Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A star is a massive, luminous sphere of plasma held together by gravity and our Milky Way galaxy alone contains a minimum of 100 billion of them. A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, some of the hydrogen is steadily converted into helium through the process of nuclear fusion. Learn more about stars, their birth, life and spectacular death as well as the science that studies them in the following pages.