ancient INTRODUCTION Many cultures cultures believed that the stars were lights attached to a huge dome the sky over Earth The stars maintained maintained fixed positions relative to each other as they moved across the heavens as if the sky dome were rotating around Earth Ancient people imagined patterns in the stars and grouped them into constellations representing representing various animals people mythological heroes and even everyday objects Some cultures attributed godlike powers to the stars and worshipped them Many also thought that the motions of the heavenly bodies bodies corresponded to or foretold events on Earth This belief shared by many cultures became the basis of astrology More practically the motions of the stars and planets during the year became the basis for calendars calendars which were crucial in the development of agriculture Also the stars became valuable tools for navigation especially for seafaring peoples such as the Phoenicians and Pacific Islanders 6
Approximately 100,000 stars at the core of the globular Omega Centauri cluster as captured by NASA’s Hubble Space Telescope In the last century scientists determined what stars are enormous balls of incandescent gas powered by nuclear fusion reactions in their cores However to just say that stars are balls of gas that shine through the workings of their internal energy does not do justice to their full nature and complexity Not all stars are like our Sun Some stars are massive giants doomed to burn away in merely millions of years Others will have violent and dramatic fates as supernovae white dwarfs neutron stars or even black holes 7
StarS and nebulae When a star goes supernova considerable amounts of its matter equaling the material of several Suns may be blasted into space with such a burst of energy as to enable the exploding star to outshine its entire home galaxy Supernovae are characterized by a tremendous rapid brightening lasting for a few weeks followed by a slow dimming A supernova explosion is a cataclysmic event for a star one that essentially ends its active i e energy-generating lifetime Supernovae release many of the heavier elements that make up the components of the solar system including Earth into the interstellar medium White dwarfs have a mass similar to that of the Sun but with a radius comparable to that of Earth making them extremely dense White dwarfs have average densities densities approaching 1,000,000 times that of water Neutron stars are any of a class of extremely dense compact stars thought to be composed primarily of neutrons neutrons Their masses range between 1 and 2 times that of the Sun Having so much mass packed within a ball on the order of 20 km 12 miles in diameter a neutron star has a density that can reach that of nuclear values which is roughly 100 trillion 1014 times the average density of solar matter or of water This approximates the density inside the atomic nucleus and in some ways a neutron star can be conceived of as a gigantic nucleus 8
IntroductIon A black hole can be formed by the death of a massive star When such a star has exhausted the internal thermonuclear thermonuclear fuels in its core at the end of its life the core becomes unstable and gravitationally collapses inward upon itself and the star’s outer layers are blown away The crushing weight of constituent matter falling in from all sides compresses the dying star to a point of zero volume and infinite density called the singularity around which nothing not even light can escape Nebulae are clouds of gas and dust that occur in the space between the stars A nebula is thus made up of the interstellar medium Some nebulae give birth to new stars and dying stars expel nebulae The Sun was formed roughly 4 5 billion years ago inside a nebula that was produced from a supernova For thousands of years people have gazed at the seemingly seemingly infinite number of stars in the night sky For most of this time they could only guess about the nature of these pinpoints of light often making them objects of wonder worship comfort or fear 9
