Chapter III: Where Stars Die
The Extinguishing Light
For centuries, the heavens have been a source of solace. Where Earthly life is ephemeral, the stars were eternal. Yet, despite this long held belief that dichotomises the heavens from the Earth, modern astronomy shows us a very different view.
The constancy of the heavens is all but an illusion, an illusion that stems from the brevity of our human lives. Peering through the lens of powerful telescopes, this view of the cosmos has been shattered by one of continuous creation and destruction.
Every second, the Sun uses up 620 million metric tons of hydrogen, converting it into helium and generating voluminous amounts of energy thereafter. While this number is large, the sheer mass of the Sun means that even at this rate, it has enough fuel to keep going for another 5 billion years. Although this means that on human timescales, there is nothing for us to worry about, this also means that the Sun will die one day, albeit in the distant future.
As the Sun uses up its nuclear fuel within, it will begin to swell and expand. One day, the Sun will rise on the last perfect day on Earth. Slowly but surely, the Sun will grow into a Red Giant, engulfing the inner planets one by one. As the Sun expands, the Earth is slowly torched into an inferno, eventually following the fate of the inner planets as Sun expands beyond Earth's orbit.
As the Sun expands, it gradually loses its ability to hold itself together. The swelling atmosphere of the Red Giant Sun is slowly blown into surrounding space, forming a shell-like structure known as a planetary nebula.
One such example of a planetary nebula is the Helix Nebula (left), which gives us a picture of the future of the Sun, long after the Earth is gone. The hot core of the star is left behind, gradually cooling and radiating energy that lights up the surrounding shell of gas. Such is the fate of the lower mass stars like the Sun.
The Most Violent Explosions in the Universe
Nature always seems to have a way of creating twists. While stars like the Sun enter a relatively quiescent death, fading gradually into the darkness, their larger and heavier counterparts seem to do quite the opposite. For stars that weigh in more than 8 times the mass of the Sun, the end is much more spectacular. Here, we enter the realm of the Supernova: explosions so powerful that the only thing more energetic is the creation of the Universe itself.
The Carina Nebula (pictured above) is the brightest nebula in the night sky, and a region of extensive star formation. Despite bring a birthplace of stars, it holds within its dense gas clouds a ticking time bomb. At the heart of the nebula lies Eta Carinae, a star so massive and luminous that it shines with over 5 million times the energy of the Sun. Like all other stars, Eta Carinae is slowly using up its fuel within. As this happens, heavier and heavier elements are produced within its core in order to produce energy to sustain itself. However, this process cannot last forever: when iron is produced, energy can no longer be produced, and the sheer weight of the star crushes the core into a single point.
As this cosmic drama ensues, the collapse of the core creates a rebound effect, sending an enormous shockwave from within the centre of the star. This shockwave reignites all the layers of the star as it travels outward, sending the atmosphere of the star hurtling outwards into space at speeds up to 30,000 kilometres a second. The amount of energy released is so great that the star, for a brief moment, outshines the rest of the hundreds of billions of stars in its galaxy.
From Death comes Life
In the ferocity of supernova explosions lies one of the most important driving forces of creation. Before the deaths of these large stars, heavier elements were forged in the nuclear cores within. These elements were essential for all life on Earth, and would not be available in such abundance if not for the dispersing effect of supernovae.
During the explosion itself, the intense shockwave also rapidly triggers the formation of new elements, especially increasing the yield of the heavier elements.
Supernovae therefore seed the Universe with ingredients necessary for life, and much of the elements in our bodies came from stars that had lived and died in these spectacular explosions in the distant past. While supernovae signify the death of these stars, they enrich the surrounding interstellar region, eventually being the life-giving force on planets orbiting subsequent generations of stars like the Sun.
Supernova remnants are of exquisite beauty, both from their fine intricate structures as well as in their role in shaping the Universe. The image above shows the Veil Nebula, which is an expanding shell of gas produced by a supernova that exploded less than 10,000 years ago: very recent history, in astronomical timescales.
The teal colours in the nebula indicate the presence of oxygen, the same element that exists in the molecular oxygen we breathe in on Earth. As the Veil Nebula expand into space, the gases may eventually find themselves within the collapsing dust clouds of a star about to be born.
This brings us to the end of Chapter III, and in a way, it closes a full circle to where we began our story. While the last star in the Universe will eventually die, the Universe still contains within itself vast quantities of star-forming material to create new stars for many trillions of years. Between now and then, however, the cycle of birth, death and rebirth will continue to hold.