September 14, 2018 N: Novas, Supernovas and Nebulas: The Life and Death of Stars (Where do all the radioactive elements come from?) 

Novas

On December 14, 2013, Australian amateur astronomer John Leach was looking through his telescope in the Constellation Centaurus when he saw what he thought was a new star come into being. Actually Nova Centauri 2013 isn’t a star at all. 

Unlike our sun, most stars come as binaries, that is, two stars that are rather close together. The stars orbit one another after they are formed, and when they get close enough, one robs the other of its accretion disk material. As the disk material spirals into the other star, it heats up and causes a fusion reaction. At this point the spent hydrogen explodes from the surface of the star it was attracted to and, at that moment, it gives off electromagnetic radiation (some visible, some not so visible: like the gamma rays which are part of the cosmic rays that are thought to nucleate storm clouds during our solar minimum). The visible part of that radiation can sometimes be seen by the naked eye. Our galaxy houses maybe ten novas each year.

Supernovas

In 1604, a star appeared in the sky in the Constellation Ophiuchus. It was brighter than the planet Venus. It was so bright that it could be seen in the daylight. That was the last supernova seen in our galaxy.

When a very large star ends its life, when it’s fuel is spent, then without heat energy, the star can no longer keep gravity from making it collapse upon itself. When that happens, material in a shell of the star explodes out, giving off lots of radiation (including light). Though not continuously visible for years, telescopes reveal the spent and somewhat lit fuel for years, maybe even centuries as what we call nebulas. (A nebula that’s easy to see in binoculars or small telescopes is the Orion Nebula, one of the “stars” that make up Orion’s sword).

The Supernova, as opposed to a nova, produces its own compilation of heavier elements. Because atomic chemists know how different elements in the heat of a supernova (like this one: Kepler’s Supernova) are produced, they know the kind of light given off by the different elements in type 1a supernovae, like this one. Using what these chemist-physicists know, they can calculate distances to other stars and other galaxies, so, sometimes, this type of supernova is thought of a yardstick to the stars, allowing us to map our universe.

Where do all the elements come from? 

When nova occur the new fusion reactions in the hydrogen (atomic weight = 1 (one proton, one neutron, one electron)) unlike our sun in its fusion reactions which produces energy for us here on Earth (fusing two hydrogen atoms into a heavier helium atoms) novas produce the heavier lithium atom by fusing a hydrogen and a helium atom.

If our sun produces helium and nova produce lithium, how are the heavier elements we find on Earth created? 

Nucleosynthesis is the process by which atom, or atom remnants, are fused to produced heavier elements. Carbon, sometimes known as the backbone of molecular life on our planet, has an atomic weight of four and can only be formed with billion-degree temperatures (our sun at its center is 27 million degrees F, not hot enough to create carbon). So to produce the heavier elements in our planet we needed really big and hot stars much bigger than our sun (Look up at the night sky at Orion. Look below his belt at his right knee. That’s Rigel. It’s a really big blue star, very hot (just like the blue flame of a candle is hotter than the yellow flames). Rigel is a blue supergiant about 100,000 times brighter than the sun. It is only millions of years old and has already burned all its hydrogen fuel and one day may become a supernova as bright as the moon, since it’s only 800 light years away. Before a giant star like Rigel explodes, its fusion heat can produce all the rest of the elements through iron (atomic weight 56) but all the other elements require the star’s death in a supernova.

Our sun because it burns at a lower temperature has a longer lifespan than a big star like Rigel. The sun is at middle age, but what happens as the sun ages further, especially if it’s not big enough to supernova? In time, it becomes hotter and hotter. The hydrogen all turns into helium and it gets hot enough to fuse helium atoms into carbon. The sun becomes a red giant but is no longer able to hold its outer shell which explodes out as a nebula. Because many more stars are small, and die this way, we are the beneficiaries of the carbon in their nebulae. Galaxies will distribute the elements to eventually orbit another small star to produce another accretion disk and another Earth. 

The only unknown, to my mind, is if we need a Jupiter in that system to create the precursors of genetic matter (panspermia) or, if genetic material will naturally evolve on Earth-like planets, then primitive life has to be abundant in our universe (and it mostly occurs on planets in the liquid water zones around smaller stars. Since smaller stars have very long lives, much longer than our suns, life has plenty of time to evolve.

All of that said, we are on this planet, around this sun, so we can expect our sun, though it goes through temperature oscillations through grand minima, to continue to get hotter and hotter through the second half of its life. So eventually our alien cousins will have to figure out ways to get off planet. Since smaller suns like Trappist I can produce many Earths, that may not be as hard as it will be for us humans. But maybe Mars and the Jupiter moons will heat up and their waters will melt, giving us a place to create a new home.