1.1 OUR SOLAR SYSTEM
Our solar system is the one we know the most about. The Sun is at its centre. Our solar system includes everything that orbits or goes around the Sun. Planets, moons, asteroids; comets and dust are all part of the solar system. Our solar system lies near the edge of the Milky Way galaxy. The Milky Way is shaped like a whirlpool. All the stars in the galaxy, including our Sun, orbit around the centre of the Milky Way. With the discovery of Pluto in 1930, astronomers considered the Solar System to have nine planets. Pluto was classified as the ninth planet and it remained so for 75 years but in 2006, International Astronomical Union classified Pluto as a ‘dwarf planet’ due to its smaller mass. In view of this, our solar system now consists of eight planets. Mercury, Venus, Earth and Mars are known as the inner planets, as they are nearest to the sun. They form a group of rocky planets. The outer planets are Jupiter, Saturn, Uranus and Neptune.
1.1.1 The Sun
The sun is a bright star, and it is the largest object in our solar system. At sunset or sunrise when it is safe to look at the sun, we can see the sun’s photosphere, the level in the sun from which visible photons escape most easily. Dark sunspots come and go on the sun but only rarely are they large enough to be visible to the unaided eye. The solar atmosphere consists of three layers of hot, low-density gas: the photosphere, chromosphere, and corona. The granulation of the photosphere is produced by convection currents of hot gas rising from below. Larger super-granules appear to be caused by larger convection currents deeper in the sun. The chromosphere is most easily visible during total solar eclipses, when it flashes into view for a few seconds. It is a thin, hot layer of gas just above the photosphere, and its pink color is caused by the Balmer emission lines in its spectrum. Filtergrams of the chromosphere reveal spicules, flame like structures extending upward into the lower corona. The corona is the sun’s outermost atmospheric layer and can be imaged using a coronagraph. It is composed of a very-low-density; very hot gas extending many solar radii from the visible sun. It’s high temperature — over 2 million K — is believed to be maintained by the magnetic field extending up through the photosphere — the magnetic carpet— and by magnetic waves coming from below the photosphere. Parts of the corona give rise to the solar wind, a breeze of low density ionized gas streaming away from the sun.
Astronomers can study the motion, density, and temperature of gases inside the sun by analyzing the way the solar surface oscillates. Known as helioseismology, this field of study requires large amounts of data and extensive computer analysis.
Nuclear reactors on Earth generate energy through nuclear fission, during which large nuclei such as uranium break into smaller fragments called fission. The sun generates its energy through nuclear fusion, during which hydrogen nuclei fuse to produce helium nuclei. In nuclear fission or nuclear fusion, the energy comes from the strong force.