A comet is both a celestial body and a celestial object at once
Astronomers draw a quiet line between a body and an object. A body is one tightly bound lump, such as a planet or star; an object can be a looser structure of many parts, such as a galaxy. A comet manages to be both: its icy nucleus is a body, the whole thing with coma and tail an object.
The universe is organised in layers. Galaxies are the basic building blocks at the largest scale, grouped into clusters and superclusters strung along vast filaments around almost empty voids, a web spanning everything we can observe. Galaxies come in irregular, elliptical and disc shapes, shaped by their histories and by collisions that can merge them, and most hide a supermassive black hole at the centre. Within them, gas collapses under its own gravity into clusters of stars, and discs of leftover material around young stars build planets, asteroids and comets.
Shape is part of how the International Astronomical Union sorts the Solar System. To count as a planet or dwarf planet, a body orbiting the Sun must be massive enough for gravity to pull it round, a state called hydrostatic equilibrium. Anything that falls short is a small Solar System body. Many are lumpy heaps of rock that never gathered enough heat to finish rounding, and some are barely held together by gravity at all. Vesta is a curious middle case, large enough to have partly separated into layers.
Stars follow their own paths, charted on the Hertzsprung-Russell diagram of brightness against surface temperature. Mass largely decides a star's fate: some shed their outer layers gently as planetary nebulae, others explode as supernovae, and the remnants end as white dwarfs, neutron stars or black holes.
People have watched these lights for thousands of years, early on treating them as gods and using them to navigate and time the planting of crops. Copernicus put the Sun at the centre in 1543, Giordano Bruno argued in 1584 that stars are distant suns, and in 1610 Galileo spotted Jupiter's four largest moons.
Source: Astronomical object
