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tokens: et al. 2007 ), disk formation in the central parsec followed by a migration process (Levin 2007 ; Madigan et al. 2009 ), and rejuvenation of older stars by mergers or severe atmosphere alteration by envelope-stripping (Lee 1987 ; Genzel et al. 2003 ; Davies King 2005 ). A successful scenario should explain the young ages and randomly inclined orbits of the S-stars, via an efficient transport/relaxation mechanism. External formation followed by binary disruption is among the favorite scenarios, as it also explains some of the hyper-velocity stars that escape the Galaxy (e.g., Brown et al. 2009 ). However, the field binary disruption scenario requires some minimum time span to explain the thermal distribution of eccentricities in the S-star cusp (Perets 2009 ). This may be incompatible with the shorter main-sequence lifetime of some S-stars. While it is plausible to assume that S-stars were initially formed in an outer stellar disk in the central parsec, it has proved difficult to find an explanation of how their original angular momenta are strongly perturbed and later redistributed. Finally, many of the scenarios that suggested environmental mechanisms to significantly alter the photosphere of in fact old S-stars (Lee 1987 ; Alexander Morris 2003 ; Hansen milosavljević 2003 ; Davies King 2005 ) either have serious problems with explaining the S-stars’ stellar properties or are not elaborated in detail. For example, it is not clear how quickly after a major atmospheric perturbation a star will relax, and whether such products experience significant mixing (e.g., Gaburov et al. 2008 ). With all the aforementioned observational constraints and theoretical complexities the question of the origin and distribution of young stars in the GC has become one of the most remarkable issues in this field. Here, we perform quantitative spectroscopy of S-stars and obtain their individual stellar parameters. This enable us to locate these stars for the first time in a Hertzsprung–Russell (HR)-like diagram and provide new insights on their evolutionary stage and their puzzling origin. However, such an analysis demands a high signal-to-noise (), comparable to that achieved in spectra of nearby Galactic OB-stars. In this study, we co-add more than 100 hr of adaptive-optics (AO) assisted spectroscopic observations of the GC over twelve years, to construct ultra-deep spectra of eight S-stars. The paper is organized as follows: in Section 2 we list the observations used in this analysis and describe the reduction of the data to obtain deep spectra on a number of stars in the GC. Section 3 describes the stellar atmosphere models used to match the data and our fitting strategy . In the results section (Section 4 ), we present the spectral and luminosity (gravity) classification of the S-stars as well as parameters of individual stars based on the results of atmosphere model fits. In the final paragraph of the results section we compare the inferred stellar