31 Aug—Mother Star

"I believe a leaf of grass is no less than the journeywork of the stars,…"
 — Leaves of Grass

Two thirds of the age of the universe has passed since the last date marked on the Cosmic Calendar, but the universe has not been idle. It has been hard at work creating the elements that made our evolution possible.

The Big Bang ended too quickly to create the complex atoms necessary to build our world. It created only hydrogen and helium, the two lightest elements, with only traces of heavier elements. Human life relies on the complex combinations of elements which the more complex make possible. With a simple palette of two elements, we could never have evolved.

The stars which formed after the Big Bang fueled their furnaces by fusing together the simple elements. Two hydrogen atoms, for example, could fuse together to create a single helium atom. In the process, the atoms released the energy that heated and illuminated the stars.

The energy released in these atomic fusion reactions also prevented the stars from collapsing. It exerted an outward pressure that balanced the inward pressure of gravity.

These earliest stars were giants and lived short, violent lives. As they aged, they exhausted their cores' hydrogen fuel. As a star exhausted its fuel, its furnace cooled and the outward pressure that prevented its gravitational collapse began to fail. The star's core would begin to collapse.

As the core collapsed, the star's atoms packed closer and closer together causing the temperature and pressure to increase. At sufficiently high temperatures and pressures, helium can fuse together. As the pressure and temperature reached this threshold, the core would reignite with with helium fusion reactions instead of hydrogen.

The star would begin forming carbon (and a few other elements such as calcium, sulfer, and magnesium) at its core. This new fusion reaction released enough energy and outward pressure to halt further collapse.

Even though hydrogen had been exhausted in the core, hydrogen fusion continued in an outer layer of the star. The star became layered with a helium fusing core and a hydrogen fusing outer layer.

Eventually, the star would run low on helium in its core, and the process of collapse, increasing pressure, and reignition would be repeated. This time, carbon atoms would begin to fuse into neon. This process repeated several times until the star is layered like an onion with an iron-nickel core surrounded by layers of silicon, oxygen, neon, carbon, helium, and hydrogen in that order. ((The nickel is formed through fusion and later decays into iron.))

When the star exhausted its silicon fuel, it would run out of options. The fusion of nickel releases no energy. Instead it absorbs energy, so this reaction could halt another core collapse. When the core runs low on silicon, the core begins to collapse again, but the star is powerless to forestall its imminent death.

The collapse of the core would create conditions favorable for the creation of elements heavier than iron. It would collapse in on itself as fast as 70,000 kilometers per second, about one-fourth the speed of light. This collapse would form a solid core of neutrons. (Stars greater than twenty times the mass of the Sun would go on to collapse into a black hole.)

After the formation of the neutron core, the star would explode in what is known as a supernova. During this explosion, it is possible to create atoms heavier than iron and nickel. A significant portion of these extra heavy elements like gold, uranium, and lead were formed during supernovae.

The supernova explosion would throw off a shock wave of material into the interstellar void, broadcasting its heavy atoms and leaving behind the dense, collapsed core. The shock wave would perturb the inert material between the stars and trigger the formation of new stars in its wake.

A new generation of stars grew from the materials that the supernovae of the first stars left in their wakes. The new generation started their lives with heavy atoms gleaned from the previous generation. The new generation continued the work of fusing together light atoms to make heavier atoms.

Not all stars end in supernovae, but when those of the new generation massive enough to create a supernova died, they too broadcast their heavy elements into the universe and the cycle of stellar life began anew. Each successive generation of stars has had an increasing concentration of heavy atoms formed by the previous generation.

Thus we owe the heavier atoms essential for our existence — such as oxygen, carbon, nitrogen, calcium, and phosphorus — to the alchemy at the heart of stars that have long since died.

Mother Star Supernova—31 August

We believe that our Sun got its start when a star somewhere in the Milky Way galaxy went supernova. About 4,600 million years ago, this unknown supernova — our Sun's mother star — came to the end of its life.^{1, 2} The shock wave of its death gave birth to our own star, the Sun. Our bodies and almost everything we see are made of the materials that the mother star broadcast into the galaxy.

Further Study

Maps of Time by David Christian

Born With a Bang by Jennifer Morgan

1. Which supernova started our solar system? We don't know. The sun has traveled around the center of the Milky Way galaxy 20–30 times since it formed. Things have become so scrambled since then that we have no way of knowing where the sun started in the galaxy.
2. There may have been more than a single supernova that played a role in creating our solar system.