| Astronomy and Space Timeline Oct 13th 2011, 10:17 - Approximately 15 Billion Years Ago: The Big Bang occurred an estimated 15 billion years ago. The universe began with a cataclysm that created space and time, as well as all the matter and energy the universe will ever hold.
- Immediately After The Big Bang: One millionth of a second after the Big Bang, the universe continued to expand but not nearly as quickly. It became less dense and cooler. Gravity emerges. Matter forms. Building block particles of quarks, leptons, photons, and neutrinos, form. The universe is now about the size of a melon. Quarks combine to form protons and neutrons. Protons and neutrons form the nuclei of all atoms. Protons and neutrons combine to form the atomic nuclei of hydrogen, helium and lithium.
- 10,000 Years After The Big Bang: Radiation Era: Most energy was in the form of radiation -- different wavelengths of light, X rays, radio waves and ultraviolet rays. As the universe expanded, these waves of radiation were stretched and diluted.
- 300,000 Years After The Big Bang: Matter Domination Era: The average temperature had cooled to a mere 5000 degrees Fahrenheit. The energy in matter and the energy in radiation were equal. But as the universe continued to expand, the effects of the stretching of the light waves continued driving them into lower and lower energy, while the matter continued outward largely unaffected. Electrons could now remain in orbit around atomic nuclei. These hydrogen and helium atoms would eventually form the fuel for the stars.
- 300,000,000 Years After The Big Bang: Stars and Galaxies Form: The force of gravity began to affect the irregularities in the density of the gaseous matter. As the universe continued to expand more rapidly, pockets of gas formed, becoming more and more dense. Within these pockets, stars ignited, and as they formed, groups of them became the earliest galaxies.
- 5 Billion Years Ago: Our Solar System Forms: Our sun formed within a cloud of gas in a spiral arm of what we now call the Milky Way Galaxy. A huge cloud of gas and debris surrounded this new star gave birth to planets, moons, and asteroids.
- 3.8 Billion Years Ago: Life Appeared: The Earth cooled and an atmosphere formed. Microscopic cells began to form, which were neither plant nor animal.
- 700,000,000 Years Ago: Animals Appeared: Mostly flatworms, jelly fish and algae. 130 million years later large numbers of creatures with hard shells suddenly appear.
- 200,000,000 Years Ago: Mammals Appeared: These early mammals evolved from some reptile who had begun developing mammalian traits.
- 65,000,000 Years Ago: Dinosaurs Became Extinct: An asteroid or comet crashed into the northern part of the Yucatan Peninsula in Mexico. This catastrophic event brought an end to the long age of the dinosaurs, providing mammals with a chance to expand their ranges.
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