*Includes pictures
*Includes a bibliography for further reading
When people look up into the night sky, the stars seem fixed and immutable, as unchanging as the darkness of space itself, but the truth is that stars are born, live and die in a never-ending cycle of creation and annihilation. These cycles stretch over such vast spans of time that, to short-lived humans, they seem to last forever.
No one knows just how many stars there are, but their number is almost beyond comprehension. When people look up into the night sky, they can see further than they might guess: up to 19 quadrillion miles, the distance to Deneb in Cygnus, a star that is visible from most inhabited parts of Earth. In total, around five thousand stars are visible to the naked eye, though only around two thousand are visible at any one time from a particular place on Earth. All the visible stars are bigger and brighter than the Sun. Of course, there are many more known stars than those that can be seen with the naked eye. Astronomers estimate that in the Milky Way, there may be more than three hundred billion stars, and every other galaxy may have a similar number of stars. How many galaxies are there in the Universe? Again, no one is certain, but most astronomers agree that there must be many billions.
Stars begin as vast clouds of dust and gas within galaxies and are known as nebulae. Due to Newton's Law of Global Attraction, the densest areas in these nebulae pull-in matter from the surrounding space. The more mass they gain, the more mass they attract. Over time, this accumulation can lead to the creation of a star. From that moment on, an eternal battle begins: gravity tends to contract the star while its growing inner pressure tends to expand it. Nebulae are stellar nurseries, the places where stars are created and an essential part of the life cycle of the Universe.
Stars do not last forever. Over time they gradually lose energy and finally die. This process of the creation of new stars and the gradual death of existing stars is part of a vast, cosmic process of recycling that continues all the time. However, that raises the question of how the very first stars were formed and that in turn leads to questions about the origin of the Universe itself.
However, the life cycle of stars also has a direct relationship to life here on Earth. Singer Joni Mitchell famously included the line "we are stardust" in her hit song "Woodstock." Surprisingly, it seems that she was absolutely right. In the beginning, the Universe comprised hydrogen, small quantities of helium, minuscule amounts of lithium and almost nothing else. Stars are the engines that provide the raw material from which life itself as well as stellar bodies are created. Each star is like a factory that uses nuclear fusion to convert hydrogen into helium and that in turn is used to create carbon, nitrogen, oxygen and many other elements. When a star dies, it ejects its outer layers, throwing these elements off as cosmic dust. The gravity of planets attracts and captures this dust which settles on the surface, introducing new elements. It is estimated that more than forty thousand tons of cosmic dust arrives on Earth every year and this process has continued as long as there has been a planet Earth.
Some of the tiny pieces of dust (most are smaller than one-hundredth the width of a human hair) are very old indeed. Scientists have found what they call "original stardust" on meteorites and asteroids. Many of these have been drifting in space since before the Sun was created. The elements in this dust are the fundamental building-blocks of life and every living organism on Earth is created from elements that were originally produced in long-dead stars. It seems that humans and everything else on the planet really did begin as stardust.