 Now that we have a handle on star development, from how older stars, and some key dates from uranium-led analysis of rocks and meteorites, we can estimate the age of the solar system as a whole, give or take a few million years here and there. Our first data point is provided by uranium decay. A neutron star merger would have seeded our cloud with relatively equal amounts of uranium-235 and uranium-238. The time it takes for the ratio to reach today's value of 1 uranium-235 for every 137.88 uranium-238s is 6 billion years. For almost 1.4 billion years, the cloud orbited the Milky Way in hydrostatic equilibrium. And then, for some as yet unknown reason, the equilibrium was broken and it started to collapse. Within a million years, the colliding giant molecular cloud broke up into fragments, with our fragment being one of them. Over the next million years, a circumstellar disk formed around a central object accreting mass from the disk material orbiting around it. From here on, we'll cover the development of the core object and the circumstellar disk in parallel. Over the course of the next million years, the central object continued to accumulate matter and its core temperature reached 10,000 degrees Kelvin. At this temperature, it began to shine by a normal, non-nuclear means. That made it a protostar. It may have looked like this one, just 950 light years away. During this time, most of the matter continues to reside in the circumstellar disk. It's losing large quantities of material to the central object, but by the time the protostar forms, the disk still has 99% of the solar system's mass. Some dust may have been colliding and sticking together, but the vast majority of whatever formed in the disk during this period was eventually lost to the forming star. The protostar phase does not last long for stars the size of our sun. Over the next million years, it accumulated massive amounts of matter from the disk and shrank significantly as gravity took hold. Its core temperature rose to 5 million degrees. This put the sun into its T-Tari phase, named after the star, T-Tari. In fact, it may have looked like T-Tari. The young sun was still growing by accumulating large amounts of material from its surroundings, so it was not yet stable. Unlike the short life span for protostars, T-Tari stars can last for 100 million years. During this phase, the disk experienced a growing solar wind from the developing star. This wind started pushing on the lighter gas and dust close to it, forming a snow line beyond which water ice could form. With the sun at only 5 million degrees Kelvin, this line would be much closer to the star than it is today. Over the next 30 million years, the sun's core temperature would have reached 10 million degrees. Throughout the disk, some dust grains began to stick together, forming larger particles. These particles continued to randomly collide and stick, creating planetesimals, reaching the size of boulders or small asteroids. The oldest of the starting material found so far was in the Elande meteorite and dated by a lead isotope contents to be 4.567 billion years old. By convention, astronomers used this date for the age of the sun and its solar system, often rounded up to 4.6 billion years. Estimates are that the sun remained in its T-Tari phase for an additional 67 million years as it migrated to the main sequence. In that time, it reached 15 million degrees Kelvin at its core. As the sun's solar wind picked up, it dispersed the remaining gas and dust around it back into the interstellar medium. This ended mass accumulation and the sun settled into hydrostatic equilibrium. During this period in the disk, the forming of planetesimals increased as the objects began to attract each other via gravity. Objects grew to planet sizes and swept out the debris in the vicinity of their orbits. The disk experienced a chaotic period of collisions that resulted in nine major planet-sized objects along with dozens of moons and millions of asteroids and comets. In addition, as the sun heated up, the snow line moved out to where we find it today, just outside the asteroid belt. This line separated the four waterless inner solar system planets from the five water-rich outer solar system planets. The giant impact hypothesis has a collision between the Earth and a Mars-sized planet liquefying the crust of both planets and forming the moon from ejected matter. Based on uranium-led dating of zircon crystals found in Australia and on the moon, this happened 4.3 billion years ago. That would be 200 million years after the original Earth formed. Given the mass of our sun, we know that in the beginning it had enough hydrogen to shine for a total of 10 billion years. We now figure that it has been burning for 4.6 billion years. Therefore we can expect that it will burn for 5.4 billion more years before it runs out of fuel. I'm impressed by how we have been able to reconstruct our solar systems formation. We started with a giant molecular cloud rotating around the Milky Way every 213 million years, 26,000 light years from the center. It was seated, with uranium around here, a little over an eighth of a revolution from our current position. It rotated an additional six and a half times before the cloud segment started to collapse here where we find the Perseus arm today. It took only half of a revolution more to form the entire solar system. Today we have a beautiful planet with a sun that will sustain us for billions of years. But even more spectacular than the rise of the Earth over a 300 million year period is what happened in the 4.3 billion years since our temperate, watery Earth formed. Life.