The Sun is the only star in the universe that astronomers on Earth can study in detail. There is no other star close enough to show features of the order of a few hundred kilometres. In the past decade or so, astronomers have modified their understanding of the Suns processes because of new discoveries made by solar satellites. This chapter introduces the reader to the key features and behaviours of the Sun in preparation for an in-depth study of the Sun.
The Solar System
The Sun, planets and their moons form a family of bodies called the Solar System. We know that the Sun is at the centre of the Solar System and that the major planets orbit the Sun in nearly circular orbits. Our understanding of the Solar System has changed dramatically over the centuries as bigger and better telescopes were developed and new data of planetary motions was collected. Although people knew the Sun and planets of the solar system existed, little was known about the nature of these worlds until space probes containing scientific instruments were sent into space to explore these objects from close range.
The Sun is a star because it emits its own heat and light through the process of thermonuclear fusion. Planets differ to stars in that they do not emit much heat and they do not have enough mass for fusion to occur in their core. We see the planets because they reflect sunlight. The Sun and planets are spherical in shape, and are held in that shape by a force called gravity.
The planets orbit the Sun in much the same plane, and because of this they all appear to move across the sky through a common narrow path. Observed from a position above the Suns north pole, all the planets orbit the Sun in an anti-clockwise direction (Fig. ).
Fig. 1.1
The Sun as seen from the space shuttle as it orbits around Earth.
The time taken by a planet to orbit the Sun is called its period of revolution . A planets period depends on its distance from the Sun: the further a planet is from the Sun, the slower its speed and the longer its period.
The four inner planets (Mercury, Venus, Earth and Mars) are smaller, denser and rockier than the outer planets of Jupiter, Saturn, Uranus, and Neptune. The inner planets are also warmer and rotate more slowly than the outer planets. The outer planets are gaseous planets, containing mostly hydrogen and helium with some methane and ammonia. They are also cold and icy with deep atmospheres.
During the 1800s, astronomers discovered a large number of small, rocky bodies orbiting the Sun between Mars and Jupiter. Bodies such as Ceres, Pallas and Vesta, that had been thought of as small planets for almost half a century, were classified as asteroids .
Fig. 1.2
The planets in front of a disc of the Sun, with all bodies drawn to the same scale (Fig. ).
Formation of the Solar System
The Solar System is thought to have formed about 4.5 billion years ago from a vast cloud of very hot gas and dust called the solar nebula . This cloud of interstellar material began to condense under its own gravitational forces. As a result, density and pressure at the centre of the nebula began to increase, producing a dense core of matter called the protosun . Collisions between the particles in the core caused the temperature to rise deep inside the protosun.
The planets and other bodies in the Solar System formed because the solar nebula was rotating. Without rotation, everything in the nebula would have collapsed into the protosun. The rotating material formed a flat disc with a warm centre and cool edges. This explains why nearly all the planets now rotate in much the same plane. Astronomers have found similar discs around other stars.
As the temperature inside the protosun increased, light gases like hydrogen and helium were forced outward while heavy elements remained closer to the core. The heavier material condensed to form the inner planets (which are mainly rock containing silicates and metals), while the lighter, gaseous material (methane, ammonia and water) condensed to form the outer planets. Thus a planets composition depends on what material was available at different locations in the rotating disc and the temperature at each location.
The formation of the Solar System took millions of years. During this time the temperature and pressure of the protosun continued to increase. Finally the centre of the protosun became hot enough for nuclear fusion reactions to begin and the Sun was born (Fig. ).
Fig. 1.3
Stages in the formation of the Solar System: ( a ) A slowly rotating cloud of interstellar gas and dust begins to condense under its own gravity. ( b ) A central core begins to form a protosun. A flattened disc of gas and dust surrounds the protosun, and begins to rotate and flatten. ( c ) The planets begin to condense out of the flattened disc as it rotates. ( d ) The planets have cleared their orbit of debris.
Stars like our Sun can take 100 million years to form from a nebula. Radioactive data of the oldest material in our Solar System suggests it is about 4.6 billion years old.
Much of the debris leftover from the formation of the Solar System is in orbit around the Sun in two regions the Asteroid Belt and the Kuiper Belt. The Asteroid belt lies between Mars and Jupiter while the Kuiper belt is a region beyond Neptune.
The Asteroid Belt contains over a million rocky bodies. About 2,000 of these have very elliptical orbits, some of which cross Earths orbit. Stony fragments ejected during asteroid collisions are called meteoroids. Some of these meteoroids enter the Earths atmosphere and burn up releasing light; such bodies are called meteors. Large meteors that impact with the ground form craters like those seen on the surface of the Moon.
The Kuiper Belt is a bit like the Asteroid Belt, except that it is much farther from the Sun and it contains thousands of very cold bodies made of ice and rock. Objects in this outer region take up to 200 years to orbit the Sun.
There are other smaller objects on the outer edge of Solar System, in a region called the Oort cloud. This roughly spherical cloud also contains many objects left over from the formation of the Solar system. Comets are icy bodies that originate from these outer regions of the Solar System. Many comets have highly elongated orbits that occasionally bring them close to the Sun. When this happens the Suns radiation vaporizes some of comets icy material, and a long tail is seen extending from the comets head. Each time they pass the Sun, comets lose about 1% of their mass. Thus comets do not last forever. Comets eventually break apart, and their fragments often give rise to many of the meteor showers we see from Earth (Fig. ).
