Short answers, pulled from the story.
An astronomical body is a single, tightly bound, contiguous physical object, such as a star, moon, or asteroid. An astronomical object admits a more complex, less cohesively bound structure that may consist of multiple bodies or substructures, such as a planetary system, galaxy, or nebula. A comet illustrates both: its frozen nucleus is a body, while the entire comet including its coma and tail is an object.
According to NASA astrophysicists, the earliest astronomical objects began to emerge roughly 13.6 billion years ago, approximately 200 million years after the Big Bang formed the early universe. Gravity drew matter together to fuse into the first stars and galaxies.
Hydrostatic equilibrium is the rounding process by which a Sun-orbiting body achieves a roughly spherical shape under its own gravity. The International Astronomical Union requires this condition for classification as a planet or dwarf planet. Any natural Sun-orbiting body that has not reached hydrostatic equilibrium is instead classified as a small Solar System body.
In 1610, Galileo Galilei used a telescope to observe the four largest moons of Jupiter, now called the Galilean moons. He also recorded the phases of Venus, craters on the Moon, and sunspots on the Sun.
The Hertzsprung-Russell diagram is a plot of absolute stellar luminosity against surface temperature that reveals the hidden order among stars. It was developed independently in 1913 by astronomers Ejnar Hertzsprung and Henry Norris Russell. Stars cluster along a band called the main-sequence, and the diagram was used as the basis for a refined stellar classification scheme published in 1943 by William Wilson Morgan and Philip Childs Keenan.
At the largest scales, galaxies are organized into groups and clusters, which gather into larger superclusters strung along great filaments between nearly empty voids, forming a web that spans the observable universe. Within galaxies, stars assemble from condensing nebulae and are often found in multi-star systems, with planetary systems and smaller bodies forming from protoplanetary disks around newly formed stars.