We review the current knowledge about nuclear star clusters (NSCs), the spectacularly dense and massive assemblies of stars found at the centers of most galaxies. Recent observational and theoretical works suggest that many NSC properties, including their masses, densities, and stellar populations, vary with the properties of their host galaxies. Understanding the formation, growth, and ultimate fate of NSCs, therefore, is crucial for a complete picture of galaxy evolution. Throughout the review, we attempt to combine and distill the available evidence into a coherent picture of NSC evolution. Combined, this evidence points to a clear transition mass in galaxies of \(\sim 10^9\,M_\odot\) where the characteristics of nuclear star clusters change. We argue that at lower masses, NSCs are formed primarily from globular clusters that inspiral into the center of the galaxy, while at higher masses, star formation within the nucleus forms the bulk of the NSC. We also discuss the co-existence of NSCs and central black holes, and how their growth may be linked. The extreme densities of NSCs and their interaction with massive black holes lead to a wide range of unique phenomena including tidal disruption and gravitational-wave events. Finally, we review the evidence that many NSCs end up in the halos of massive galaxies stripped of the stars that surrounded them, thus providing valuable tracers of the galaxies’ accretion histories.

我们回顾了有关核星团（NSC）的当前知识，核星团是在大多数星系中心发现的极其密集和巨大的恒星集合。最近的观测和理论工作表明，许多 NSC 特性，包括质量、密度和恒星数量，随着宿主星系的特性而变化。因此，了解 NSC 的形成、生长和最终命运对于全面了解星系演化至关重要。在整个综述中，我们试图将现有证据结合并提炼成 NSC 进化的连贯图景。综合起来，这些证据表明\(\sim 10^9\,M_\odot\)星系中存在明显的过渡质量，其中核星团的特征发生变化。我们认为，在较低质量下，NSC 主要由吸入星系中心的球状星团形成，而在较高质量下，核内的恒星形成构成了 NSC 的主体。我们还讨论了 NSC 和中心黑洞的共存，以及它们的增长如何联系起来。 NSC 的极端密度及其与大质量黑洞的相互作用导致了一系列独特的现象，包括潮汐破坏和引力波事件。最后，我们回顾了许多NSC最终出现在被剥夺了周围恒星的大质量星系晕中的证据，从而为星系吸积历史提供了有价值的示踪剂。