Supplementary Figure: Conventional Schematic Diagrams of the Evolution of a Protoplanetary Disk around a Sun-like Star.
The top two diagrams relate to newborn stars called "proto-stars". Stars are born when a molecular cloud, consisting of cold gas and dust, collapses due to the gravitational pull of its own mass. The proto-star grows by gathering material from its parental molecular cloud. The accumulated material falls onto a disk surrounding the new star rather than coming down directly onto the proto-star. The energy released by the falling material heats up the disk and its light can be observed at infrared and sub-millimeter wavelengths. Such stars are less than 100,000 years old.
The third diagram from the top is a schematic of a Sun-like star when it is about a million years old, what astronomers call a T-Tauri star. Strong stellar winds and other processes have now blown away most of the parental material. Only a dense disk of material remains. This is a protoplanetary disk.
The next diagram (the fourth) represents a Sun-like star at an age of about 10 million years. The protoplanetary disk has become thinner, and planets may have formed.
The bottom (the fifth) diagram is a schematic of a Sun-like star in its long and stable middle age, when it is 100 million years old and fueled by nuclear fusion at its core. The original protoplanetary disk is almost gone, but the collision of asteroids may form secondary disks. This process may be the origin of the dust disks surrounding stars like Beta Pictoris, or of zodiacal dust, dust in the plane of the planets in our own solar system.