ABSOLUTE BEGINNERS

This article featured in the July 2002 Beginners Magazine

THE SUN - OUR STAR

THE BEGINNING

Before thinking about our Sun the family of planets, moons Asteroids, Comets and Meteors we must go back long before the Sun was formed to when the Universe began. The almost universally accepted theory of how the universe was formed is the Big Bang Theory. This states that every thing we can see and even time itself came into being about 15 billion (15 million million) years ago in an enormous bang. After about a million years atoms had formed from the fireball and light was able to shine. Nearly all matter at that time was in the form of the basic element Hydrogen.

From huge swirls of Hydrogen gas, galaxies and stars began to form. It is believed that these early stars were very large compared to the stars we see today. Because they were formed of almost pure Hydrogen they could disperse their energy into space quicker and easier. Some of these old stars are thought to have been up to 300 times as massive as our Sun. Very few stars today achieve a size of more than 50 times the Sun.

 

GIANT STARS

These huge stars consumed hydrogen at an enormous rate, converting it into heavier elements. Being so large these stars could only survive for a short time (probably less than a million years) then they destroyed themselves in colossal explosions. Much of the new elements formed in these stars was blasted off into space along with the remaining Hydrogen. Great shock waves blasted across space from these unbelievably violent explosions. These shock waves pushed the gas and dust from other exploding stars into clumps which formed into new stars but these stars were not pure Hydrogen they were contaminated with Helium, Carbon, Oxygen, Calcium, Iron and even Gold and all the other elements we see around us today.

 

OUR SUN FORMS

About 4500 million years ago the blast from an exploding star formed a swirling mass of gas and dust into a spinning disc. At the centre the gas became so compressed and hot that a nuclear fusion furnace ignited and our Sun was born. The remaining heavier dust particles drifted together to form larger and larger lumps until planet sized lumps had grown. At first the Sun was much more powerful and violent than it is today. It probably had huge jets of material blasting out of its poles and the radiation created a powerful solar wind shining out in all directions.

 

THE PLANETS FORM

The solar wind blasted away all the light volatile material from the disc closest to the Sun, leaving only the heavier materials in the inner planets. The lighter gases were pushed to the outer parts of the disc where it was cooler and they froze into various kinds of ices which in turn coalesced into the gas giant planets. These gas giants were so big that the compression caused them to produce internal heat and the ices again became gas. It is thought that there may have been many more planets in the beginning but there would have been many collisions and near misses. Some planets may have been destroyed and the debris reformed into new planets or may have been absorbed by other planets which grew bigger. Close encounters could move planets out of their orbits and driven some to their death in the Sun. Others may have been thrown out of the Sun's family to wander through empty space forever. Eventually all calmed down and the planets cooled and the Sun became the quiet stable star we see today. Now let us consider our Sun, the dominating matriarch of the family.

 

THE STABLE SUN

The Sun is about a million kilometres in diameter and contains nearly all the material of the solar system. It is hard to imagine but the Sun, like all the other stars, is a cloud of gas. The gravity of the cloud has compacted the cloud into the smallest volume possible, a sphere. A question which must be considered is ' what stops the Sun from collapsing even more under the enormous forces of its own gravity.' As a result of the incredible pressure at the centre of the Sun the temperature rises to millions of degrees centigrade. At this temperature and pressure a nuclear fusion reaction is generated (i.e. four hydrogen atoms are combined into one helium atom, and a large amount of energy is released). This energy creates an outward pressure at the centre of the Sun which pushes out against the force of gravity pushing in. The energy is transported by radiation in the inner 70% and by convection in the outer 30% of the Sun. On the surface the convection can be seen as granulation, which resembles bubbles. Moving currents and the rotation of the Sun form a complex magnetic field, whose strength varies and poles change in 11-year cycles. Where the lines of magnetism break through the surface, Sun Spots form.

Sometimes huge flares, known as 'prominances' erupt from the surface of the Sun throwing millions of tonnes of superheated material called 'plasma' out into space. The surface has a temperature of about 6400 degrees centigrade.

Virtually all the light and heat in the solar system comes from the Sun , without that light and heat even the atmospheres of all the planets would be frozen solid.

 

SOHO

One of the most important space missions ever launched is SOHO the Solar and Heliospheric Observatory. The spacecraft's payload module includes 650 kilograms of very advanced instrumentation for observing the Sun from its deep core to the outer corona, and outwards to the solar wind. The service module distributes power from the solar panels and provides thermal control, pointing and telecommunications. The building of these modules was done under the leadership of the prime contractor, Matra Marconi, at factories in Portsmouth, UK, and Toulouse, France where the modules were joined in August 1994.

SOHO was launched into its three-month journey to Lagrangian point No.1 (L1) (The gravitational equilibrium point between Earth and the Sun) on December 2nd, 1995, from Cape Canaveral by an Atlas-IIAS launcher. The whole mass of the spacecraft when launched was 1610 kg plus 240 kg propellant which is sufficient for at least six years of operation. When orbiting L1 between the Earth and the Sun the spacecraft can enjoy an uninterrupted view of the Sun, the heliosphere, and the solar wind particles. One L1 orbit takes about 6 months. The information from the SOHO spacecraft is controlled in NASA's Goddard Space Flight Centre near Washington DC, where the satellite's scientific data is gathered via the three ground stations of NASA's Deep Space Network (DSN) system. The overall responsibility for the spacecraft and its operations belongs to ESA.

SOHO has a robotic control system, which allows it to look after itself up to 48 hours. An onboard computer controls the orientation of the spacecraft relative to the Sun, using a fine-pointing sun sensor and star-tracking system, and three reaction wheels which keep it pointing at the Sun. The 70-metre antenna of DSN station in Goldstone, California.

 

SCIENTIFIC AIMS AND CAPABILITIES OF SOHO

Soho aims to answer the following three fundamental questions about the Sun :

What is the structure and what are the dynamics of the solar interior ?
Why does the corona exist, and how it is heated ?
Where and how is the solar wind accelerated ?

THE MAIN RESULTS IN BRIEF SO FAR:

The first images of a star's convection zone and the subsurface structure of sunspots
The most precise measurements of the temperature structure and rotation profile in the solar interior, including the discovery of a polar jet stream
The discovery of a magnetic carpet on the solar surface
The first measurements of the acceleration profile of the slow and fast solar wind
The identification of the source regions of the fast solar wind in the magnetically "open" regions at the Sun's poles
The most detailed view to date of the dynamics in the outer solar atmosphere, including the discovery of coronal "Moreton waves" and solar tornadoes
The most spectacular images and movies of coronal mass ejections

SOHO is able to monitor Coronal Mass Ejections (CME) and provide a warning if Earth is in the path of a dangerous one. If these CME's hit the Earth's magnetic field huge electric storms may be generated causing failure of satellites and even overloading of power lines. During the last maximum of solar activity a large area of Canada was 'blacked out' when one such CME struck.

 

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