between the predictions for radial and non-radial modes is very different if we consider the heights in the PS instead of the amplitudes. This is important as the heights (not the amplitudes) are used as detection criterion.

In main-sequence stars, the periods of high-order gravity modes are sensitive probes of stellar cores and, in particular, of the chemical composition gradient that develops near the outer edge of the convective core. We present an analytical approximation of high-order g modes that takes into account the effect of the μ gradient near the core. We show that in main-sequence models, similarly to the case of white dwarfs, the periods of high-order gravity modes are accurately described by a uniform period spacing superposed to an oscillatory component. The periodicity and amplitude of such component are related, respectively, to the location and sharpness of the μ gradient.We investigate the properties of high-order gravity modes for stellar models in a mass domain range between 1 and 10 M , and the effects of the stellar mass, evolutionary state and extra-mixing processes on period spacing features. In particular, we show that for models of a typical Slowly Pulsating B (SPB) star, a chemical mixing that could likely be induced by the slow rotation observed in these stars is able to significantly change the g-mode spectra of the equilibrium model. Prospects and challenges for the asteroseismology of γ Doradus and SPB stars are also discussed.

Context. The detection of oscillations with a mixed character in subgiants and red giants allows us to probe the physical conditions in their cores. Aims. With these mixed modes, we aim at determining seismic markers of stellar evolution. Methods. Kepler asteroseismic data were selected to map various evolutionary stages and stellar masses. Seismic evolutionary tracks were then drawn with the combination of the frequency and period spacings. Results. We measured the asymptotic period spacing for 1178 stars at various evolutionary stages. This allows us to monitor stellar evolution from the main sequence to the asymptotic giant branch and draw seismic evolutionary tracks. We present clear quantified asteroseismic definitions that characterize the change in the evolutionary stages, in particular the transition from the subgiant stage to the early red giant branch, and the end of the horizontal branch. Conclusions. The seismic information is so precise that clear conclusions can be drawn independently of evolution models. The quantitative seismic information can now be used for stellar modeling, especially for studying the energy transport in the heliumburning core or for specifying the inner properties of stars entering the red or asymptotic giant branches. Modeling will also allow us to study stars that are identified to be in the helium-subflash stage, high-mass stars either arriving or quitting the secondary clump, or stars that could be in the blue-loop stage.

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Asteroseismology Delivers Using asteroseismology—the study of stellar oscillations, it is possible to probe the interior of stars and to derive stellar parameters, such as mass and radius (see the Perspective by Montgomery ). Based on asteroseismic data from the NASA Kepler mission, Chaplin et al. (p. 213 ) detected solarlike oscillations in 500 solartype stars in our Galaxy. The distribution of the radii of these stars matches that expected from stellar evolution theory, but the distribution in mass does not, which challenges our knowledge of star formation rates, the mass of forming stars, and the models of the stars themselves. Derekas et al. (p. 216 ) report the detection of a triple-star system comprising a red giant star and two red dwarfs. The red giant star, instead of the expected solarlike oscillations, shows evidence for tidally induced oscillations driven by the orbital motion of the red dwarf pair. Finally, Beck et al. (p. 205 ) describe unusual oscillations from a red giant star that may elucidate characteristics of its core.

The life of a star is dominantly determined by the physical processes in the stellar interior. Unfortunately, we still have a poor understanding of how the stellar gas mixes near the stellar core, preventing precise predictions of stellar evolution 1 . The unknown nature of the mixing processes as well as the extent of the central mixed region is particularly problematic for massive stars 2 . Oscillations in stars with masses a few times that of the Sun offer a unique opportunity to disentangle the nature of various mixing processes, through the distinct signature they leave on period spacings in the gravity mode spectrum 3 . Here we report the detection of numerous gravity modes in a young star with a mass of about seven solar masses. The mean period spacing allows us to estimate the extent of the convective core, and the clear periodic deviation from the mean constrains the location of the chemical transition zone to be at about 10 per cent of the radius and rules out a clearcut profile.The young massive star HD 50230 is poorly studied, but it is known to have spectral type B3V and a visual magnitude of 8.95. HD 50230 is in its central nuclear-burning phase, just like the Sun, transforming hydrogen into helium in its core (the so-called main sequence phase). This important evolutionary phase covers about 90% of the life of the star, and the detailed internal structure of the star during this phase determines its ultimate fate 1,2 . This star is at the limiting mass of ending either as core-collapse supernova or as a white dwarf, enriching the interstellar medium with helium and heavy metals in the former case and carbon in the latter. A highquality continuous photometric light curve with 32-s sampling and with a span of 137 days has been obtained for the star by the Convection Rotation and Planetary Transits (CoRoT 4 ) satellite. A linear downward trend was removed from the data, after outliers were excluded from the light curve. The duty cycle of the final set of measurements analysed here is 88.6% and the noise has an amplitude of 1 mmag (Fig. 1). The light curve reveals the presence of numerous oscillation modes.Stellar oscillations have been found to occur at different stages of stellar life, for a range of stellar masses 5 . The best known case is that of the solar oscillations, which are acoustic modes with periods near 5 min (refs 6-9). The seismic interpretation of the detected solar oscillations led to a drastic improvement in the knowledge of the internal structure of the Sun 10,11 . Meanwhile, similar acoustic modes have been detected in various types of distant stars [12][13][14] . Gravity modes, on the other hand, probe much deeper layers inside stars and in principle allow the study of the core properties of stars far better than acoustic modes. Although such modes have been detected in massive stars similar to HD 50230 15,16 , where they have typical periods of half a day to a few days, their seismic potential could not be exploited because the number of detected modes in one star was far too lo...