A Mars-sized impactor smacks the embryo Earth, heating and deforming both bodies and spewing ejecta into space. The impactor rebounds and hits Earth again. Most of its metallic core gets incorporated into Earth's core. An orbiting ring of very hot ejecta, very little of it metallic, eventually cools and condenses into discrete particles.

The Moon's Violent Birth About 4.5 billion years ago, somewhere between the present orbits of Earth and Mars, a planet about the size of Mars probably struck the embryonic Earth in a collision that ultimately created Earth's only satellite. Known as the giant impact theory, this scenario largely accounts for the particular chemical composition of both bodies. It explains, for instance, why the Moon has only a tiny metallic core and Earth has a considerable one. It also accounts for the amount of angular momentum in the Earth­ Moon system. (Angular momentum is the measure of motion of objects in curved paths. In this case it means the spin of each body plus the orbital motion of the Moon around Earth.) As shown here, the young Earth was probably almost completely molten during this process. As particles accrete, they sweep up the disk of ejecta. Within about 10 years, the largest body sweeps up the remaining debris to become our Moon.

this pressure halts gravity's march. The dwarf star glows with a fierce white-hot light from the energy of gravitational collapse and leftover thermonuclear fusion.

Astronomers discovered white dwarfs in 1862 when they spotted a dim star orbiting Sirius, the Dog Star . Called Sirius B or the "Pup," the white dwarf is 10,000 times fainter than Sirius, the brightest star in the sky. By applying Kepler's laws of planetary motion to the two stars, we can deduce that Sirius B contains almost as much mass as the Sun and is nearly 5,000 times denser than lead. It will glow for billions of years as it radiates its heat into empty space. White dwarfs behave exactly like coals in your fireplace: They turn orange, then red, and then black as they inexorably cool. We know of thousands of them, but billions surely populate the galaxy. Since all they do is cool down, they are the closest things to cosmic chronometers that we have for estimating the age of the Milky Way.

A white dwarf can flare back to life explosively if it orbits around another star. Sirius and Sirius B form such a binary pair, but the stars are too far apart to interact directly. However, many other pairs of stars in the galaxy orbit each other more closely than the Dog Star and its Pup. If one star becomes a white dwarf and the other is a bloated giant, the dwarf can pull some of its companion's atmosphere onto its surface. This material--mostly hydrogen--can coat the dwarf in a layer thick enough to ignite in a thermonuclear flash. Such stellar hydrogen bombs are called novas . They can flare up every few weeks to every few centuries as new layers of hydrogen accumulate. In spectacular cases the thermonuclear flash can spark the entire white dwarf to collapse and then explode in a supernova that shines as brightly as its entire galaxy of stars.

A star much larger than our Sun suffers a similar fate when it runs out of fuel. If the core of the star exceeds a critical threshhold of mass--1.4 times the mass of our Sun--it spawns a supernova blast. At the heart of the explosion, the Ping-Pong ball shells of the atoms all collapse. Atomic nuclei disintegrate at the centers of the crushed shells. Electrons, neutrons, and protons all jam together, and gravity eradicates the spaces between them. The electrically charged electrons and protons cancel each other out, forming an extraordinarily compact glob of neutrons. A "neutron star " is born. But there's a limit to this squeeze: The pressure of the quarks within the neutrons, a consequence of the strong nuclear force, holds the line. This is neutron degeneracy, matter's last stand against gravity.