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W

Watt (W)

Unit of power in the SI unit system. 1 Watt = 1 Joule per second.

Wavelength

The distance between two peaks of a wave. See also Electromagnetic radiation and Frequency.

White dwarf

A very dense star with a mass below 1.4 solar masses that is no longer burning nuclear fuel. The Sun will one day evolve into a white dwarf with a diameter of 10 000 km.

Wirtanen (comet)

Periodic comet which orbits the Sun once every 5.45 years. Discovered in 1948 at the Lick Observatory, California, by Carl A. Wirtanen. It is a so-called `Jupiter-type' comet, whose orbit is strongly influenced by that planet. Perihelion is at 159 million km (1.06 AU) from the Sun, i.e. just outside the orbit of the Earth. Aphelion is at a distance of about 768 million km (5.13 AU), near the orbit of Jupiter. Target of ESA's Rosetta mission, which will go into orbit around the nucleus and deploy a lander on its surface.

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