Cosmic history has witnessed the lives and deaths of multiple generations of massive stars, all of them invigorating their host galaxies with ionizing photons, kinetic energy, fresh material, and stellar-mass black holes. Ubiquitous engines as they are, astrophysics needs a good understanding of their formation, evolution, properties and yields throughout the history of the Universe, and with decreasing metal content mimicking the environment at the earliest epochs. Ultimately, a physical model that could be extrapolated to zero metallicity would enable tackling long-standing questions such as “What did the first, very massive stars of the Universe look like?” or “What was their role in the re-ionization of the Universe?” Yet, most of our knowledge of metal-poor massive stars is drawn from one single point in metallicity. Massive stars in the Small Magellanic Cloud (SMC, \(\sim \)1/5Z_⊙ ) currently serve as templates for low-metallicity objects in the early Universe, even though significant differences with respect to massive stars with poorer metal content have been reported. This White Paper summarizes the current knowledge on extremely (sub-SMC) metal poor massive stars, highlighting the most outstanding open questions and the need to supersede the SMC as standard. A new paradigm can be built from nearby extremely metal-poor galaxies that make a new metallicity ladder, but massive stars in these galaxies are out of reach to current observational facilities. Such a task would require an L-size mission, consisting of a 10m-class space telescope operating in the optical and the ultraviolet ranges. Alternatively, we propose that ESA unites efforts with NASA to make the LUVOIR mission concept a reality, thus continuing the successful partnership that made the Hubble Space Telescope one of the greatest observatories of all time.

宇宙历史见证了多代大质量恒星的生与死，所有这些恒星都以电离光子、动能、新鲜物质和恒星质量的黑洞为宿主星系注入活力。尽管引擎无处不在，但天体物理学需要很好地了解它们在整个宇宙历史中的形成、演化、特性和产量，并且随着最早时期模仿环境的金属含量的减少。最终，一个可以外推到零金属丰度的物理模型将能够解决长期存在的问题，例如“宇宙中第一批非常大的恒星是什么样子的？” 或者“它们在宇宙重新电离中的作用是什么？” 然而，我们对缺乏金属的大质量恒星的大部分知识都来自金属丰度的一个点。\(\sim \) 1/5Z _⊙) 目前作为早期宇宙中低金属度天体的模板，尽管已经报道了金属含量较低的大质量恒星的显着差异。本白皮书总结了当前关于极度（亚 SMC）金属贫乏的大质量恒星的知识，突出了最突出的开放性问题以及取代 SMC 作为标准的必要性。可以从附近的金属含量极低的星系构建新的范式，这些星系构成新的金属丰度阶梯，但这些星系中的大质量恒星无法通过当前的观测设施进行观测。这样的任务需要一个 L 尺寸的任务，包括一个在光学和紫外线范围内运行的 10m 级太空望远镜。或者，我们建议 ESA 与 NASA 共同努力，使 LUVOIR 任务概念成为现实，