Astronomy should never be confused with astrology. Astronomy (law of the stars), is the scientific study of celestial objects. Astrology is the belief system which claims that human affairs are correlated with the positions of celestial objects.
In early times, astronomy was based on the observation and prediction of motions of objects visible to the eye. There are some ancient artifacts that more than likely had something to do with astronomy. The most notable is Stonehenge.
As ancient civilizations flourished and grew, observatories were built, the universe started to be observed and explored in earnest.
Because planets and stars involve such great distances, a new method of measuring distance was developed. Because stars and planets are so far away, distance is measured in light-minutes, or the more common light-year. The speed if light is 186,000 miles per second. So a light-minutes 186,000 miles times sixty. A light-year is how far light travels in one year.
The invention of the telescope by Galileo Galilee went a long way to help. Sir Issac Newton invented the reflective telescope which improved telescopic observation tremendously. The advent of photography brought another boost to astronomy. With that, stars were proved to be similar to the Sun.
The Existence of the Milky Way, Earth's own galaxy, as a separate group of a star and planets, was only proved in the 20th century. Also came the existence of "external galaxies", those outside of our own, and the expansion or recession of most galaxies from us. The theory of the Big Bang has gained a lot of support from astronomy and physics. The concept of the Big Bang goes back to 1965 with the discovery of microwave radiation.
The Sun which is about eight light-minutes from us is probably the most studied star. We know there are periodic changes we call sunspot activity. This is an 11-year fluctuation in sunspot numbers.
Astronomy also examines the planets, moons, comets, and asteroids.
Planets reach sufficient mass during planetary differentiation. This process can form a stoney, metallic core, surrounded by a mantle and an outer surface. A moon's heat is generally produced from the collisions that created it.
Astronomy is one field in which amateurs can make a difference. Most observation is done by telescope of the Moon, planets, stars, comets, etc. But some use ultraviolet wavelengths, or infrared, or radio telescopes.
As you can probably tell, astronomy can be quite complex.