Originally published 25 July 1983.
On the evening of November 11 in the year 1572, the Renaissance astronomer Tycho Brahe, as was his custom, contemplated the stars in the clear evening sky. Suddenly he noticed, almost directly over his head, a new star, surpassing in brilliance all the others. Tycho was an exacting mapper of the heavens and had known the constellations since boyhood. He knew immediately that no star, not even the most faint, had previously occupied the position of the intruder. New stars in the heavens were unheard of, and Tycho briefly doubted his own eyes. He called to his friends and assistants and they too verified the “new star,” or nova. It was a miracle, Tycho believed, the first of its kind since the Creation.
He was wrong, of course. The “new star” that appeared in the constellation Cassiopeia in 1572 was what astronomers now call a supernova, the explosive death of a massive star scattering most of the star’s substance into space. There are several hundred billion stars in the Milky Way galaxy, and every century two or three of them blow themselves to bits.
In the fury of a supernova detonation, a dying star briefly shines with a dazzling intensity. The fireworks quickly fade as the star blows off its outer layers. Within a few thousand years the expanding shell of stardust has swept out past neighboring stars and their planets. As the blast moves forward, it sweeps up and concentrates dust and gas from the space between the stars.
It is now widely believed that supernova shock waves, by concentrating interstellar matter, can trigger the formation of new stars.
Star-making wave
One supernova remnant which has been carefully studied by astronomers lies in the constellation Canis Major, the larger dog of Orion. The supernova that produced this ghostly wreath of stellar debris appeared in earth’s sky at about the time humans were learning the use of fire. Today, the remnant of glowing gas is visible only on observatory photographs. It is expanding into a particularly dusty corner of the galaxy. Along the forward edge of the remnant lies a string of hot young stars. The origin of the stars seems to be related to the supernova event.
As the blast of the Canis Major supernova expanded into space, it swept up matter in its path like dust before a broom. Eventually, the dust and gas along the shock wave was sufficiently dense for gravity to pull the material together into clumps. Those clumps became the stars we now see burning near the edge of the nebula.
If this scenario is correct, the death of one massive exhausted star was the trigger for the birth of a cluster of new stars.
Could a supernova explosion four and a half billion years ago have triggered the formation of our star, the sun, and its attendant planets? Some astronomers say yes. They cite evidence that literally fell from the sky. But before the evidence, a short digression.
A sense of proportions
Isotopes are different forms of the same element. They differ only in the number of neutrons (uncharged subatomic particles) in the nucleus of an atom. For example, oxygen-16 has 8 protons and 8 neutrons in the nucleus. Oxygen-17 has an extra neutron, and oxygen-18 has two extra neutrons. The extra neutrons do not effect the chemical identity of the element.
On Earth, the relative proportions of the different isotopes of an element are unvarying. For example, in any sample of oxygen 99.756 percent of the atoms are oxygen-16, 0.039 percent are oxygen-17, and the rest are oxygen-18. It doesn’t matter if the oxygen is collected from the atmosphere above Boston or from the rocks of Australia, the mix of isotopes is constant.
The same relative proportions of isotopes are found in rocks the astronauts brought back from the moon. This is what we would expect if the earth and the moon formed from the same well-mixed store of elements.
Like the other stars and their planets, the solar system condensed by gravity from an interstellar cloud of dust and gas. That pre-solar nebula was mostly hydrogen and helium, but contained a small amount of heavier elements like oxygen, carbon and iron. The heavy elements in the cloud were contributed by stars that had lived and died earlier in the history of the universe. Stars fuse heavy elements from light ones as they burn, and they disperse those elements to space when they die as supernovas.
There is no reason to believe that any two supernovas contributed elements to the pre-solar cloud with exactly the same mix of isotopes. But if the cloud had been around long enough, its material would have been thoroughly stirred and the contributions of all sources blended together. We would expect to find the same mix of isotopes in all bodies of the solar system that condensed from the cloud.
Rocks from somewhere else
Now for the evidence. Several researchers claim to have found isotope concentrations in certain recently fallen meteorites that differ significantly from the earth and moon. The meteorites presumably had their origin elsewhere in the pre-solar nebula and subsequently made their way to earth. The unusual concentrations of isotopes in the meteorites suggest that the pre-solar nebula was not thoroughly mixed after all.
The pre-solar cloud might not have been thoroughly blended if fresh materials were injected into it by a nearby supernova just before it condensed to form the sun and planets. It is only one further step to suggest that the supernova which injected the fresh assortment of isotopes was also the trigger that caused the cloud to collapse.
Life on earth is the child of the stars. We are made of the stuff of stars, of the ash of star-shine. The atoms in our bodies were cooked up in stars and blasted into space by supernovas millions or billions of years before the earth was born.
And now, out of the sky fall chunks of stone that carry a startling message of the earth’s beginning. It is a message that would have satisfied Tycho Brahe’s taste for the fabulous. A supernova was midwife to the arrival of our planet on the cosmic scene.
