Aside from being the spectacular finale to a massive star’s life, what good is a supernova? Honestly, we wouldn’t be here without them. Both types of supernovae are nature’s mechanisms to both produce and disperse all of the heavier elements needed for life. As we have discussed, massive stars can produce elements via fusion burning with atomic numbers as high as that of iron (number 26), but if a supernova didn’t climax the process, all of those useful elements would forever be locked up deep within the stellar core. In addition, all of the elements heavier than iron (up to uranium, number 92) have no way of being produced in nature except during a star’s supernova explosion, when the vast amount of extra energy being released can be tapped to produce the heavier elements. These elements never form during the normal part of a star’s life, since their formation only proceeds by reactions that consume energy rather than producing it.
Since the elements heavier than iron are only produced during the explosion phase of a supernova, we would expect that these elements are less abundant in our universe than the lighter elements, and such proves to be the case.1 Another prediction of the formation process of elements inside of stars, called stellar nucleosynthesis, is that the even-numbered elements (those with an even number of protons) will be more abundant than the odd-numbered elements. This also matches observational evidence, and the reason for it is that fusion most easily proceeds by adding a helium nucleus onto an already existing nucleus. The helium nucleus, with 2 protons and 2 neutrons, is like a brick in the process of element construction.
Throughout the history of a galaxy, supernovae serve to enrich the interstellar medium with the full array of elements needed for life. The preponderance of material in nebulae is still made up of just hydrogen and helium, but over time, about 2% of the total mix is built up to consist of elements heavier than helium. This enriched content of interstellar gases contains the right mixture to form not only a star like our sun, but also the variety of planets in our solar system, including ones with solid surfaces composed primarily of the heavier elements.
A Delicate Balance
Although supernovae serve a vital role in manufacturing life-essential elements, they also pose a serious danger to life. A supernova explosion releases prodigious amounts of radiation that would sterilize any planet orbiting around a nearby star system. Therefore, it’s crucial for us that the rate of supernovae in our galaxy has diminished throughout eons of time to a low level today—only 3 have been close enough to be observed with the naked eye in the last 1000 years, although others likely occurred further away. For this reason, our planet with its great variety of life could not have formed much earlier in the history of the galaxy than it did.2
Supernovae also play a role in determining where in the Galaxy the conditions are most ideal for a planet like Earth. Close in to the Galactic center, the density of stars in space is much higher than it is in our location (our sun is about two thirds of the way to the edge from the center of the galaxy). The higher density of stars near the central region of the Galaxy would increase the probability of sterilization events from supernova explosions. More crowded star conditions would also increase the risk of a gravitational encounter with another star, which could serve to disrupt our solar system’s planetary orbits. On the other hand, too far from the Galactic center the initial frequency of supernovae would have been too low to sufficiently enrich the interstellar medium with life-essential heavy elements. In actuality, our Solar System resides within a narrow band circling about 26,000 light years from the Galactic core, known to astronomers as the “Galactic Habitable Zone.”3 Earth’s location within our Galaxy’s habitable zone is ideal for allowing the long-term existence of life on our planet.
When a massive star reaches the end of its life and its iron core is crushed by gravity into a sphere of neutrons, if the mass of the resulting sphere is more than about twice the mass of our sun, gravity becomes so strong that the neutrons cannot withstand its crushing force. They then implode their mass into an even smaller volume, which pushes the escape velocity of the object past the speed of light, forming a black hole. Our current physics knowledge cannot adequately explain the properties of matter inside of a black hole, but perhaps that’s inconsequential, since no information from inside the black hole can ever reach the outside universe.
Black holes remind us of the delicate balance within the forces of nature. Although the universe needs the force of gravity to coalesce gases together in order to form galaxies and stars to give light, too much matter packed together in one place results in gravity forming a black hole from which no light emerges. Black holes also show us that our universe is not cyclical or eternal, for the formation of black holes is not reversible.
Black holes also spur us on to further understand the laws of physics of our universe, for although the General Theory of Relativity adequately describes and predicts the external properties of black holes, the conditions at the center of a black hole defy our current understanding of physics. Interestingly, the conditions at the beginning of our universe were similar to those at the center of a black hole, so a theory of physics which accurately described the center of a black hole would also help us to understand the first moments in the history of our universe. The needed theory would be a combination of quantum mechanics and gravity, accurately describing a state of matter with ultra-high mass in an ultra-small volume. Einstein sought to formulate such a theory, and many others have worked on it since his day, but so far to no avail. Undiscovered frontiers in science keep things interesting, and who knows when another under-employed “patent examiner” might surprise the world with the needed breakthrough.
1. Jeffrey Bennett, Megan Donahue, Nicholas Schneider, and Mark Voit, The Cosmic Perspective: The Solar System, 7th ed. (Pearson, San Francisco, 2014), p. 351.
2. Hugh Ross, Why the Universe is the Way it is (Baker Books, Grand Rapids, 2008) pp. 35, 177.
3. Guillermo Gonzalez, Donald Brownlee, and Peter Ward, “The Galactic Habitable Zone: Galactic Chemical Evolution,” Icarus 152 (July, 2001), pp. 185-200, cited in Hugh Ross, Why the Universe is the Way it is, p. 66.
Adapted from Canceled Science: What Some Atheists Don’t Want You to See, by Eric Hedin.