2006年9月17日 13:59 (0 评论)
- Avijit
- 26, 英国
The history of Earth covers approximately 4.567 billion years (4,567,000,000 years), from Earth’s formation out of the solar nebula to the present. This article presents a broad overview, summarizing the leading scientific theories. Due to the difficulty of comprehending very large amounts of time, the analogy of a single 24-hour period will be used, beginning exactly 4.567 billion years ago, at the formation of Earth, and ending now. Each second of this period represents approximately 53,000 years. The Big Bang and origin of the universe, estimated at occurring 13.7 billion years ago,[1] is equivalent to taking place almost three days ago—two whole days before our clock began to tick.
Solar System:-
The Solar System, or solar system, is the stellar system comprising the Sun and the retinue of celestial objects gravitationally bound to it: the eight planets, their 162 known moons[1], three currently identified dwarf planets and their four known moons, and thousands of small bodies. This last category includes asteroids, meteoroids, comets, and interplanetary dust.
The principal component of the Solar System is the Sun (astronomical symbol ); a main sequence G2 star that contains 99.86% of the system's known mass and dominates it gravitationally.[2] Because of its large mass, the Sun has an interior density high enough to sustain nuclear fusion, releasing enormous amounts of energy, most of which is radiated into space in the form of electromagnetic radiation, including visible light. The Sun's two largest orbiting bodies, Jupiter and Saturn, account for more than 90% of the system's remaining mass. (The currently hypothetical Oort cloud, should its existence be confirmed, would also hold a substantial percentage).[3]
In broad terms, the charted regions of the Solar System consist of the Sun, four rocky bodies close to it called the terrestrial planets, an inner belt of rocky asteroids, four gas giant planets, and an outer belt of small, icy bodies known as the Kuiper belt. In order of their distances from the Sun, the planets are Mercury ( ), Venus ( ), Earth ( ), Mars ( ), Jupiter ( ), Saturn ( ), Uranus ( ), and Neptune ( ). All planets but two are in turn orbited by natural satellites (usually termed "moons" after Earth's Moon), and every planet past the asteroid belt is encircled by planetary rings of dust and other particles. The planets, with the exception of Earth, are named after gods and goddesses from Greco-Roman mythology.
From 1930 to 2006, Pluto ( ), one of the largest known Kuiper belt objects, was considered the Solar System's ninth planet. However, in 2006 the International Astronomical Union (IAU) created an official definition of the term "planet"[4]. Under this definition, Pluto is reclassified as a dwarf planet, and there are eight planets in the Solar System. In addition to Pluto, the IAU currently recognizes two other dwarf planets: Ceres ( ) , the largest asteroid, and Eris, which lies beyond the Kuiper belt in a region called the scattered disc. Of the known dwarf planets, only Ceres has no moons.
For many years, the Solar System was the only known example of planets in orbit around a star. The discovery in recent years of many extrasolar planets has led to the term "solar system" being applied generically to all stellar systems. Technically, however, it should strictly refer to Earth's system only, as the word "solar" is derived from the Sun's Latin name, Sol. Other stellar systems or planetary systems are usually referred to by the names of their parent star; "the Alpha Centauri system" or "the 51 Pegasi system".
Capitalization of the name varies. The IAU, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects (Solar System). However, the name is commonly rendered in lower case (solar system) including in the Oxford English Dictionary, Merriam-Webster's 11th Collegiate Dictionary, and Encyclopædia Britannica.
Formation:-
The current hypothesis of Solar System formation is the nebular hypothesis, first proposed in 1755 by Immanuel Kant and independently formulated by Pierre-Simon Laplace.[6] The nebular theory holds that the Solar System was formed from the gravitational collapse of a gaseous cloud called the solar nebula. It had a diameter of 100 AU and was 2–3 times the mass of the Sun. Over time, a disturbance (possibly a nearby supernova) squeezed the nebula, pushing matter inward until gravitational forces overcame the internal gas pressure and it began to collapse. As the nebula collapsed, conservation of angular momentum meant that it spun faster, and became warmer. As the competing forces associated with gravity, gas pressure, magnetic fields, and rotation acted on it, the contracting nebula began to flatten into a spinning protoplanetary disk with a gradually contracting protostar at the center.
From this cloud and its gas and dust, the various planets formed. The inner solar system was too warm for volatile molecules like water and methane to condense, and so the planetesimals which formed there were relatively small (comprising only 0.6% the mass of the disc) and composed largely of compounds with high melting points, such as silicates and metals. These rocky bodies eventually became the terrestrial planets. Farther out, the gravitational effects of Jupiter made it impossible for the protoplanetary objects present to come together, leaving behind the asteroid belt. Farther out still, beyond the frost line, Jupiter and Saturn developed as large gas giants, while Uranus and Neptune captured much less gas and are known as ice giants because their cores are believed to be made mostly of ice, that is, hydrogen compounds.
The gas giants were massive enough to retain a "primary atmosphere" of hydrogen and helium captured from the surrounding solar nebula. The terrestrial planets eventually lost their retained hydrogen and helium, and subsequently generated their own "secondary atmospheres" via volcanism, comet impacts, and, in Earth's case, the evolution of life.
After 100 million years, the pressure and density of hydrogen in the centre of the collapsing nebula became great enough for the protosun to begin thermonuclear fusion, which increased until hydrostatic equilibrium was achieved. The young Sun's solar wind then cleared away all the gas and dust in the protoplanetary disk, blowing it into interstellar space, thus ending the growth of the planets.
Sun:-
The Sun is the Solar System's parent star, and far and away its chief component. It is classed as a moderately large yellow dwarf. However, this name is misleading, as on the scale of stars in our galaxy, the Sun is rather large and bright. Stars are classified based on their position on the Hertzsprung-Russell diagram, a graph which plots the brightness of stars against their surface temperature. Generally speaking, the hotter a star is, the brighter it is. Stars which follow this pattern are said to be on the main sequence, and the Sun lies right in the middle of it. This has led many astronomy textbooks to label the Sun as "average;" however, stars brighter and hotter than it are rare, whereas stars dimmer and cooler than it are common. The vast majority of stars are dim red dwarfs, though they are under-represented in star catalogues as we can observe only those few that are very near the Sun in space.
The Sun's position on the main sequence means, according to current theories of stellar evolution, that it is in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion, and been forced, as older red giants must, to fuse more inefficient elements such as helium and carbon. The Sun is growing increasingly bright as it ages. Early in its history, it was roughly 75 percent as bright as it is today.[7]Calculations of the ratios of hydrogen and helium within the Sun suggest it is roughly halfway through its life cycle, and will eventually begin moving off the main sequence, becoming larger, brighter and redder, until, about five billion years from now, it too will become a red giant.
The Sun is a population I star, meaning that it is fairly new in galactic terms, having been born in the later stages of the universe's evolution. As such, it contains far more elements heavier than hydrogen and helium ("metals" in astronomical parlance) than older population II stars such as those found in globular clusters. Since elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, the first generation of stars had to die before the universe could be enriched with them. For this reason, the very oldest stars contain very little "metal", while stars born later have more. This high "metallicity" is thought to have been crucial in the Sun's developing a planetary system, because planets form from accretion of metals.[8]
The heliospheric current sheetThe Sun radiates a continuous stream of charged particles, a plasma known as solar wind, ejecting it outwards at speeds greater than 2 million kilometres per hour, creating a very tenuous "atmosphere" (the heliosphere), that permeates the solar system for at least 100 AU. This environment is known as the interplanetary medium. Small quantities of cosmic dust (some of it arguably interstellar in origin) are also present in the interplanetary medium and are responsible for the phenomenon of zodiacal light. The influence of the Sun's rotating magnetic field on the interplanetary medium creates the largest structure in the solar system, the heliospheric current sheet.[9]
Earth's magnetic field protects its atmosphere from interacting with the solar wind. However, Venus and Mars do not have magnetic fields, and the solar wind causes their atmospheres to gradually bleed away into space.
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Solar System:-
The Solar System, or solar system, is the stellar system comprising the Sun and the retinue of celestial objects gravitationally bound to it: the eight planets, their 162 known moons[1], three currently identified dwarf planets and their four known moons, and thousands of small bodies. This last category includes asteroids, meteoroids, comets, and interplanetary dust.
The principal component of the Solar System is the Sun (astronomical symbol ); a main sequence G2 star that contains 99.86% of the system's known mass and dominates it gravitationally.[2] Because of its large mass, the Sun has an interior density high enough to sustain nuclear fusion, releasing enormous amounts of energy, most of which is radiated into space in the form of electromagnetic radiation, including visible light. The Sun's two largest orbiting bodies, Jupiter and Saturn, account for more than 90% of the system's remaining mass. (The currently hypothetical Oort cloud, should its existence be confirmed, would also hold a substantial percentage).[3]
In broad terms, the charted regions of the Solar System consist of the Sun, four rocky bodies close to it called the terrestrial planets, an inner belt of rocky asteroids, four gas giant planets, and an outer belt of small, icy bodies known as the Kuiper belt. In order of their distances from the Sun, the planets are Mercury ( ), Venus ( ), Earth ( ), Mars ( ), Jupiter ( ), Saturn ( ), Uranus ( ), and Neptune ( ). All planets but two are in turn orbited by natural satellites (usually termed "moons" after Earth's Moon), and every planet past the asteroid belt is encircled by planetary rings of dust and other particles. The planets, with the exception of Earth, are named after gods and goddesses from Greco-Roman mythology.
From 1930 to 2006, Pluto ( ), one of the largest known Kuiper belt objects, was considered the Solar System's ninth planet. However, in 2006 the International Astronomical Union (IAU) created an official definition of the term "planet"[4]. Under this definition, Pluto is reclassified as a dwarf planet, and there are eight planets in the Solar System. In addition to Pluto, the IAU currently recognizes two other dwarf planets: Ceres ( ) , the largest asteroid, and Eris, which lies beyond the Kuiper belt in a region called the scattered disc. Of the known dwarf planets, only Ceres has no moons.
For many years, the Solar System was the only known example of planets in orbit around a star. The discovery in recent years of many extrasolar planets has led to the term "solar system" being applied generically to all stellar systems. Technically, however, it should strictly refer to Earth's system only, as the word "solar" is derived from the Sun's Latin name, Sol. Other stellar systems or planetary systems are usually referred to by the names of their parent star; "the Alpha Centauri system" or "the 51 Pegasi system".
Capitalization of the name varies. The IAU, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects (Solar System). However, the name is commonly rendered in lower case (solar system) including in the Oxford English Dictionary, Merriam-Webster's 11th Collegiate Dictionary, and Encyclopædia Britannica.
Formation:-
The current hypothesis of Solar System formation is the nebular hypothesis, first proposed in 1755 by Immanuel Kant and independently formulated by Pierre-Simon Laplace.[6] The nebular theory holds that the Solar System was formed from the gravitational collapse of a gaseous cloud called the solar nebula. It had a diameter of 100 AU and was 2–3 times the mass of the Sun. Over time, a disturbance (possibly a nearby supernova) squeezed the nebula, pushing matter inward until gravitational forces overcame the internal gas pressure and it began to collapse. As the nebula collapsed, conservation of angular momentum meant that it spun faster, and became warmer. As the competing forces associated with gravity, gas pressure, magnetic fields, and rotation acted on it, the contracting nebula began to flatten into a spinning protoplanetary disk with a gradually contracting protostar at the center.
From this cloud and its gas and dust, the various planets formed. The inner solar system was too warm for volatile molecules like water and methane to condense, and so the planetesimals which formed there were relatively small (comprising only 0.6% the mass of the disc) and composed largely of compounds with high melting points, such as silicates and metals. These rocky bodies eventually became the terrestrial planets. Farther out, the gravitational effects of Jupiter made it impossible for the protoplanetary objects present to come together, leaving behind the asteroid belt. Farther out still, beyond the frost line, Jupiter and Saturn developed as large gas giants, while Uranus and Neptune captured much less gas and are known as ice giants because their cores are believed to be made mostly of ice, that is, hydrogen compounds.
The gas giants were massive enough to retain a "primary atmosphere" of hydrogen and helium captured from the surrounding solar nebula. The terrestrial planets eventually lost their retained hydrogen and helium, and subsequently generated their own "secondary atmospheres" via volcanism, comet impacts, and, in Earth's case, the evolution of life.
After 100 million years, the pressure and density of hydrogen in the centre of the collapsing nebula became great enough for the protosun to begin thermonuclear fusion, which increased until hydrostatic equilibrium was achieved. The young Sun's solar wind then cleared away all the gas and dust in the protoplanetary disk, blowing it into interstellar space, thus ending the growth of the planets.
Sun:-
The Sun is the Solar System's parent star, and far and away its chief component. It is classed as a moderately large yellow dwarf. However, this name is misleading, as on the scale of stars in our galaxy, the Sun is rather large and bright. Stars are classified based on their position on the Hertzsprung-Russell diagram, a graph which plots the brightness of stars against their surface temperature. Generally speaking, the hotter a star is, the brighter it is. Stars which follow this pattern are said to be on the main sequence, and the Sun lies right in the middle of it. This has led many astronomy textbooks to label the Sun as "average;" however, stars brighter and hotter than it are rare, whereas stars dimmer and cooler than it are common. The vast majority of stars are dim red dwarfs, though they are under-represented in star catalogues as we can observe only those few that are very near the Sun in space.
The Sun's position on the main sequence means, according to current theories of stellar evolution, that it is in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion, and been forced, as older red giants must, to fuse more inefficient elements such as helium and carbon. The Sun is growing increasingly bright as it ages. Early in its history, it was roughly 75 percent as bright as it is today.[7]Calculations of the ratios of hydrogen and helium within the Sun suggest it is roughly halfway through its life cycle, and will eventually begin moving off the main sequence, becoming larger, brighter and redder, until, about five billion years from now, it too will become a red giant.
The Sun is a population I star, meaning that it is fairly new in galactic terms, having been born in the later stages of the universe's evolution. As such, it contains far more elements heavier than hydrogen and helium ("metals" in astronomical parlance) than older population II stars such as those found in globular clusters. Since elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, the first generation of stars had to die before the universe could be enriched with them. For this reason, the very oldest stars contain very little "metal", while stars born later have more. This high "metallicity" is thought to have been crucial in the Sun's developing a planetary system, because planets form from accretion of metals.[8]
The heliospheric current sheetThe Sun radiates a continuous stream of charged particles, a plasma known as solar wind, ejecting it outwards at speeds greater than 2 million kilometres per hour, creating a very tenuous "atmosphere" (the heliosphere), that permeates the solar system for at least 100 AU. This environment is known as the interplanetary medium. Small quantities of cosmic dust (some of it arguably interstellar in origin) are also present in the interplanetary medium and are responsible for the phenomenon of zodiacal light. The influence of the Sun's rotating magnetic field on the interplanetary medium creates the largest structure in the solar system, the heliospheric current sheet.[9]
Earth's magnetic field protects its atmosphere from interacting with the solar wind. However, Venus and Mars do not have magnetic fields, and the solar wind causes their atmospheres to gradually bleed away into space.
continue.........next post