Have you ever wondered where the expression “we’re all made of stardust” came from? Well, I’m not sure who first coined the phase, but we are indeed made of stellar stuff. Carbon-based lifeforms (us), silica in the rocks, and oxygen in the atmosphere – all of the elements found within our bodies and in the very fabric of the Earth were forged in the stars.
Solar systems across the Universe are created from collapsing interstellar clouds of gas and dust – remnants of long-dead suns. To have sown the seeds of the Solar System, the star responsible for our own protosolar cloud would have needed to be at least eight times the size of our Sun; the minimum size to achieve supernova status. Our Sun – a yellow dwarf star – is rather small and insignificant compared to the stars responsible for gargantuan cosmic explosions, and will end its life as a beautiful planetary nebula, much like the Helix and Cat’s Eye nebulae.
Supernovae come in a variety of types: Type I which contain hydrogen, and Type II which are hydrogen deficient. Type I supernovae are further categorised as Type Ia, Ib, and Ic. And biggest of all are the spectacular but rare hypernovae, created from hypergiants like VY Canis Majoris (one of the largest known stars). As well as the distinctions of type, the death throes of large stars are also categorised as either ‘thermonuclear’ (Type Ia) or ‘core collapse’ (Type Ib, Ic and Type II) and all will outshine their host galaxies in their moment of glory.
A Type II core collapse happens when a star runs out of fuel, the fuel being nuclear fusion in its core. The lighter elements – hydrogen and helium – run out first and after a few cycles of expansion and contraction, the star ends up with an iron core, at which point nuclear fusion stops. Radiation pressure – the energy responsible for preventing the star from collapsing under its own weight – has now vanished and at this point the inward force of gravity takes over, causing a rapid collapse followed by a cataclysmic explosion. Under the extreme conditions of the blast, heavier and heavier elements are created, including Barium, Mercury, and Lead. Gas and dust are blasted into interstellar space, creating a fast-expanding cloud of material known as a ‘supernova remnant’; well illustrated by the Crab Nebula in the constellation of Taurus, which was created from a bright supernova observed and recorded by Arab, Chinese and Japanese astronomers in 1054. At the centre of the Crab lies a pulsar – an incredibly dense and fast-spinning neutron star – all that’s left of a core collapse, indicating that this particular supernova remnant is the result of a Type II event.
Type Ia supernovae are described as thermonuclear. These sudden bursts of stellar radiation occur when a pair of white dwarf stars (the burned-out cores of relatively small stars) smash together in a tragic orbital embrace. Their carbon hearts combine under crushing heat and pressure creating a massive explosion, elements of which are spread through the interstellar medium. Another method for producing a Type Ia is when a highly dense white dwarf pulls material away from a companion star, the former’s carbon core being re-ignited and eventually blown up. Since the 1960s, Type Ia supernovae have been used as ‘standard candles’ to make measurements over intergalactic distances and to calculate the expansion of the Universe. Astronomers had worked on the theory that these explosive events had very uniform properties, but recent studies have shown a variability of up to 40% in their brightness. And although these models are mostly accepted within the astronomy community, the processes are not fully understood.
It’s thought that a galaxy will host a supernova event roughly twice a century, which means that the Milky Way is well overdue this cosmic spectacle. A number of unstable red giant stars are of particular interest to astronomers, including Betelgeuse (Beetlejuice), Antares and Eta Carinae. Variable stars like this are supernova candidates, likely to explode soon (in astronomical terms, soon is within the next million years!). If a nearby star within the Milky Way were to go supernova, the result would be awesome in the true sense of the word. The resulting glow could, for a short time, be as bright as the Sun – both day and night. After a few days, the light would begin to fade, with the afterglow visible (akin to a very bright star, still visible in daylight), fading after a period of several months.
Supernova remnants are caused when the debris thrown outwards by the explosion crashes into surrounding material, generating a shell of hot gas and high-energy particles that glow brightly in X-rays, radio waves and other wavelengths for thousands of years.
In the early Universe, stars formed from the most abundant element: hydrogen. As the Universe has matured, so has the variety of elements within it, meaning that new stars (and their protoplanetary discs) have richer ingredients from which to form. A cyclic pattern of stellar evolution has emerged over the near 14-billion-year history of the cosmos – starbirth, supernova, interstellar cloud, starbirth – introducing new ingredients along the way. Which brings us back to the rich and diverse elements found throughout our Solar System… salts in the oceans of the Earth and icy moons; hydrogen and methane in planetary atmospheres; precious metals, both mined from terra firma and delivered from space; silica in the rocky planets, moons and comets; oxygen in the air we breathe. All have condensed from a historic supernova remnant – the phoenix rising from stellar ashes of a once-vibrant star. Recycling at its best!
© 2016 Melanie Davies