What is
the life history of stars?
Our galaxy is actually just one of several hundred billion in the Universe. The Universe is BIG. Every spec of light in the picture above is a galaxy, possibly with billions of stars of its own.
Stars do not stay the same. They are born, they get older and they eventually die. From observing thousands of stars at different stages in their lifetime astronomers have figured out their life cycles.
Our Sun's Life Cycle
Our Sun like all stars started in a cloud of hydrogen, the simplest element. (The cloud may also contain other elements left after the death of earlier stars.)
Gravity pulls the hydrogen together into clumps where the pressure and temperature gets hotter and hotter. When it gets hot enough for the protons to get close enough to fuse together then the star comes alive. Some of the dust and gas around the baby star clumps together and forms planets.
Nuclear fusion in the core of the star generates huge amounts of radiation as matter is changed into energy. This has been happening in our Sun for over 5 billion years and will continue to happen for another 5 billion. Radiation pressure pushes outwards and gravity pulls inwards. In a stable star the two are balanced. Hydrogen in the core is fused into Helium. This nuclear reaction ticks over relatively slowly so that the star's fuel does not run out quickly.
Eventually the Hydrogen in the core will start to run out. When this happens the fusion reaction will begin to spread outwards from the core. The star expands rapidly to become a massive red giant. The Earth would be swallowed by our Sun when this happens. It is now hot enough for Helium to fuse into nuclei of Carbon, Nitrogen and other smallish elements.
The red giant now collapses. It ejects its matter into space as a huge cloud called a planetary nebula leaving a very small and very hot white dwarf. Eventually this will cool to become a brown dwarf.
The Life Cycle of Big Stars
Big stars are much hotter and brighter than our Sun. Because of this they burn their fuel at a much faster rate and have much shorter lives.They are blue in colour unlike or yellow sun.
When big stars die (stars greater than 10 solar masses) they do so spectacularly in an explosion called a super nova. It is so hot that heavier elements are now formed then ejected out into space. What is left is a very small and incredibly dense object with gravity so strong that even light cannot escape. These are called black holes.
Elements
Shortly after the Big Bang the only element in the whole Universe was Hydrogen, just single protons with single electrons in orbit around them. Small elements have been put together by nuclear fusion in stars like our own. Larger elements were put together in the massive temperatures created in super nova explosions. These elements were ejected into space and some went on to form solar systems such as our own.