Acknowledgements

I am particularly grateful to the colleagues who have accepted to participate in my HDR committee and to read this whole manuscript: Véronique BUAT, Vassilis CHARMANDARIS, Thomas HENNING, Laurent VERSTRAETE, Stéphane CHARLOT and François-Xavier DÉSERT. I thank them for their comments and the interesting discussions we had. I also thank Hervé DOLE for handling the administrative procedure of this accreditation.

I thank the people who willingly provided me with useful comments, clarifying the text and correcting some errors: Dangning HU, Anthony JONES, Vianney LEBOUTEILLER, Suzanne MADDEN, Marc-Antoine MIVILLE-DESCHÊNES and Takashi ONAKA. I also thank Christine JOBLIN for her insight on the 3.4 $\mu m$ feature, and Matteo BUGLI for a useful discussion about SNe and collapsars. I thank Anthony JONES and Nathalie YSARD for providing me with their DDA results, plotted in Fig. 1.21, and Chia-Yu HU for his sputtering yield polynomial fits used to make Fig. 4.13. I am finally grateful to all the people who have granted me the permission to reproduce their figures in this manuscript: Lou ALLAMANDOLA, Jean-Philippe BERNARD, Robert GENDLER, Peter HOPPE, Tom JARRETT, Andy MATTIODA and Marc-Antoine MIVILLE-DESCHÊNES.