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Glossary: W
| Wavelength | The distance between consecutive crests of a wave. This serves as a unit of measure of electromagnetic radiation. |
| White Dwarf | A very small, white star formed when an average sized star uses up its fuel supply and collapses. This process often produces a planetary nebula, with the white dwarf star at its center. |
Glossary: X
| X-ray | Electromagnetic radiation of a very short wavelength and very high-energy. X-rays have shorter wavelengths than ultraviolet light but longer wavelengths than cosmic rays. |
| X-ray Astronomy | The field of astronomy that studies celestial objects by the x-rays they emit. |
| X-ray Star | A bright celestial object that gives off x-rays as a major portion of its radiation. |
Glossary: Y
| Yellow Dwarf | An ordinary star such as the Sun at a stable point in its evolution. |
Glossary: Z
| Zenith | The point in the sky directly overhead. |
| Zodiac | An imaginary belt across the sky in which the Sun, moon, and all of the planets can always be found. |
| Zodiacal Light | A faint cone of light that can sometimes be seen above the horizon after sunset or before sunrise. Zodiacal light is caused by sunlight reflecting off small particles of material in the plane of the Solar System. |