Why Does E=mc2_ - Brian Cox [67]
With no other source of energy available to prevent the inevitable, a star that has an iron-rich core is really at the point of no return, and gravity resumes its relentless work. There is now only one last chance for the star to prevent total collapse. It becomes so dense that the electrons that have been hanging around ever since they were ripped off the hydrogen atoms during its birth resist further collapse as a result of the Pauli exclusion principle. The principle is an important one in quantum theory and it is crucial for the stability and structure of atoms. Crudely put, it says that there is a limit to how closely you can pack electrons together. In a dense star, the electrons exert an outward pressure that increases as the star collapses until it is eventually so large that it can prevent any further gravitational collapse. Once that happens, the star is trapped in an enfeebled but incredibly long-lived state. It has no fuel to burn (that is why it was collapsing in the first place) and it cannot collapse any further because of the electron pressure. Such a star is called a white dwarf—a slowly fading memorial to a majesty irredeemably diminished—the once-bright creator of the elements of life compressed into a remnant the size of a small planet. In a time far longer than the age of the universe today, the white dwarfs will have cooled so much that they fade from view. We are reminded of the beautiful sentiments of the father of the big bang theory, Georges Lemaitre, when reflecting on the inevitable universal journey from light into darkness from which even stars cannot escape: “The evolution of the universe can be likened to a display of fireworks that has just ended: some few wisps, ashes and smoke. Standing on a well-cooled cinder, we see the fading of the suns, and try to recall the vanished brilliance of the origins of the worlds.”
It has been our goal throughout this book to be careful to explain why things are as they are and to provide arguments and evidence as we progress. The description we presented here of how a star works might seem fanciful, and we have certainly deviated from our careful, explanatory style. You might even object that since it is not possible to do laboratory experiments directly on stars, we cannot possibly be certain how they work. But that isn’t why we were brief. We have been brief because it would take us too far from the point to go into more detail. The remarkable work of Hoyle and the success of experiments like Super-Kamiokande will have to suffice by way of supporting evidence, along with one last beautiful prediction made by Indian physicist Subrahmanyan Chandrasekhar. In the early 1930s, armed only with already well-established physics, he predicted that there should be a largest possible mass for any (nonrotating) white dwarf star. Chandrasekhar originally estimated the largest mass to be around 1 solar mass (i.e., the mass of the sun), and more refined calculations later led to a value of 1.4 solar masses. At the time of Chandrasekhar’s work, only a handful of white dwarf stars had been observed. Today, around 10,000 white dwarf stars have been observed, and they typically have a mass close to that of the sun. Not a single one has a mass that exceeds Chandrasekhar’s maximum value. It is one of the true joys of physics that laws discovered in tabletop experiments in a darkened laboratory on earth pertain throughout the universe, and Chandrasekhar exploited that universality to make his prediction. For that work he received the 1983 Nobel Prize. The validation of his prediction is one of the pieces of evidence that allows physicists to be very confident that they really know how stars work.
Are all stars fated to end their lives as white dwarf stars? The narrative in the previous paragraph suggests so, but it is not the whole story