g our galaxy alone. Being a star the Sun is an example of the fundamental building blocks of our universe. It formed, 4.5 billion years ago, as the nucleus of a cloud of gas which was collapsing under its own gravitational attraction. At formation, hydrogen was the most abundant gas, as elsewhere in the universe, and accounted for three quarters of the Sun's original material. This will changes as the hydrogen is burned, and within the very central regions virtually all the hydrogen has been converted to helium. The outer regions have not yet taken part in hydrogen burning. Astronomers have measured the chemical composition of the Sun, and can thus estimate that of the initial solar nebula from which the sun and planets formed. In addition to 78 percent by weight of hydrogen, they find 20 percent to be helium, while only 2 percent remains for other elements such as oxygen, carbon, nitrogen, and iron.Later, as its hydrogen becomes depleted, it will evolve into a giant red star, swelling to engulf the earth and the inner planets. The remnant Sun will fade gradually to oblivion, passing through the white dwarf stage on its way. MercuryMercury is named after the messenger of the Roman gods. It is the smallest of the inner planets and the second smallest in the whole Solar System. It has a very weak magnetic field and - being the closest planet to the Sun - has only a very thin atmosphere of helium captured from the solar wind. The surface of Mercury is very much like the Moon's - with craters, mountains and valleys. Since there is no form of atmosphere, life on Mercury is impossible. Nor will there be any manned flights in the forseeable future. However, there will be new unmanned probes sent out, if only to complete the map of the surface.Although Mercury has only a third the diameter of Earth, its density is about the same. This suggests that 65 to 70 percent of Mercury's weight is made up of a heavy material, probably iron. This is conce...
