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