Comets (“tailed stars”)
A comet nucleus is a mixture of ice, dust and rock a few km across (a “dirty snowball”). As it approaches the Sun the ice evaporates, forming a coma around the nucleus and then tails: the straight, bluish ion tail is shaped by the solar wind, the curved, whitish dust tail by radiation pressure; both point away from the Sun, even when the comet is receding. Short-period comets (T < 200 years) come from the Kuiper belt, long-period comets from the Oort cloud. Halley’s comet has a period of ≈ 76 years, a perihelion of ≈ 0.59 AU, was last at perihelion on 9 February 1986 and is due again in 2061; Edmond Halley predicted its return in 1705. In 1994 fragments of comet Shoemaker–Levy 9 hit Jupiter. In 2014 Rosetta went into orbit around comet 67P and landed the Philae probe. So far three interstellar objects have been recorded: 1I/ʻOumuamua (2017), 2I/Borisov (2019) and 3I/ATLAS (2025). Look for comets near the Sun with binoculars or a telescope only after the Sun has fully set.
Demonstrate a tail: hold a light piece of cotton wool in front of a fan (not a hair dryer) and watch how the “wind” carries it away; the cotton is the tail and the wind the solar wind. An adult operates the electrical devices.