As we saw in last month’s article, stars are not infinite objects. They live out their lives, just as we do. They are born, reach maturity, and ultimately die. Stars can be seen as great stellar engines – they are reliant on a constant supply of fuel to shine. Once that fuel is depleted, the star can no longer sustain itself.
So, what is star-fuel? Ironically, these massive objects are powered by the simplest element in the periodic table – hydrogen. Hydrogen clumps together under the force of gravity within enormous nebulae to form great swirling masses of gas. Should they attain a sufficient mass, the hydrogen in their core is crushed to the point that individual atoms fuse together to form helium (the second simplest element). The resulting energy, and light, pushes back against the relentless gravity, keeping the star stable as it enters its mature years.
Stars are great nuclear reactors that produce more energy every second than it is possible to fully understand. Every second, the Sun fuses 600 million tons of hydrogen into helium … that’s a lot! To put it into perspective, each year the entire human population uses around 17 trillion watts of power. Our, Sun creates 380 sextillion (380 000 000 000 000 000 000 000) watts per second! To put it another way, the Sun generates the equivalent of 20 million hydrogen bombs per second. And it is worth noting that our stellar parent is by no means a ‘massive’ star!
A star’s lifespan is ultimately determined by the amount of fuel that is has. Like any engine, once exhausted, it will stall. Small stars have efficient engines that slowly sip their hydrogen and may last for trillions of years; whereas supergiant stars, such as Rigel and Betelgeuse in Orion, gulp their fuel so quickly that it is depleted in only a few million years.
The mass of the star will also dictate its ultimate demise. First, let us look at what happens to ‘low mass’ stars, like our Sun.
Once the Sun has converted all the hydrogen in its core into helium, its energy output will diminish. Its gravity will not be enough to squash (fuse) the helium into the next element, but it will be heated to tens of thousands of degrees. This will be enough to briefly ignite some of the remaining hydrogen in its atmosphere, causing the star to swell in size. The larger surface area will spread out the remaining heat, thus cooling the star and turning it red in colour. The Sun will then have entered its ‘red giant’ phase.
It is worth noting at this point that in space, the colour of a star is indicative of its temperature, and that our general understanding of this relationship is wrong. We tend to associate red with heat, and blue with cold (bathroom taps are a perfect example of this) but in science it is the opposite. Think of a welding torch, or a Bunsen burner from school: at their lowest temperature, they burn orange, but when the temperature increases, they burn blue. Red stars are thus cool, while blue stars are hot.
This ignition process will continue, until the Sun can no longer sustain itself and its atmosphere will bleed into space leaving a colourful, diffuse ring called a planetary nebula. The remaining scolded core is known as a white dwarf. These objects are unimaginably beautiful in long exposure photography. The accompanying photo shows the closest planetary nebula to Earth, the Helix Nebula, poignantly known as the ‘Eye of Sauron’ after the Lord of the Rings villain.
However larger stars suffer a far more cataclysmic demise. Supergiant stars are capable of fusing hydrogen into a succession of elements in their core, until they reach iron, and the star balloons into a red supergiant. After a while, the process stalls again, but the incredible gravity pushing down on the core is so great, that the entire star collapses. Trillions and trillions of tons rain down on the core before rebounding into the cosmos in a fabled supernova explosion. The resulting energy release is so bright that it outshines all the other stars in the galaxy and can be witnessed by amateur telescopes from millions of light years away.
The star’s remaining Earth-sized core, compressed into only a few kilometres wide, is now known as a Neutron Star. These objects are so dense, that a piece the size of a sugar cube would weigh more than 1 billion tons! Even larger stars have so much gravity, that not even a neutron star can survive, and their entire mass is squashed in a single point of space and time. Its gravity is so insanely strong that not even light can escape its pull, thus creating a ‘black hole’. Quite what happens within these inky depths remains a mystery, since literally no information can escape them, but they may hold the key to understanding the secrets of space-time and the very nature of Universe.
Supernovae have been recorded in the Milky Way, the last one being witnessed in 1604. It was so bright that it was visible in the daylight for three weeks and was bright enough to cast shadows at night. There are two or three supernovae remnants in our night sky that can be seen and captured with the correct equipment, the most famous being the Crab Nebula in Taurus, and the Veil Nebula in Cygnus. The primary candidates for our next supernova event include Betelgeuse, Antares and the massive hypergiant star, Eta Carina. When it will happen is not an exact science, but it will be a sight to behold!
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