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Open Questions in Massive Star Research across Cosmic Scales

Andreas A. C. Sander

Abstract

Massive stars are the engines of the Cosmos, shaping their environments and driving galaxy evolution across cosmic time. Yet, this general textbook picture faces many challenges when trying to turn abstract insights into quantitative predictions. Recent discoveries, such the surprisingly high metallicity and early nitrogen enrichment in high-redshift galaxies discovered by JWST, are challenging current descriptions of massive star evolution and add new pieces to a puzzle that is yet everything but complete. The oncoming era of large surveys and advances in computational modeling create the potential to reach breakthroughs in our understanding. Yet, to resolve current problems and conflicting conclusions, we will also need to reconsider what we think we know. Are the objects we observe what we think they are? Are the models we use describing what is actually going on? And what can we learn from previous misconceptions? This short review highlights major open questions from individual stars and stellar systems back to the first galaxies while also discussing two examples - the weak-wind problem as well as the different flavours and impact of Wolf-Rayet stars - where recent discoveries might point in a new direction.

Open Questions in Massive Star Research across Cosmic Scales

Abstract

Massive stars are the engines of the Cosmos, shaping their environments and driving galaxy evolution across cosmic time. Yet, this general textbook picture faces many challenges when trying to turn abstract insights into quantitative predictions. Recent discoveries, such the surprisingly high metallicity and early nitrogen enrichment in high-redshift galaxies discovered by JWST, are challenging current descriptions of massive star evolution and add new pieces to a puzzle that is yet everything but complete. The oncoming era of large surveys and advances in computational modeling create the potential to reach breakthroughs in our understanding. Yet, to resolve current problems and conflicting conclusions, we will also need to reconsider what we think we know. Are the objects we observe what we think they are? Are the models we use describing what is actually going on? And what can we learn from previous misconceptions? This short review highlights major open questions from individual stars and stellar systems back to the first galaxies while also discussing two examples - the weak-wind problem as well as the different flavours and impact of Wolf-Rayet stars - where recent discoveries might point in a new direction.
Paper Structure (7 sections, 1 equation, 2 figures, 2 tables)

This paper contains 7 sections, 1 equation, 2 figures, 2 tables.

Table of Contents

  1. Open Questions across cosmic scales
  2. Puzzling discoveries at high redshift
  3. The need to connect the scales and uncover multiplicity
  4. Selected recent discoveries leading us to reconsider our assumptions
  5. Weak winds: A problem or a matter of diagnostics?
  6. The different roles and faces of Wolf-Rayet stars
  7. Summary: Pathways to answer the many open questions about massive stars

Figures (2)

  • Figure 1: Galaxies with high nitrogen enrichment Pascale+2023Castellano+2024Marques-Chaves+2024Schaerer+2024Senchyna+2024Topping+2024Ji+2025Napolitano+2025 compared to typical abundances in star-forming regions Nicholls+2017 in the C-O-mass-fraction plane, normalized to solar abundances from Magg+2022. Green curves show CN (dashed) and CNO (dashed-dotted) equilibrium abundances Nicholls+2017. For comparison, abundances of few individual stars (R136: Brands+2022; HD93129Aa: Gruner+2019; SSN9: Bouret+2021; AB4: Pauli+2023; MBO3: Ramachandran+2021) from different environments are shown as black, 4-pointed stars. Baseline abundances (purple dotted lines) are from Kniazev+2005, Skillman+2013, and Vink+2023.
  • Figure 2: Part of the normalized UV and optical spectra from three hot WR stars to demonstrate the direct spectral transition from WN2 to WO. The transition-type object M33WR 206 Sander+2025 combines the WN2 and WO spectral features while also showing intermediate surface abundances.