Spinning Stars: Exploring Rotation Across Stellar Spectral Types
Stars are celestial engines, each with unique characteristics that shape their evolution. Among these, rotational velocity—how fast a star spins—plays a critical role. This study by Mert et al. investigates the relationship between spectral type (ST), which categorizes stars by temperature and color, and their projected rotational velocity. Using data from nearly 50,000 stars spanning spectral types O0 to M9, the researchers explored how rotation varies across evolutionary stages and luminosity classes (LCs), such as main sequence, giants, and supergiants.
The Data Set
To ensure reliability, the study excluded stars with peculiar traits, like binaries or variable stars. The sample size of 48,639 stars provided a comprehensive overview of stellar rotation across spectral and luminosity classifications. The distribution showed more stars of cooler spectral types like F and G, with fewer hot (O-type) and cool M-type stars due to observational challenges.
Methodology
The team divided stars into two groups: one by spectral type and the other by spectral type combined with luminosity class. They averaged the rotational velocity for each group, applying Gaussian models to identify patterns. Spectral types were encoded numerically for clarity, and distinct trends emerged in rotation rates.
Results: Rotation and Spectral Type
Hot, massive stars (O0 to F2) exhibit significantly faster rotation than cooler stars (F2 to M9). A dramatic decrease in rotation—over 100 km/s—occurs as stars transition from hot to cool types. This slowdown links to structural differences, with hot stars having weaker magnetic fields and less convection compared to cooler stars. Cooler stars also experience magnetic braking, where stellar winds and magnetic fields slow their rotation over time.
Results: Rotation and Luminosity Class
Stellar rotation decreases as stars evolve. Main-sequence stars spin faster, but as they age into giants or supergiants, their rotation slows due to increased size and angular momentum loss. For cool stars, magnetic braking contributes to this reduction, while for hot stars, stellar winds play a significant role.
Conclusion
The study highlights the intricate relationship between a star's spectral type, evolutionary stage, and rotation. Cooler stars slow dramatically over time, while hotter stars maintain faster speeds but still experience rotational decline with age. Future research aims to extend this analysis to chemically peculiar stars and stellar clusters, further unraveling the mysteries of stellar rotation.
Source: Mert
