Wonders of the Universe - Brian Cox [53]
There are over sixty elements heavier than iron in the Universe, some are valuable, such as gold, silver and platinum; some are vital for life, such as copper and zinc; and some are just useful, such as uranium, tin and lead. Very massive stars can produce very tiny amounts of the heavier elements up to bismuth-209 (element number 89) in their cores by a process called neutron capture, but it is known that this makes nowhere near enough to account for the abundances we observe today. There simply haven’t been enough massive stars in the Universe.
The conditions necessary to produce large amounts of the elements beyond iron are only found in the most rare of all celestial events. Blink and you’ll miss them, because in a galaxy of 100 billion stars the conditions violent enough to form substantial amounts of these elements will exist on average for less than two minutes in every century
SUPERNOVA: LIFE CYCLE OF A STAR
All stars are born from clouds of gas, but the length of their life and their eventual fate are governed by their mass (i.e. how much gas they contain). Stars dozens of times heavier than the Sun live for only a few million years before swelling into supergiants and exploding as supernovae (top row). However, stars like the Sun live longer and die more gently, shining steadily for billions of years before swelling into red giants and losing their outer layers as a planetary nebula (middle row). The core of the star, exposed as a white dwarf, then continues to glow for billions of years more before gradually fading out. The least massive stars, the red dwarfs (bottom), simply fade out over tens of billions of years.
Nathalie Lees © HarperCollins
THE BEGINNING AND THE END
This computer-generated sequence of images shows what will happen when Betelgeuse goes supernova. Deep in the heart of the star, the core will succumb to gravity and fall in on itself, then rebound with colossal force. The blast wave emitted generates the highest temperatures in the Universe. Over millions of years the scattered elements of the exploded star will become a nebula, at the heart of which is a super-dense core that is Betelgeuse the neutron star.
After a few million years of life, the destiny of the largest stars in our universe is a dramatic one. Having run out of hydrogen and burnt through the elements all the way to iron, giant stars teeter on the edge of collapse. Yet even in this dilapidated state these stars have one last violent act, and it is a generous one. It occurs with such intensity that it allows for the creation of the heavy elements.
If we could gaze deep into the heart of one of these dying giants, we would see the core finally succumb to gravity. As fusion grinds to a halt, this giant ball of iron falls in on itself with enormous speed, contracting at up to a quarter of the speed of light. This dramatic collapse causes a rapid increase in temperature and density as the core shrinks to a fraction of its original size. The inner core may eventually shrink to 30 kilometres (19 miles) in diameter. At this point, with temperatures nearing 100 billion Kelvin and densities comparable to those inside an atomic nucleus, quantum mechanics steps in to abruptly halt the collapse. By now most of the electrons and protons in the core have been literally forced to merge together into neutrons. Neutrons, in common with protons and electrons, obey something called the Pauli exclusion principle, which effectively prevents them from getting too close to one another (in more technical terms, no two neutrons can be in the same quantum state). This has the effect of making a ball of neutrons the most rigid material in the Universe – 100 million million million times as hard as a diamond. When the neutrons can be compressed no more, the contraction must stop and all the superheated collapsing matter rebounds with colossal force. A shockwave shoots out through the star and as this blast wave runs into the outer layers of the star it generates the highest temperatures in the Universe – 100 billion degrees.