CHILDREN OF THE STARS

ByANDY FLEMING

It’s the story of how we, and all of the creatures with whom we share the Earth came to be. It’s an epic tale to rival the best Shakespearean tragedy or our best works of literature. It’s the story of how we and everything we see was literally ‘made in heaven’, and it confidently predicts what our fate may be...

Stars do not live forever, and our Sun will one day die, and with it all life on Earth. Five billion years from now, when our planet has been incinerated to a crisp, our local star will have run out of the fuel that powers its nuclear fusion. Its hydrogen depleted and all consumed, it will have metamorphosed from the relatively stable yellow dwarf star that we see today into a bloated angry red giant, its outer layers and atmosphere occupying most of the inner solar system.

Indeed, the Sun is already imperceptibly increasing in temperature – it’s 20 per cent hotter now than when the Earth coalesced out of the Sun’s proto-planetary disk 4½ billion years ago, and within a couple of hundred million years the Earth will become uninhabitable. This chain of events is inevitable and, over different time periods, happens to all stars.

Stars coalesce by gravity out of clouds of interstellar gas, made up largely of the original constit-uent elements of the universe: about 75% hydrogen and 25% helium, plus trace amounts of lithium – the latter two termed ‘metals’ in the unorthodox nomenclature of astronomy.

Hertzsprung-Russell Diagram detailing stellar luminosity v surface temperature.

The force of gravity contracts the proto-star to the point where pressure and temperature dictate that nuclear fusion starts and it begins to shine. The star is then said to gain membership of the ‘main sequence’ (see below), and will spend the vast majority of its life undergoing the nuclear fusion of hydrogen into helium. A small amount of mass is converted in the process into energy (under E = mc2, mass is equivalent to energy). A hydrostatic equilibrium is achieved whereby the gravity of the star is counterbalanced by the energy, in the form of photon and radiation pressure from the nuclear reactions within its core.

At this stage, the star gives the appearance of stability in terms of shape and size. In truth, however, it is a magnetically contorted and tortured object, erupting frequently and spewing out solar flares, charged particles such as protons, and coronal mass ejections. Stars the mass of our Sun will be in this hydrostatic equilibrium for about 10 billion years, but all the while, the concentration of helium in their cores continues to increase. As it does so, the star begins to contract to maintain hydrostatic equilibrium, and temperatures in the core gradually increase over billions of years.

Finally, when the star’s nuclear fuel is exhausted, gravity exceeds radiation pressure and the core contracts much further, until spiralling temperatures and pressures succeed in igniting helium, which is then fused and transmuted into carbon via the triple-alpha process. The loosely gravitationally bound outer layers are puffed out, and the star becomes a red giant.

Gravity has won -- the progenitor star, having had insufficient initial mass to fuse heavier elements, ends its life as a white dwarf, within which, owing to a quantum effect (the Pauli exclusion principle), the repulsive negative electric charge of tightly bound electrons in the star is sufficient to resist further collapse. These sub-atomic particles are not morally reprehensible, but this energy has the unfortunate title of ‘electron degeneracy pressure’!

A white dwarf is a truly bizarre, object – a star of diamond, a dense hot carbon star the size of the Earth, but with the mass of the Sun. It puffs off its outer layers to interstellar space as a beautiful planetary nebula, and over billions of years it will cool off to become a black dwarf.

Larger blue giant hot stars, however, with a mass range between a couple and one hundred solar masses, burn through their fuel at a much more prodigious rate. They live fast and die young: they are the James Deans of the star family, with life-spans in millions rather than billions of years. One such well-known star is Rigel in the constellation of Orion the Hunter.

Neither does the nuclear fusion process within the core of such a star terminate at the element carbon. With its high mass and thus much higher gravity, the result is a much more compact stellar core with higher temperatures and pressures, thus ensuring the nucleosynthesis of ever heavier elements.

The process only stops with the exceedingly stable atomic nucleus of element number 26, iron. Up to this point, the fusion process in all stars has been exothermic, i.e. during nuclear fusion energy has been constantly released as elements have been transmuted via E = mc2. However, the dying star is about to undergo one of the most spectacular phenomena in the cosmos.

At this point, one may ask, just as the great British astronomer and astrophysicist Fred Hoyle did, how are the remaining natural chemical elements formed – those most essential for life, from cobalt at atomic number 27, right up to uranium with atomic number 92? The answer is: with an event of gargantuan violence – a Type II supernova.

The contorted, twisted iron star collapses under its own gravity, and detonates and explodes with a shockwave and explosion of gargantuan proportions. The awesome energy released fuses the heavier elements in a final endothermic nuclear fusion reaction, and these elements are then vomited back into space, enriching the clouds of gas and dust that in the future will coalesce to form another generation of stars. In this manner, successive generations of stars become ever richer in heavier elements from nitrogen to iron, and then right up the periodic table to uranium.

The process that follows this cataclysmic event is intrinsically bound up with the mass of the progenitor star. If this was less than ten solar masses, it collapses into a neutron star, a hideous and fascinating object so dense that it resembles a giant atomic nucleus with a diameter of 20 kilometres, about the size of Greater London. Stabilised by neutron degeneracy pressure and with insufficient gravity, this neutron star can collapse no further. However, its huge angular momentum from energy produced by its gravitational collapse ensures that it spins as a pulsar, often at near-relativistic speeds (speeds approaching that of light), with intense jets of charged particles emanating from its poles due to intense magnetic fields generated in the star.

The frequency of the pulsar’s spin is constant over human timescales, and varies in value depending on the size of the neutron star and the mass of its progenitor. It can range from a few seconds to milliseconds. Pulsars are in effect Nature’s successful attempts at exquisitely accurate timepieces – outclassed only by our modern atomic clocks. When first detected by Anthony Hewish and Jocelyn Burnell at Cambridge University in 1967, the radio signature of the first pulsar stellar remnant was thought to be artificial in original... and was duly labelled on the computer print out as ‘LGM’, standing for Little Green Men! Well-known pulsars include that in the centre of the Crab Nebula (M1), resulting from a supernova witnessed by Chinese astronomers in 1054 AD, and the Vela Pulsar.

Finally, progenitor stars with masses of between 10 and 100 solar masses end their lives as probably the most bizarre object known, one that is the subject of endless speculation and folk-lore: a black hole. For a star to become a black hole, its mass, and hence gravity, must be large enough to ensure that its final contraction overcomes neutron degeneracy pressure, the stellar remnant collapsing into a singularity, a point of infinite density. Its gravity is so great that its escape velocity is more than the speed of light. This point surrounding the singularity, beyond which no light or indeed any electromagnetic radiation can escape, is termed its ‘event horizon’.

Nobel Prize-winning astrophysicist
Subrahmanyan Chandrasekhar.

One final type of important star detonation that must be mentioned is a Type Ia supernova. The mechanism for its explosion is very different from that of a Type II event. A Type Ia supernova occurs when one of the stars in a closely bound binary system is a dense white dwarf. Its immense gravity-well draws material from the atmosphere and outer layer of its companion main sequence star, and deposits it onto the surface of the white dwarf.

When the mass of the white dwarf finally equals approximately 1.4 solar masses (the so-called Chandrasekhar Limit, named after the brilliant Indian American Nobel Prize-winning astrophysicist Subrahmanyan Chandrasekhar (see left)), it will collapse under its own gravity and detonate as a Type Ia supernova, again spewing heavy elements into the interstellar medium.

In addition, such exploding stars are of immense importance to cosmologists – they always detonate at the same mass and have approximately the same intrinsic brightness. As such they can be used as ‘standard candles’ for measuring cosmic distances over inter-galactic scales.

It is truly astounding that some of the most awesomely powerful and destructive phenomena in the heavens are simultaneously responsible for creating a habitable universe. For if it were not for massive stars that ended their brief lives as supernovae, there would be no Milky Way, no solar system, no Earth, and no homo sapiens. Everything from which we are made, from the iron in our blood to the calcium in our bones, can all be directly traced back to the fiery cores of long-dead stars.

You see, you are a child of the stars. You were quite literally ‘made in heaven’. As the late NASA astronomer and astrobiologist Carl Sagan so eloquently stated, 'You are the cosmos with consciousness, a way for the cosmos to know itself, star stuff harvesting starlight'. There can be no more profound or spiritually uplifting a thought than that.

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