Current Unsolved Problems in Planetary Nebulae Research
Abstract
1. Introduction
2. What Is a Planetary Nebula?
3. The Distance Problem
4. Morphological Structure of Planetary Nebulae




4.1. Wind Shaping
4.2. Collimated Flows
4.3. Episodic Outflows
4.4. Microstructures
4.5. Spirals and Disks
5. Dust and Molecular Components of Planetary Nebulae
5.1. Molecular Torus as a Collimation Agent
5.2. Confinement of Optical Lobes by Neutral Envelope
5.3. Circumstellar Chemistry
6. Large-Scale Structures of Planetary Nebulae
6.1. Outer Haloes
6.2. Large Bipolar Nebulae
6.3. Large Outer Structures
7. The Relevance of Binarity to PN Evolution and Morphology
7.1. Search for Binary Central Stars in Proto-Planetary Nebulae
7.2. Hierarchical Systems
7.3. The Abundance Discrepancy Problem and Possible Connection to Binarity
8. Planetary Nebulae in Extragalactic Systems
9. Summary
Author Contributions
Funding
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Kwok, S.; Balick, B.; Chu, Y.-H.; Hrivnak, B.J.; López, A.; Parker, Q.; Sahai, R.; Zijlstra, A. Current Unsolved Problems in Planetary Nebulae Research. Galaxies 2026, 14, 30. https://doi.org/10.3390/galaxies14020030
Kwok S, Balick B, Chu Y-H, Hrivnak BJ, López A, Parker Q, Sahai R, Zijlstra A. Current Unsolved Problems in Planetary Nebulae Research. Galaxies. 2026; 14(2):30. https://doi.org/10.3390/galaxies14020030
Chicago/Turabian StyleKwok, Sun, Bruce Balick, You-Hua Chu, Bruce J. Hrivnak, Alberto López, Quentin Parker, Raghvendra Sahai, and Albert Zijlstra. 2026. "Current Unsolved Problems in Planetary Nebulae Research" Galaxies 14, no. 2: 30. https://doi.org/10.3390/galaxies14020030
APA StyleKwok, S., Balick, B., Chu, Y.-H., Hrivnak, B. J., López, A., Parker, Q., Sahai, R., & Zijlstra, A. (2026). Current Unsolved Problems in Planetary Nebulae Research. Galaxies, 14(2), 30. https://doi.org/10.3390/galaxies14020030

