Are we made of star dust or nuclear waste?

The first stars, formed about 400 million years after the formation of the universe, were made of hydrogen and helium and were drawn together by gravity inside the clumps of dark matter, that probably owe their origin to quantum fluctuations, when the universe was less than a billion, billionth of a second old. Because these stars were made of only hydrogen and helium, they could be 1000 times the mass of the sun and lived short lives before creating and spreading the first batch of heavy elements into space. When they died they left behind black holes that may be the origin of the 1000 million solar mass black holes we see in the centre of galaxies today.

Successive generations of stars have been born out of the giant molecular clouds we see  orbiting galaxies. Each generation incorporates material processed by previous generations of stars and as a result, we see a steady increase in the  number of atoms of elements heavier than helium, in interstellar space.

The more massive a star, the brighter it shines and the shorter it lives. Stars more than about 5 times the mass of the sun build up successive nuclear fusion layers inside their cores, containing elements all the way up to iron. Each element, that is the end product of the previous series of nuclear fusion reactions, becomes the fuel for the next.  Once the core contains iron a problem arrises because, it can not be fused to release energy. With no source of energy, gravity takes over, compresses the core until it becomes so hot the iron is destroyed and turned into neutrons, that continue the collapse to form a tiny neutron star, or if the star is massive enough, a black hole.

The outer layers of the star collapse inwards, bounce off the neutron star, creating an out going shock wave that races back to the surface, exploding the star, when we call it a supernova. For a few days it outshines an entire galaxy of 100 thousand million stars.
The high temperatures  produced by the shock, make elements all the way up the periodic table, which are spread into space by the explosion. Stars like the sun will not explode and will only manufacture elements in their cores up to carbon or nitrogen, before their outer layers detach to become beautiful planetary nebula and later fade away and become inert white dwarfs.

However, many stars form close pairs orbiting around each other. If one star has already become a white dwarf and its companion evolves to be a red giant and enough of its extended atmosphere spills over onto  the white dwarf, it can push the white dwarf over what is known as the Chandrasekhar (Subrahmanyan Chandrasekhar 1910 – 1995) limit of 1.4 solar mass. If this happens, nuclear fusion will again burst into life making elements all the way up to iron, creating an explosion known as a Type I Supernova, that leaves no remnant and again enriches interstellar space with heavy elements.

The most extreme examples are neutron stars, themselves left over from supernova explosions in. If they were formed in double star systems they will eventually spiral in towards each other, merge and in the resulting explosion produce heavy elements including gold.This cycle of star birth and death had been going on for two thirds the present age of the universe, before our sun and planets and eventually ourselves were born. So we are indeed made of star dust or the nuclear waste of stellar evolution.

This blog was written by Robin Michael Catchpole who works as an astronomer at the Institute of Astronomy, Cambridge, having retired as Senior Astronomer at the Royal Observatory Greenwich in July 2004.

If you would like to download a 15 minute presentation given by Robin please:

click here