**12. Summary and Conclusions**

The Luminous Blue Variable phase is a short phase in the life of massive stars. It may be passed by stars with an initial mass as low as 21 M. LBVs have a specific variability the S Dor variable, can undergo giant eruption and have very high mass loss rate. The one and only way to pinpoint and truly classify LBVs is by the variability and/or giant eruption This asks for the detection of at least one S Dor cycle the star passes or to catch it in a giant eruption. These variabilities also subdivide the LBV class in classical (S Dor variable) LBVs and giant eruption LBVs. With the variability as the only clear classification method many LBVs in a quiescence state might be overlooked and not be identified as such. It is therefore not trivial to describe the LBV population in a galaxy. In that connection not knowing the true amount of LBVs and non-LBVs makes it hard to give an estimate for the real duration of the LBV phase. This again is directly linked to uncertainties of the total mass loss rate of massive stars. Even small changes of the phase length are linked to large changes in the mass total loss of the stars, given LBVs have very high mass loss rates. Last but not least that implies that the final mass of stars that pass a LBV phase could be much lower as thought so far. In that case this would even effect amounts and ratios of different SN types.

The path is therefore clear, to better characterize the LBV population and the underlying physics more long-term variability studies of nearby galaxies are needed. Spanning the parameter space especially towards lower metalicities will potentially clarify the importance of opacity effects and rotation for the S Dor variability. Also analyses of the long-term variability of massive stars in all the most metal-rich spiral galaxies in the Local Universe are not really done yet. First attempts are already ongoing, partly using data from well maintained archives, and the time-domain section of future large survey projects like LSST will be a major step forward. This will also be true for a better understanding of eruption LBVs. Another promising avenue will be the "archaeology" of the mass-loss

of LBVs and related stars using their circumstellar nebulae. In this way, information on energy, mass, and chemical composition of earlier mass-loss of the stars can be investigated, again providing clues about the underlying mechanism of instability and the evolutionary state of the stars. With the rise of integral field spectrographs, even with AO support (e.g., MUSE at the ESO/VLT), such analyses should be possible in all Local Group galaxies and the nearest galaxy groups. First such analyses are already appearing for galaxies in the Scultor group: NGC 300 [184] and NGC 7793 [185]. An unfortunate weakness in the currently available instrumentation are high-dispersion spectrographs fed by long-slits and IFUs, an important capability for kinematics/energetics of nebulae, which is becoming rare [186] at the intermediate and large telescopes. High-multiplex spectroscopic survey instruments at large telescopes, like e.g., Hectospec at the MMT, and soon MOONS and 4MOST at ESO telescopes, as well as WEAVE at the WHT, can be very useful tools to set LBVs in context to their massive star environment, as they are capable of providing good quality spectra for many photometrically selected LBV candidates (as well as other supergiants). This still requires that starforming, nearby galaxies will be targeted in the upcoming large surveys at these facilities.

Taking this all together, one can be optimistic, that in the coming years many more good quality observational data will be available to improve our understanding of the LBV phenomenon and its importance for the evolution of massive stars.

**Author Contributions:** K.W. planned the review structure. Both authors contributed then roughly equally to writing this review, with slightly different relative amounts depending on the topics covered in each chapter. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported through the Astronomical Institute of the Ruhr University Bochum, the Ruhr University Research Department Plasmas with Complex Interactions, and DFG Research Unit FOR 1254.

**Acknowledgments:** The authors thank Roberta Humphreys for many discussions, and her many comments and suggestions for this text; Kris Davidson for several helpful comments; Jochen Heidt and Alexander Becker for their contributions to the LBT AO observations of P Cygni; and our students for their contributions to many aspects of LBV related research here at Bochum during the last years. Thanks also go<sup>t</sup> to two anonymous referees whose suggestions improved this paper.

**Conflicts of Interest:** The authors declare no conflict of interest.
