Pulse of the Spheres
We begin our "Walk Through Time" with
the formation of the Sun and Earth, two-thirds of the way through the history of the universe and 4,600 million years before
human beings appear.
About 15,000 million years before humans evolve (two miles behind you, at one million years to the
foot), the universe exploded out of the void in a "Big Bang." Early stars cycled through life and death. Supernova explosions spewed elements cooked in their interiors
into space, creating dense clouds of molecules and dust.
New stars and planets form as nebulae contract and condense under the force of their own gravity. In
a galaxy we call the Milky Way, the massive center of one such nebula contracts to form our Sun. Orbiting gas and dust accrete (grow by being added to) into planetesimals which then collide to build the planets and moons of our solar system.
Space. (Photography, top and bottom, courtesy NASA)
Middle: Crashing planetesimals begin to build Earth. (Illustration by William K. Hartmann, from The
History of the Earth, by William K. Hartmann and Ron Miller © 1991)
Bottom: In 1054, an Earthling in China records the explosion of a massive supernova 6,300 light years
away. The explosion creates the Crab Nebula, which is still expanding 50 million miles per day.