The formation of the planets is viewed as a by-product of the formation of the Sun. Stars are formed by the condensation of interstellar matter -- clouds of atoms, molecules, and dust. Star formation is an ongoing process --- the highest mass stars consume their nuclear fuel in a very short time compared to the age of the Universe (or even the Earth). In addition, what we think are the youngest stars have a higher concentration of higher mass elements; presumably material formed by nuclear processes in the interior of stars and and spewed by supernovae back into interstellar space.

Cool giant interstellar clouds, shielding their interiors from the exciting radiation of nearby stars, are dense and cool enough to permit the formation of molecules and to permit the adsorption of hydrogen onto dust grains.

To be stable, these interstellar clouds balance the internal thermal pressure of the gas and the gravitational force attempting to collapse the cloud. As the cloud contracts, the Virial Theorem requires that half of the energy goes into the thermal energy of the cloud. This increases the internal pressure which, if unchecked, would dissipate the cloud.

What triggers the ultimate collapse of the cloud is still unknown. Like most astrophysical phenomena, the time scales for this process are much longer than human history. So we must induce what happens from the few scant clues provided by nature. Some evidence suggests that some collapses are triggered by the radiation pressure of nearby supernovae.

The star rapidly collapses to less than a millionth of its original size. During this collapse, the object is known a protostar. The collapsing system becomes a Bok Globule, still cold and dark. The rotation of the system causes the globule to flatten; the more rapidly rotating clouds forming into multiple stars systems to conserve the angular momentum.

During this protostar phase, the warming object becomes visible in the infrared region of the spectrum. Narrowly collimated jets often project out of the poles enormous distances into space -- often a few parsecs. Finally, the temperature core of the collapsing central protostar reaches millions of degrees Kelvin and nuclear fusion commences. At this point the protostar becomes a true star and displays the peculiar visual spectrum of a T Tauri star.

The age of the Solar System is deduced from the ratio of parent- daughter ratios of decaying radioactive elements found in the oldest rocks from the Earth, the Moon, and meteorites. These three sources give a consistent age of 4.6 billion years.