The Sun PDF

THE SUN INTRODUCTION Sun is a G2-type Main Sequence Star (MSS). G2-type based on spectral classification: The spectral class of sun is G2 (at the hotter end of the G type stars). MSS because it is fusing hydrogen into helium in its core: A star that is burning Hydrogen in its core is known as a M...

THE SUN INTRODUCTION Sun is a G2-type Main Sequence Star (MSS). G2-type based on spectral classification: The spectral class of sun is G2 (at the hotter end of the G type stars). MSS because it is fusing hydrogen into helium in its core: A star that is burning Hydrogen in its core is known as a Main Sequence Star. It is located in the Milky Way Galaxy. It revolves around the centre of the galaxy orbiting at a distance of about 25,000 light-years from the galactic core and completing one revolution in around 250 million years. Sun generates energy by nuclear fusion. Life on Earth could not exist without the heat and light that Sun provides. The connection and interactions between the Sun and Earth drive our seasons, ocean currents, weather, climate, radiation belts and auroras. Though it is special to us, there are billions of stars like our Sun scattered across the Milky Way galaxy. The average temperature of the Sun is around 5500 °C. Sun is sometimes informally referred to as a yellow dwarf star. Compared to the age of other stars, the Sun is not all that old. It formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Size and Distance With a radius of 695,508 kilometres, our Sun is far more massive than our home planet: 332,946 Earths would equal the mass of Sun. Sun’s volume would need 1.3 million Earths to fill it. The Sun is 150 million kilometres from Earth. Its nearest stellar neighbour is the Alpha Centauri triple star system: Proxima Centauri being the closest star at a distance of 4.24 light years, and Alpha Centauri A and B—two stars orbiting each other—next at 4.37 light years. Note: A light year is the distance light travels in one year; 1 light year = 9,460,528,400,000 km. AST-152 1 MAQ Formation & Evolution The sun was born about 4.6 billion years ago. Many scientists think the sun and the rest of the solar system formed from a giant, rotating cloud of gas and dust known as the solar nebula. As the nebula collapsed because of its gravity, it spun faster and flattened into a disk. Most of the material was pulled toward the centre to form the sun. The sun has enough nuclear fuel to stay much as it is now for another 5 billion years. After that, it will swell to become a red giant. Eventually, it will shed its outer layers (forming a planetary nebula), and the remaining core will collapse to become a white dwarf. Slowly, this will fade, to enter its final phase as a dim, cool theoretical object known as a black dwarf. Rotation of Sun The Sun rotates on its own axis. Its spin has an axial tilt of 7.25 degrees with respect to the plane of the planets’ orbits. Since the Sun is not a solid body (it is gaseous – very hot ionized gases – plasma), different parts of the Sun rotate at different speeds. It completes one rotation at its equator in 25 Earth days, while at its Poles one rotation takes about 35 - 36 Earth days. AST-152 2 MAQ Composition: Internal Structure & Atmosphere The Sun is a giant ball of gases with very high density and temperature. The Sun's enormous mass is held together by gravitational attraction. In terms of the number of atoms, Sun consists of 91.0% hydrogen and 8.9% helium. By mass, it is made of almost 70.6% hydrogen and 27.4% helium; the remaining 2% being made up of trace elements: oxygen, carbon, iron, neon, nitrogen, magnesium, iron and silicon. Because of the conditions of extreme temperature and pressure on the Sun, these elements are in a plasma state – so hot that the electrons are ripped away from the atoms forming ionized gases. The sun and its atmosphere are divided into several zones and layers. The solar interior is made up of the core, radiative zone and the convective zone. The visible portion of the Sun, which can be referred to as the surface of the Sun is called photosphere and is made of boiling gases emanating from the solar core. Above the photosphere lies the solar atmosphere, consisting of the chromosphere and the corona. SUN’S INTERIOR CORE – The core extends from the sun's centre to about a quarter of the way to its surface. It is 27 times wider than the Earth. The core has a temperature of over 15 million °C. It occupies only 2 % of the total volume of the Sun, but in it is concentrated about half the total mass of the Sun. Hence it is very dense: almost 15 times the density of lead. The resulting great pressures and temperatures in the core produce thermonuclear fusion. This is a process in which atoms combine to form larger atoms and in the process release staggering amounts of energy. Specifically, in the Sun’s core, hydrogen atoms fuse to make helium. RADIATIVE ZONE – It surrounds the core. Energy from the core is carried outward by radiation, which bounces around the radiative zone, taking about 170,000 years to get from the core to the top of the convective zone. The radiative zone extends from the core to 70 % of the way to the sun's surface, making up 32 % of the sun's volume and 48 % of its mass. TACHOCLINE – It is a transition layer that separates the radiative zone and the convective zone. Presently, it is hypothesized that a magnetic dynamo within this layer generates the Sun's magnetic field. CONVECTIVE ZONE – The region, about one-third of the Sun’s radius in width, between the outer edge of the radiative zone and the surface, is called the convective zone. It makes up 66 % of the sun's volume but only a little more than 2 % of its mass. It carries the Sun’s energy up to the surface via gas currents (through large and small convective cells). The temperature of the plasma gradually drops from 15 million °C in the core down to less than 1.5 million °C in the convective zone. AST-152 3 MAQ ATMOSPHERE OF SUN When the Sun is observed (taking great care to use the appropriate filters) it appears to have a sharp edge but there is, of course, no actual ‘surface’. We are, in fact, just seeing down through the solar atmosphere to a depth where the gas becomes what is called ‘optically thick’. PHOTOSPHERE  It is the visible surface of the Sun as well as the first layer of Sun’s atmosphere.  It is about 500 km thick.  Temperature ranges from 6,125 °C at the base of the photosphere to 4,125 °C at its top. The region has an effective temperature of 5,504 °C.  The sunlight we see radiates from this region, & takes a little over 8 minutes to reach Earth.  The spectrographic study of this layer has allowed scientists to confirm that the main components of the Sun are hydrogen and helium. CHROMOSPHERE  It is a reddish gaseous layer immediately above the photosphere of the sun.  It is an irregular layer, about 2000 km thick.  Temperatures in this region increase from 4,125 °C to about 20,000° C with increasing altitude.  Chromosphere is 10,000 times less dense than the photosphere.  It is apparently made up entirely of spiky structures known as spicules. CORONA  Located above the chromosphere, it is the third layer of the Sun's atmosphere.  It extends millions of miles into space and the temperature increases very steeply reaching 1,000,000° C.  It has some holes, or low density regions, through which gases flow into the solar wind. It can only be seen during a total solar eclipse. AST-152 4 MAQ