Until the End of Time: Mind, Matter, and Our Search for Meaning in an Evolving Universe

by Brian Greene

What we can say with more confidence is that some 4.7 billion years ago a supernova shock wave likely plowed through a cloud containing hydrogen, helium, and small quantities of more complex atoms, compressing part of the cloud, which, now being denser than its surroundings, exerted a stronger gravitational pull and thus began to draw material inward. Over the next few hundred thousand years, this region of the gas cloud continued to contract, rotating slowly at first and then more rapidly, like a graceful skater pulling in her arms while spinning. And much as the spinning skater experiences an outward pull (which splays out any loose fringe on the skater’s costume), so did the spinning cloud, which spread and flattened its outer regions into a rotating disk, which surrounded a smaller spherical region at the core. During the following fifty to one hundred million years, the gas cloud then performed a slow and steady rendition of the gravitational entropic two-step discussed in chapter 3: The gravitational force squeezed the spherical core, which grew ever hotter and denser, while the surrounding material cooled and thinned. The entropy of the core decreased; the entropy of the surroundings offset that by a more than compensating increase. Ultimately, the core’s temperature and density crossed the threshold for igniting nuclear fusion. The sun was born.