Big History: The Big Bang, Life on Earth, and the Rise of Humanity - David Christian

Page 43

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,

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Glossary

31min
pages 250-272

Bibliography

23min
pages 273-288

Big History—Humans in the Cosmos

7min
pages 233-237

Permissions Acknowledgments

1min
pages 289-290

The Next Millennium and the Remote Future

6min
pages 229-232

The Next 100 Years

6min
pages 224-228

Human History and the Biosphere

6min
pages 219-223

The World That the Modern Revolution Made

6min
pages 214-218

The 20th Century

6min
pages 209-213

The Early Modern Cycle, 1350–1700

5min
pages 195-198

Threshold 8—The Modern Revolution

7min
pages 185-189

The Medieval Malthusian Cycle, 500–1350

6min
pages 190-194

Spread of the Industrial Revolution to 1900

6min
pages 204-208

Breakthrough—The Industrial Revolution

7min
pages 199-203

The Americas in the Later Agrarian Era

7min
pages 180-184

The World That Agrarian Civilizations Made

6min
pages 156-159

Long Trends—Rates of Innovation

6min
pages 165-169

Comparing the World Zones

7min
pages 175-179

Long Trends—Expansion and State Power

7min
pages 160-164

Long Trends—Disease and Malthusian Cycles

7min
pages 170-174

Agrarian Civilizations in Other Regions

6min
pages 152-155

Sumer—The First Agrarian Civilization

7min
pages 147-151

From Villages to Cities

6min
pages 142-146

Homo sapiens—The First Humans

6min
pages 104-108

The First Agrarian Societies

6min
pages 128-132

Early Power Structures

6min
pages 137-141

Power and Its Origins

5min
pages 133-136

The Origins of Agriculture

7min
pages 123-127

Threshold 7—Agriculture

6min
pages 118-122

Change in the Paleolithic Era

7min
pages 113-117

Paleolithic Lifeways

6min
pages 109-112

Life on Earth—Single-celled Organisms

5min
pages 82-85

Life on Earth—Multi-celled Organisms

6min
pages 86-90

Threshold 6—What Makes Humans Different?

7min
pages 99-103

Hominines

5min
pages 91-94

Evidence on Hominine Evolution

6min
pages 95-98

The Origins of Life

7min
pages 77-81

The Evidence for Natural Selection

6min
pages 73-76

Darwin and Natural Selection

6min
pages 69-72

Threshold 5—Life

6min
pages 64-68

Plate Tectonics and the Earth’s Geography

6min
pages 59-63

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