|Matter | Pages 94-95 ||
In ordinary solid matter, atoms bind together in gridlike structures called lattices. The space between each atom is large compared to the sizes of the individual atoms. It's as though a network of interconnecting toothpicks holds the Ping-Pong balls in fixed positions relative to one another. The toothpicks represent the electrostatic repulsion among the clouds of electrons around the atoms. They resist compression by any force we could muster on Earth, even if we ground the brick to dust.
However, stars can turn that trick readily. We know that during its lengthy adulthood, a star like our Sun balances the tremendous inward crush of its own gravity with outward pressure released by its fires of thermonuclear fusion. This equilibrium will exist in our Sun for 5 to 7 billion years more. Observers in our future solar system will see the Sun's energy output decline temporarily as helium ash builds up in the interior. Then the full weight of the star's outer layers starts to bear down upon the core. For a time the Sun staves off implosion by fusing helium into carbon. This transition pumps out energy at a thousand times the previous rate, resulting in a spectacular swelling of the Sun's outer layers (like our balloon in the hot sunlight). Our home star expands to hundreds of times its current diameter, becoming a "red giant" that engulfs Mercury, Venus, and possibly Earth and Mars within its scorching atmosphere.
Sadly for the Sun, those helium fires die down in about 100 million years. Then, gravity wins in a dramatic fashion. The core collapses under the pressure, as surely as a steamroller flattens an egg. In our analogy the Ping-Pong ball atoms pour into the center of the star. The force of gravity shatters the toothpicks between the atoms until they push against one another in a dense globe--far more densely packed than any state of matter on Earth.
At that point the basic rules of quantum mechanics prevent a further collapse. Electrons in the star's core obey the Pauli exclusion principle: As the gravitational bear hug crams the electron shells closer together, the electrons orbit their atoms ever more excitedly to avoid falling into the same physical state. This creates a new kind of outward pressure. The shells of the Ping-Pong balls grow vastly more rigid than occurs in normal matter. Finally, when the star shrinks to an object the size of Earth, (continued)