In ancient times astronomy was simply the study of planetary motion. The stars themselves appeared motionless to the astronomers, except on the rare occasion when a new star suddenly appeared and then faded away. Astrophysics in this setting hardly existed. Astronomers and philosophers tried to create models of the solar system, many of which were fanciful and totally at odds with the observed motions of the planets. For instance, followers of Pythagoras were motivated by their numerology to create a ten-body Solar System, with the central body a ball of fire that is not the Sun and the tenth body a counter-Earth that is always hidden from view. The best of the ancient cosmologies was created by Ptolemy. This Earth-centered system describes a planet's motion on the sky as the motion of a rotating sphere attached to a second sphere. The sphere centered on the Earth describes the planet's motion on the sky caused by the planet's orbit around the Sun, and the sphere attached to the Earth-centered sphere describes planet's motion on the sky caused by Earth's orbit around the Sun. This cosmology produces an excellent fit to the observations of that era. Not until the era of Copernicus was there a reason to change the cosmology to a Sun-centered Solar System.
The early cosmologies had an element of physics to them. In the Ptolemaic cosmology, the Earth was fixed, while the stars were attached to a sphere that rotated once in a day. But why not let the Earth spin once in a day, and leave the stellar sphere fixed? Among the arguments given by Ptolemy is that if the Earth rotated, there would be a constant wind caused by this rotation.
Astrophysics was born when Newton used his theory of gravity to correctly derived Kepler's laws of planetary motion. Kepler's laws of planetary motion around the Sun are the simplest empirical equations that describe the motion of the planets. These laws describe each planet's orbit as an ellipse, and they gave a simple relationship between the size and the period of a planet's orbit, but astronomers of that era had no explanation of why each planet obeys them. Newton provided an explanation when he showed that a simple equation for the gravitational attraction between the Sun and a planet gives Kepler's laws.
Over the past two hundred years astronomy and astrophysics have advanced to the point that the planets constitute only a tiny area of astronomical research, an area that is of more interest to geologists and meteorologists than to most astronomers. Astronomy has not only moved to the stars, but beyond the stars to the galaxies and to the edge of our observable universe. Astrophysics is concerned not only with Newtonian gravity, but also with general relativity—our modern theory of gravity. Other types of physics comes into play in our attempts to understand how the universe works. Nuclear fusion was developed to explain the longevity of stars. Plasma physics is used to describe the gas within a star and in the emission nebula floating between stars. Our universe provides conditions not achievable in the laboratory, so we see physical processes in space that we cannot duplicate on Earth.
Astronomy today is principally the study of four types of object: planets, stars, galaxies, and the universe as a distinct whole. These different objects form a hierarchy from the smallest and least massive to the largest and most massive. While gravitational physics is a critical element for each of these objects, other types of physics makes each class distinct.
The planets are small masses of material that are cooling or are cold. They differ from stars in that stars can generate internal energy through the nuclear fusion of hydrogen, helium, and heavier elements, while planets cannot; whatever energy is generated internally by a planet comes from gravitational collapse, phase transitions, or the nuclear decay of radioactive elements. The composition of a planet in our Solar System and in other star systems is set by the planet's distance from the central star. It is assumed that many planets have formed far away from any star. These Jupiter-like planets are invisible to us, but they provide part of our Galaxy's gravitational force.
The nuclear fusion within a star creates an object that can remain hot for billions of years. While all stars begin life with almost the same composition, nuclear fusion changes this composition. This transformation through nuclear fusion can cause a star to undergo violent changes, with the supernova the most dramatic change. Eventually a star reaches its end point as a degenerate dwarf, a neutron star, or a black hole.
Galaxies are gravitationally bound systems of stars. While a star or a planet is stabilized against gravitational collapse by gas pressure, a galaxy is stabilized by the angular momentum of its constituents. The difference is one of density. The mass density of a galaxy is too low to trap radiation for extended times, so the gas within a galaxy gravitationally collapses until the angular momentum carried by the gas balances gravitational attraction. Galaxies come in many flavors, with some composed of ancient stars and having an elliptical structure, and others composed of young stars and gas and having a disk or irregular structure.
Our universe is expanding. When we look out beyond our own galaxy, we see the surrounding galaxies rushing away from us, with the most distant galaxies moving away at nearly the speed of light. Beyond these galaxies we see a photosphere, a surface that is emitting radiation, much as we see when we look at the Sun. But unlike the Sun, this photosphere is shell that completely surrounds us, and the radiation it is emitting is in the microwave. In the past gravity played a role in the evolution of the universe by slowing the rate of expansion, but the observational evidence suggests that gravity is no longer sufficiently strong to slow the expansion.