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Universe and the Solar system - Part 1
1.0 INTRODUCTION
The Universe is everything there is, that we humans can see and comprehend. So, it includes living things, planets, stars, galaxies, dust clouds, light, and even time! Scientists say that before the birth of the Universe, time, space and matter did not exist. The Universe contains billions of galaxies, each containing millions or even billions of stars. The space between the stars and galaxies is largely empty but contains scattered particles of dust or a few hydrogen atoms per cubic centimeter. Space is also filled with radiation (e.g. light and heat), magnetic fields and high energy particles (e.g. cosmic rays). Since the Universes is very huge, a spacecraft made by humans, travelling at the speed of light (300,000 km per second) would take 100,000 years to cross our Milky Way galaxy alone! No one knows the exact size of the Universe, because we cannot see the edge, even if there is one! The visible Universe is at least 93 billion light years across. A light year is the distance light travels in one year – about 9 trillion km. The Universe has not always been the same size. Scientists believe it began in a Big Bang, which took place nearly 14 billion years ago. Since then, the Universe has been expanding outward at very high speed. So the area of space we now see is bigger than it was when the Universe was very young. Today, the galaxies are also moving further apart as the space between them expands.
2.0 MAJOR THEORIES OF FORMATION OF THE EARTH
2.1 Nebular Theory (Kant and Laplace)
The nebular hypothesis was first developed in the 18th century by Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace.
According to this theory, the Solar System began as a nebula, a huge mass of swirling cold gas and dust, in an area in the Milky Way Galaxy. Due to some perturbation, possibly from a nearby supernova, this cloud of gas and dust began to condense, or pull together under the force of its own gravity. As more and more material was drawn towards the centre, gravity became stronger due to which the speed of the condensation increased.
As the nebula conserved the angular momentum of the material drawn towards the centre, it began spinning anticlockwise. Due to this the material around the center of the condensing nebula flattened out into a disk-like shape. The nebula at this stage had a centre and was a roughly, spherical core, surrounded by a disk. This has been observed by the Hubble Space Telescope. The remainder of the nebula theory is based more on modeling and indirect evidence.
The center of the nebula continued to contract due to gravity. Eventually, pressure and temperatures in this mass became high enough that nuclear fusion started. The central mass became a star, the Sun.
While this was happening, condensation was also occurring in the disk. Gas and dust came together to make tiny particles, which gradually joined with other particles, making larger and larger objects. These objects grew to be several hundred kilometers in diameter; they became protoplanets. The protoplanets had much stronger gravity than the very small particles of gas and dust around them. They began to behave almost like vacuum cleaners, attracting the small particles around them. Protoplanets also collided from time to time. These collisions, plus the "vacuuming" of small particles, formed the planets of the Solar System. Gravity pulled these bodies into their current spherical shapes.
As the protoplanets were formed, the disk of the nebula was whirling around the core. The protoplanets continued this motion by revolving around the newly evolved Sun. In addition, the protoplanets, and the planets, as they formed, began to rotate, or spin on an internal axis. This took place as some of the force from collisions was converted into rotational energy.
The large moons of the gas giant planets (Jupiter, Saturn, Uranus, and Neptune) formed in a similar fashion to the planets. The small moons of the gas giants, as well as the moons of Mars and Pluto are probably leftover debris from formation of the planets that were captured by their respective planets' gravity. They are captured moons. The Earth's Moon probably formed a third way, from a collision between the Earth and a large protoplanet.
The solar system is dynamic. Evolution of the solar system is still continuing. It is likely that the orbits of the planets were originally more oval-shaped, and have changed to their current nearly circular shapes with time. The number of moons around some planets has increased through gravitational capture and collisions. The strength of the Sun (the amount of solar radiation emitted) has also possibly changed.
2.2 Chamberlin-Moulton planetesimal hypothesis
The Chamberlin-Moulton planetesimal hypothesis was proposed in 1905 by geologist Thomas Chrowder Chamberlin and astronomer Forest Ray Moulton which contrasted with the more-popular nebular-gas-cloud theory.
According to this hypothesis the origin of the planets has been due to severe tidal eruptions and disruption of the sun's mass. The tidal eruptions were caused due to an approaching star due to its gravitational powers. The disrupted solar matter was thrown to long distances from the sun. The larger nuclei of the tidal ejection gathered together. As the passing star moved away, the material separated from the solar surface continued to revolve around the sun and it slowly condensed into planets.
These large bodies gathered smaller scattered bodies or Planetesimal and eventually grew into the mature planets of the solar system. Planetesimals are the tiny planets which coalesced owing to gravitational attraction of collision.
According to this theory "the total mass of the planets is only a small fraction (1/700) of the mass of the whole solar system, but they carry nearly 98% of its energy of revolution". (Wooldridge and Morgan)
The Planetesimal theory discusses how the different constituent parts of the earth evolved and how it grew from small beginnings by the addition of Planetesimal matter, rapidly at first, but with decreasing speed. It also mentions how the internal heat arose from condensation of its mass during the period of growth. When the mass of the earth condensed and became compact, molten pockets of the more readily fusible constituents were formed and forced outwards. The metallic core, the stony crust, the atmosphere, the oceans and even the lithosphere are considered to have been derived from the matter of Planetesimal.
2.3 Russel's binary star hypothesis
A binary star is a star system consisting of two stars orbiting around their common centre of mass. These systems may have two, three, four or multiple star systems. They often appear to the unaided eye as a single point of light, and are then revealed as double (or more) by other means. Research over the last two centuries suggests that half or more of visible stars are part of multiple star systems.
In order to explain the causes due to which planets have been thrown to such great distances from the sun, Prof. H.N. Russel, an American astronomer, made a suggestion in 1937 that the sun was part of a binary star system and had a companion star.
In the beginning the planets were closer together and the satellites owe their birth to the mutual gravitational attraction between them. This third star was too far away from the sun to have any impact on the latter. The third star happened to pass close to the companion star of the sun which resulted in the ejection of gaseous matter from the latter in the form of a filament which ultimately separated from it. In course of time, the planets were formed from this gaseous filament. The suggestion that the primitive sun was a binary star cannot be dismissed as mere imagination. At least 10 per cent of the stars in the universe are binary stars. In fact, in the opinion of some scholars, the number of binary stars is probably 30 per cent of the total. This hypothesis helps us to explain the great distances of the planets from the sun as well as their high angular momentum.
3.0 The Big Bang theory
The Big Bang theory is one of the most widely accepted theories for evolution of the Earth's origin. This theory states that the universe began 13.8 billion years ago by expanding from an infinitesimal volume with extremely high density and temperature. The universe was initially significantly smaller than even a pore on your skin. With the big bang, the fabric of space itself began expanding like the surface of an inflating balloon - matter simply rode along the stretching space like dust on the balloon's surface. The big bang is not like an explosion of matter in otherwise empty space; rather, space itself began with the big bang and carried matter with it as it expanded. Physicists think that even time began with the big bang. Today, just about every scientist believes in the big bang model. (In 1951, the Catholic Church officially pronounced the big bang model to be in accordance with the Bible.)
In 1915, Einstein formulated his famous general theory of relativity that describes the nature of space, time, and gravity. This theory allowed for expansion or contraction of the fabric of space. In 1917, astronomer Willem de Sitter applied this theory to the entire universe and boldly went on to show that the universe could be expanding. Aleksandr Friedmann, a mathematician and Georges Lemaître, a cosmologist and a Jesuit, reached the same conclusion in a more general way in 1922 and 1927 respectively. This step was revolutionary since the accepted view at the time was that the universe was static in size. Tracing back this expanding universe, Lemaître imagined all matter initially contained in a tiny universe and then exploding. These thoughts introduced amazing new possibilities for the universe, but were independent of observation at that time.
There are three basic arguments in support of the big bang:
- Due to the expansion of the universe, the distances between galaxies are becoming larger and larger. This leads us to the deduction that everything used to be extremely close together before some kind of explosion.
- The big bang perfectly explains the abundance of helium and other nuclei like deuterium (an isotope of hydrogen) in the universe. A hot, dense, and expanding environment at the beginning could produce these nuclei in the abundance in which they exist today.
- Astronomers can actually observe the cosmic background radiation-the afterglow of the explosion-from every direction in the universe.
4.0 The Sun
The Sun was formed when a swirling cloud of dust and gas contracted, pulling the matter into its centre. When the temperature at the centre rose to 10,00,000°C, nuclear fusion - the fusing of hydrogen into helium, creating energy - occurred, releasing a constant stream of heat and light. The sun has the following layers.
4.1 The Core
The innermost layer of the sun is the core. With a density of 150 g/cm3, 10 times that of lead, the core might be expected to be solid. However, the core's temperature of 15.7 million kelvins (27 million degrees Fahrenheit) keeps it in a gaseous state.
In the core, fusion reactions produce energy in the form of gamma rays and neutrinos. Gamma rays are photons with high energy and high frequency.
The gamma rays are absorbed and re-emitted by many atoms on their journey from the envelope to the outside of the sun. When the gamma rays leave atoms, their average energy is reduced. However, the first law of thermodynamics (which states that energy can neither be created nor be destroyed) plays a role and the number of photons increases. Each high-energy gamma ray that leaves the solar envelope will eventually become a thousand low-energy photons.
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with half-integer spin. Neutrinos do not carry electric charge, which means that they are not affected by the electromagnetic forces that act on charged particles such as electrons and protons. Neutrinos are created as a result of certain types of radioactive decay, or nuclear reactions such as those that take place in the Sun, in nuclear reactors, or when cosmic rays hit atoms. Most neutrinos passing through the Earth emanate from the Sun. About 65 billion (6.5×1010) solar neutrinos per second pass through every square centimeter perpendicular to the direction of the Sun in the region of the Earth.
The neutrinos are extremely nonreactive. To stop a typical neutrino, one would have to send it through a light-year of lead! Several experiments are being performed to measure the neutrino output from the sun. Chemicals containing elements with which neutrinos react are put in large pools in mines, and the neutrinos' passage through the pools can be measured by the rare changes they cause in the nuclei in the pools. For example, perchloroethane contains some isotopes of chlorine with 37 particles in the nucleus (17 protons, 20 neutrons). These Cl-37 molecules can take in neutrinos and become radioactive Ar-37 (18 protons, 19 neutrons). From the amount of argon present, the number of neutrinos can be calculated.
Outside of the core is the radiative envelope, which is surrounded by the convective envelope. The temperature is 4 million kelvins (7 million degrees F). This is known as the solar envelope. The density of the solar envelope is much less than that of the core. The core contains 40 percent of the sun's mass in 10 percent of the volume, while the solar envelope has 60 percent of the mass in 90 percent of the volume.
The solar envelope puts pressure on the core and maintains the core's temperature.
The hotter a gas is, the more transparent it is. Hence the solar envelope is cooler and more opaque than the core. It becomes less efficient for energy to move by radiation, and heat energy starts to build up at the outside of the radiative zone. The energy begins to move by convection, in huge cells of circulating gas several hundred kilometers in diameter. Convection cells nearer to the outside are smaller than the inner cells. The top of each cell is called a granule. Seen through a telescope, granules look like tiny specks of light. Variations in the velocity of particles in granules cause slight wavelength changes in the spectra emitted by the sun.
4.2 Photosphere
The photosphere is the bright outer layer of the Sun that emits most of the radiation, particularly visible light. It consists of a zone of burning gases 300 km thick. The photosphere is an extremely uneven surface. The effective temperature on the outer side of the photosphere is 6000°K (11,000°F). The photosphere is the zone from which the sunlight we see is emitted. The photosphere is a comparatively thin layer of low pressure gasses surrounding the envelope. The composition, temperature, and pressure of the photosphere are revealed by the spectrum of sunlight. In fact, helium was discovered in 1896 by William Ramsey, when in analyzing the solar spectrum he found features that did not belong to any gas known on earth. The newly-discovered gas was named helium in honor of Helios, the mythological Greek god of the sun.
4.3 Chromosphere
It is the second of the three main layers in Sun’s atmosphere and is approx. 2000 kms deep. Its density is only 1/10000 that of the photosphere and below the solar transition region. Its temperature reduces from 6000 k to approx 3800 k before growing to more than 35,000 k.
4.4 Sunspot
Sunspots are dark spots on the photosphere, typically with the same diameter as the Earth. They have cooler temperatures than the photosphere. The center of a spot, the umbra, looks dark gray if heavily filtered and is only 4500 K (as compared to the photosphere at 6000K). Around it is the penumbra, which looks lighter gray (if filtered). Sunspots come in cycles, increasing sharply (in numbers) and then decreasing sharply. The period of this solar cycle is about 11 years.
The sun has enormous organized magnetic fields that reach from pole to pole. Loops of the magnetic field oppose convection in the convective envelope and stop the flow of energy to the surface. This results in cool spots at the surface which produce less light than the warmer areas. These cool, dark spots are the sunspots, cooler than the surrounding chromospheres. The individual sunspot has a lifetime ranging from a few days to a few months. Each spot has a black centre or umbra, and a lighter region or penumbra, surrounding it. The number of visible sunspots fluctuates in an eleven year cycle. It has been suggested that the Sun is 1% cooler when it has no spot, and that this variation in solar radiation might affect the climates of the Earth.
4.5 Corona
The outermost layer of the sun is the corona. Only visible during eclipses, it is a low density cloud of plasma with higher transparency than the inner layers. The white corona is a million times less bright than the inner layers of the sun, but is many times larger.
The corona is hotter than some of the inner layers. Its average temperature is 1 million K (2 million degrees F) but in some places it can reach 3 million K (5 million degrees F).
Temperatures steadily decrease as we move farther away from the core, but after the photosphere they begin to rise again. There are several theories that explain this, but none have been proven.
4.6 Solar flares
In the corona, above sunspots and areas of complex magnetic field patterns, are solar flares. These sparks of energy sometimes reach the size of the Earth and can last for up to several hours. Their temperature has been recorded at 11 million K (20 million degrees F). The extreme heat produces X-rays that create light when they hit the gasses of the corona.
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