Above: Keplers Supernova. Image © NASA
Look up at the stars. They may seem like permanent fixtures in the night sky, but did you know that stars eventually die? The life cycle and death of stars form the ingredients that make up Earth, making stars critical to life as we know it.
Before stars, the universe contained nothing but hydrogen, helium, and tiny amounts of lithium and beryllium. During their life cycles, stars create some of the light (low mass) elements that are found on Earth. When the star dies, those light elements spread across the universe. Some stars die out quietly and form beautiful astronomical bodies called planetary nebula. Others die in a spectacular explosive event called a supernova. Scientists believe that many of the elements found on Earth were formed during supernovae explosions. So how do stars and supernovae create new elements?
Did you know? A supernova is so powerful that it can outshine an entire galaxy of 100,000,000,000 stars!
The Hubble Planetary Nebula: The gaseous outer layers of a Sun-like star glow in space after a star, near the end of its life, releases them. Image © NASA, Wikimedia Commons
The life cycle of a star
Early in the history of the universe, before stars and planets existed, giant clouds of hydrogen and helium began to form. Slowly, these clouds collected enough mass for their own gravity to create an extremely dense ball of gas: a star.
When a new star is formed, its core is under extreme gravitational pressure. This pressure is so great that the star is in danger of collapsing in on itself. Luckily, nuclear fusion provides the energy the star needs to push back against the collapsing core. Nuclear fusion is a process where two or more elements combine to produce heavier elements plus energy.
In the core of the newly formed star, hydrogen atoms begin to fuse into helium. The inward pull of gravity and the outward push of nuclear fusion equalize. For a time, hydrogen fusion prevents the collapse of the star.
Did you know: The closest star to Earth, our Sun, is currently fusing hydrogen atoms into helium.
When the young star runs out of hydrogen, its core will once again begin to collapse. The extreme compression of the core causes it to heat up. Soon, the core is hot enough that it can begin to fuse helium into carbon and oxygen. Once again, nuclear fusion pushes back against gravity to prevent the star from collapsing. One by one, the star fuses each new element. This successively produces light elements like carbon, oxygen, and neon. Not only does nuclear fusion keep stars from collapsing, it enabled the first stars in the universe to create new elements that had never existed before!
The death of a star
What happens to a star when it dies depends on the size of the star. Low mass stars, about the mass of the Sun, grow up to become red giant stars. As they age, their outer layers expand and eventually drift out into the universe. Between a few thousand and a billion years later, the red giant will become a planetary nebula.
Did you know: Five billion years from now, the Sun will become a red giant star. It will grow so huge that it will swallow Mercury, Venus and possibly Earth before becoming a planetary nebula.
Large stars mature much differently. Their cores are hot enough that they can continue fusing heavier and heavier elements. Millions of years after the star formed, it will have fused the lightest elements on the periodic table, up to iron. After iron, fusion reactions do not release energy and the star’s core can no longer push back against gravity. In a fraction of a second, the core collapses inward. The outer layers implode with so much energy that the star rebounds off itself, creating a supernova.
The initial explosion of a supernova has so much energy that it can split atoms from the core apart, creating an abundance of protons and neutrons. In the moments following the explosion, these particles crash into each other with enough energy to fuse back together. Light elements continue colliding with protons and neutrons this way, constantly growing larger and larger. This process, called nucleosynthesis, occurs when two or more atomic nuclei combine with large amounts of energy to form heavier nuclei. The nucleosynthesis that occurs during a supernova produces elements heavier than iron, which cannot be created by nuclear fusion. When the first stars died out this way, brand new elements including gold were formed. Eventually, those elements ended up here on Earth.
After a supernova, all that remains of the star is a dead core. Usually, the core becomes a neutron star, an extremely small and dense type of star. For the largest stars of all, the remnant core is so massive and has such a strong gravitational pull that not even light can escape. This is called a stellar black hole.
No matter how a star dies, its life cycle can transform the universe. Without stars, the universe would contain nothing but clouds of hydrogen and helium. It is the life and death of stars which is responsible for the atoms that make up everything you see on Earth!
Did you know: Models of supernova explosions predict the creation of elements that aren’t even found on Earth! Scientists call them exotic nuclei.