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This is a list of named time periods defined in various fields of study. 13.7 Billion (13,700,000,000) years ago the Big Bang. Because of the scales involved (both very large and very small), cosmological time periods are usually described in seconds. In this table, each row is defined in seconds after the Big Bang, with earliest at the top of the chart. 1 second to 100 seconds after the big bang - the Lepton Epoch 100 seconds to 300,000 years after the big bang - Epoch of Nucleosynthesis 300,000 years on from the Big Bang - Epoch of Galaxies Formation of Population III stars The first stars were formed from the Hydrogen and Helium formed in the Big Bang were short lived massive Population III stars. Nuclear processes in these stars converted the Hydrogen and Helium into metals and other heavier elements. As the Population III stars died these heavier elements were released. Formation of Population II stars Population II stars contain metals formed in the Population III stars]. These were longer lived than the Population III stars and some of them are still around. In addition to the metals these inherited from the Population III stars the Population II stars also formed metals by nuclear reactions and when the stars died much of that meterial was returned to be used as the building blocks for the next generation of stars. 5 Billion Years ago - Formation of Population I stars Population I stars are also known as metal rich stars. Our own sun is a Population I star and was formed about 5 billion years ago. Geologic time periods The geologic time scale covers the extent of the existence of Earth, from about 4600 million years ago to the present day. It is marked by Global Boundary Stratotype Sections and Points. Geologic time units are (in order of descending specificity) eons, eras, periods, epochs, and ages; and the corresponding chronostratigraphic units, which measure "rock-time", are eonothems, erathems, systems, series, and stages. The second and third timelines are each subsections of their preceding timeline as indicated by asterisks. The Cenozoic is sometimes divided into the Quaternary and Tertiary periods, although their use is no longer official. Human time periods The human timescale covers the time that humans have existed, usually taken to be from about 250,000 years ago - when homo sapiens began to develop. It is broadly divided into prehistorical (before history began to be recorded) and historical periods (when written records began to be kept). Prehistorical periods In archaeology and anthropology, prehistory is subdivided around the three-age system. The dates for each age can vary by region, although the beginning of Mesolithic is usually considered to correspond to the beginning of the Holocene epoch on the geologic time scale at the end of the most recent Ice age, about 9400 BC. Historical periods Calendar#Calendar systems|Calendar systems Various societies in the past have created calendars to record events, such as religious observances and agricultural tasks. A common characteristic of most known calendars is that they measure time in relation to a particular point in history, known as the epoch date. A period between epoch dates is known as a calendar era. Mythological and astrological time periods See also | |||||||
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