Rayl - rela
- Rayleigh
- Rayleigh criterion
- Rayleigh limit
- Rayleigh number
- Rayleigh scattering
- Rayleigh taylor instability
- rays
- Razin effect
- RCB stars
- RCMA stars
- real anomaly
- receivers
- recombination
- recombination lines
- recurrent novas
- red dwarf flare stars
- red dwarf stars
- red giant stars
- red giant tip
- red stars
- red supergiant stars
- reddened stars
- reddening
- reddening law
- redshift
- redshift distance relation
- redshift magnitude relation
- reduction methods
- reference frames
- reference systems
- reflecting telescopes
- reflection
- reflection gratings
- reflection nebulae
- reflector antennas
- reflectors
- refracting telescopes
- refraction
