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THE SUN - OUR STAR

INTRODUCTION

 The Sun is a star, no different from the many stars we see at night in the sky. It looks different simply because it is closer. It is 150 million Km from the Earth and has a diameter of 1,392,000 Km. Over 1,300,000 Earths would fit inside. It has a surface gravity 28 times that of the Earth. All the energy we use comes ultimately from the Sun, even that warm log fire is releasing energy from the Sun put into the tree by photosynthesis many years ago.

 At the centre is the core where the sun’s energy is generated. Hydrogen atoms under enormous pressure are forced together to form helium - a nuclear reaction called the Proton-Proton reaction that produces energy. A hydrogen bomb operates on the same principle, in effect every star is a huge continuously exploding hydrogen bomb. Such is the density of the sun’s material that the heat energy produced at the core takes millions of years to reach the surface. Next is the radiation zone where the energy travels upward as heat and light radiation. Then is the convection zone where energy is carried up by the movement of the solar material, just like water boiling in a kettle. Finally, the surface at a temperature of 6000 degrees. The surface and the atmosphere above it has many features like sunspots, flares, prominances etc. most of which are the result of the sun’s magnetic field which ranges from twice that of the earth at the sun’s polar regions to 6000 times in a sunspot.

 THE SURFACE

 Because the Sun is made up of gas it doesn’t have a solid surface. The part of the Sun that emits light and heat is called the photosphere, it is this that we see as the surface. The photophere is at a temperature of 6000 degrees C and at those high temperatures the electrons around the nucleus of the atoms are stripped away from the atoms leaving the nucleus with a positive charge, the negatively charged electrons wander freely between the nuclei. This state of matter is called a plasma and interacts strongly with the Sun’s magnetic field. It is this interaction that produces most of the phenomena that we observe on the surface of the Sun.

 Another result of the gaseous nature of the Sun is differential rotation where different latitudes of the Sun rotate at different rates. At it’s equator the Sun rotates once every 25 days but near the poles it can take as long as 36 days. So different latitudes of the Sun are moving at different speeds. Because the surface of the Sun is a plasma and interacts strongly with the Sun’s magnetic field, the Sun’s magnetic field is locked into the Sun’s surface. As the surface is not rotating as a solid body the magnetic field becomes stretched and distorted as the Sun rotates. This creates kinks in the field. These kinks will protrude out of the Sun’s surface and extend into the solar atmosphere. The magnetic field that penetrates the surface has the effect of slowing down the movement of the atoms in that part of the surface. This cools that part of the surface by about 2000 degrees and creates a dark area we call a sunspot at a temperature of about 4000 degrees. This is still very hot but cooler than the surrounding surface.

 The number of sunspots varies with a maximum occurring every 11 years. At the start of a cycle there may be no sunspots for some months then one or two will appear at high latitudes in the northern and southern hemispheres. As the cycle progresses the number of sunspots increases and moves closer to the equator. The picture shows a recent large group of sunspots on the surface of the Sun. (Image provided by Space Environment Centre. Boulder. CO. USA) We are at present in cycle 23, which is the 23rd cycle since detailed records have been kept. From the position of the sunspots in relation to the Sun’s equator ( across the centre of the image) what point are we at in the cycle? Start, middle or end? The spots are near the equator therefore we are near the end of the cycle. Over the next year or two the number of spots will continue to decrease until the first spots of the new cycle appear at high latitudes and cycle 24 begins.

Do not attempt to look at the Sun with the naked eye or through a telescope without the correct eye protection. Sun glasses will not protect you. The Sun is very powerful and if you attempt to look at it through a telescope without the correct protection you could be blinded in seconds.

THE ATMOSPHERE

 Above the photosphere is the first layer of the Sun’s atmosphere known as the chromosphere about 2500 km thick. For reasons not fully understood the temperature of the chromosphere is higher than photosphere, the layer below it, reaching some 10,000 degrees. Faculae are bright clouds of hot Hydrogen that form in the chromosphere above sunspots. More spectacular are solar flares which can release enormous energy in a short time. They are caused by the magnetic field. As the field becomes stretched and distorted by differential rotation, it reaches breaking point and like an overstretched elastic band it will snap. The energy released creates a flare.

Above the chromosphere is the mysterious corona. It is extremely hot, in fact a lot hotter than it should be reaching temperatures of over one million degrees. As with the chromosphere the process by which the corona reaches such high temperatures is not yet understood but almost certainly involves the magnetic field. It is not normally visible but during a solar eclipse can be seen as a faint pearly glow surrounding the Sun

 Prominces are large features that form above the photosphere and often extend into the upper parts of the corona. They take the form of large loops of hot gas travelling at speeds in excess of 1000 km per second and reaching over a million km above the surface. They follow the magnetic field lines that arch up from the sunspot zones produced by the distortion of the Sun’s magnetic field resulting from the differential rotation of the Sun. They can hang in the Sun’s atmosphere for days.

THE SOLAR WIND

The corona is not even, there are large areas called coronal holes through which matter to flows out from the Sun, this is called the solar wind. It consists of sub-atomic particles such as electrons and protons with some heavier charged atoms called ions which travel away from the Sun with speeds varying between 450 and 750 kilometres per second. The solar wind fills the solar system reaching far beyond the orbit of Pluto. It is the solar wind that allows the Sun to influence the Earth.

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HISTORY OF THE UNIVERSE 2 - DARK AGES

For 200 million years after the fading of the Big Bang fireball the universe lay in total darkness. This period is called the “Dark Ages”. If human eyes could have been there they would have detected nothing, all would have seemed quiet, dead, and dark. However, within the clouds of Hydrogen that existed throughout the universe mighty forces were stirring. Gravity did its work causing the dark clouds to slowly collapse under their own weight forming embrionic galaxies. Compared to modern galaxies these were small, only 30 – 100 light years across. Under the influence of gravity the gas fragmented to form stars. As these fragments collapsed and became denser they heated up, as any gas does when it is compressed. The collapse continued, the fragments became denser and hotter. Eventually the centre of the collapsing fragment reached thousands of degrees but still the collapse continued.

The fragment had taken on a spherical shape and had started to spin, it had become a “Proto-Star.” Magnetic fields sprang from the swirling hot gas creating jets of material shooting from the north and south poles of the proto-star.

Finally core of the proto-star reached a temperature of many millions of degrees. The Hydrogen atoms travelling at high speed because of the extreme temperatures slammed into one another. Some stuck together and through a nuclear process became Helium atoms. Substantial energy was released causing the star to shine. The proto-star had become a star.

Throughout the universe stars were starting to shine. The first generation of stars lit up the universe dispelling 200 million years of darkness.

© Matthew Wallace 2008

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HISTORY OF THE UNIVERSE 1-THE BEGINNING

The universe is really big. It is so mind-numbingly huge that the human mind cannot grasp it. Most astronomers believe that it goes on and on forever.

If you get the chance to get away from the light polluted skies of the towns and cities to a place where the skies are dark, look up. Consider the thousands of points of light, the moon, the vast star clouds of the Milky Way. Take time to let it all sink in. There it is – right in your face: infinity.

It all began 13.7 billion years ago. The universe came into existence infinitely hot, infinitely dense and began to expand. This is called the “Big Bang”, the beginning of the universe, the start of physical existence, the beginning of time and space. The expansion continues to this day, the galaxies are moving apart, the further away they are the faster they seem to recede. In fact the rate of expansion is increasing driven by a little understood factor called “Dark Energy”. This dark energy acts as a sort of negative gravity – pushing the universe apart rather than drawing it together as gravity would do.

For the first few millionths of a second the universe was so hot and so dense that the type of matter we are familiar with could not exist. Instead of the electrons, protons and neutrons of normal matter there existed particles called quarks and gluons. The extreme energies of the very early universe allowed these particles to have an independent existence forming a “Quark – Gluon soup”. These particles still exist today but only as constituents of the more familiar protons and neutrons.

As the universe continued to expand and cool protons, neutrons, and electrons formed. The protons and neutrons joined together in various combinations to form the nuclei of Hydrogen, Deuterium, and Helium with traces of Tritium and Lithium. The electrons were not able to join with the nuclei until the universe had cooled to about 3000 degrees. This happened when the universe was 380,000 years old. The free ranging electrons were then captured by the various nuclei and atoms of Hydrogen and Helium were formed.

So ended the first phase of the universe’s development. The universe was dark and filled with clouds of Hydrogen and Helium.

The stage was set for the next act in the cosmic drama: the formation of the stars.

. . . . . . . . . . . . . . .

Over the next few months we will be looking at the history of the universe from the big bang to the present day.

© Matthew Wallace 2008

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