I
- Inertia
- Inferior Conjunction
- Inferior Planet
- Infrared
- Instability Strip
- Interacting Galaxies
- Intercloud Gas
- Intergalactic Medium
- Interstellar Absorption Lines
- Interstellar Gas Cloud
- Interstellar Reddening
- Intra-cluster Medium
- Ion
- Ionian Philosophers
- Ionisation
- Ionised Hydrogen
- Irregular Galaxy
- Isolated Galaxies
- Isotope
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