The Milky Way is one of the larger spiral galaxies. They are bright and distinctly disk–shaped, with hot gas, dust and bright stars in the spiral arms and a central concentration of stars known as bulge.
The force that binds the planets, billions of stars, including the sun and solar system is the same force that holds the moon in orbit and you on earth – gravitational force.
Polish astronomer Nicolaus Copernicus and later Danish astronomer Tycho Brahe, made extensive research regarding the positions of the planets in the sky and their motions. Brahe recorded the planet's positions for 20 years to 1/60 of a degree using a huge brass protractor–like instruments called quadrants. Brahe's younger colleague, German Johannes Kepler, edited and published Brahe's planetary tables. From the data, Kepler discovered some very remarkable and simple laws regarding planetary motion.
Soon after Kepler, Galileo did some remarkable observation regarding motion of Jupiter's moon. He noticed that they obeyed Kepler's laws too. After Galileo, another scientist Isaac Newton, with his innovative mind, unravelled what was happening to the motion of heavenly bodies. He was able to recognize that the force which was responsible for the motion of the planets around the Sun, or the Moon around the earth, was the same force that was responsible for making the apple fall towards the earth. This force is the famous gravitational force!