LECTURE

Gravity magni ed these differences, splitting vast clouds of matter into billions of smaller clouds. As each cloud contracted, the pressure and temperature at its center rose, and atoms collided with increasing frequency and violence.

Eventually, in a sudden phase change, the violence of these collisions overcame the positive electric charges between protons. Hydrogen nuclei fused to form helium nuclei, and the rst stars were formed. Once the center of a cloud of matter reached a minimum of about 10 million degrees Celsius, hydrogen nuclei collided so violently that some fused together, forming helium nuclei. As they did so, a tiny amount of matter was transformed into huge amounts of energy, in accordance with Einstein’s formula E = mc2. (The energy released when matter is converted into pure energy is equivalent to the mass of the converted matter multiplied by the speed of light squared—a colossal amount!) Hydrogen bombs use the same fusion reaction.

The colossal energy produced at the center of each star created a sort of furnace, which prevented any further gravitational collapse and stabilized the newborn star. From now on, its stability would depend on a constant negotiation between the heat at its center, which prevented further contraction, and the force of gravity that pressed it together. Fusion at the center explains why stars emit energy. Heat and light travel from the star’s core until eventually they are released into nearby space. Suddenly, the Universe lit up with billions of hot spots, each pumping energy into the cold of surrounding space. Fusion can continue within a star until it has used its stores of hydrogen. This may take millions or billions of years. The next lecture describes what happens when a star runs out of fuel. Recent evidence suggests that the rst stars lit up about 200 million years after the big bang.

As billions of stars lit up, gravity herded them together into larger “societies” of stars. First they formed into galaxies of hundreds of billions of stars. Gravity herded galaxies into clusters, and herded clusters into even larger membrane-like structures known as superclusters. However, at scales of millions of galaxies, gravity is too weak to overcome the expansion of the Universe. Only at these colossal scales can we observe the expansion of the Universe. Stars represented a new level of complexity. They are structured,