based microwave radio telescopes. Later observations with satellites showed that the background radiation in the universe has exactly the properties that would be predicted from the Big Bang.
As the universe expanded, the matter in it gathered, by way of gravity and other processes that are not yet fully understood, into immense structures that became galaxies. Within these structures, much smaller clumps of matter collapsed into whirling clouds of gas and dust. When the matter in the center of an individual cloud became sufficiently compressed by gravity, the hydrogen atoms in that cloud began to fuse into helium atoms, giving off visible light and other radiation — the origin of a star.
Astrophysicists also have found that some stars form in the middle of a flattened spinning disk of matter. The gas and dust within such disks can aggregate into small grains, and these grains can form larger bodies called planetesimals. Computer simulations have indicated that planetesimals can coalesce into planets and other objects (such as moons and asteroids) orbiting a star. Our own solar system is likely to have formed in this way, and careful measurements have detected large planets orbiting stars in other parts of the Milky Way. These findings imply that billions of planets are orbiting the many billions of stars in our galaxy.
Astrophysicists and geologists have developed a variety of ways to measure the ages of the universe, our galaxy, the solar system, and the Earth. By measuring