Let's consider from now on a Sun-like star formation scenario. Molecular clouds usually rotate, during their collapse the angular momentum must be conserved. This leads to the formation of an accretion disk around the star: the so called protoplanetary disk. The disk is composed of both dust and gas, as the natal molecular cloud was. The presence of the dust (solid particles with sizes ranging from few nanometers to millimeters) is actually crucial for the planet formation. Processes like coagulation causes dust growth, until a planetesimal (kilometer sized object) is formed. Subsequent mass accretion on these big bodies, in less than few million years, leads to planet formation.
Depending on their distance from the central star the planetesimals can then still grow until they accrete an icy envelope (far away), a lot of gas (a bit closer), or just keep growing to the size of the Earth (very close to the star). After this stage, the gas in these system should be gone, either accreted from the forming giant planets (see Jupiter), from the star or photo-evaporated by the stellar radiation. After 10 million years we should then be left with something similar to our solar system, with little amount of free gas around and maybe some dust leftovers.
Let's see now how astronomers observe the stages listed above. To do so we need to define the SED, which stands for: Spectral Energy Distribution. It is a simple plot where one has in the y-axis the flux or luminosity emitted from these objects and in the x-axis the wavelength at which that radiation is emitted. Let's have a look at this figure:
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| Evolution of an SED. Class 0: only the dust respons to the obscured star is visible. Class I: the star looks for its way out. Class II: star+disk system. Class III: star only? Maybe a planetary system, with dust leftovers (debris disk). |
This is the timeline of what happens.
