Intermediate Mass Stars

4-5 Intermediate Mass Stars and Supernovae

Introduction: We’ve now followed the life of a low-mass star from its birth as a main sequence star to its death as a black dwarf or a Type Ia supernova. Low-mass stars only have the chance to go out with a bang if they are fortunate enough to collect a large amount of mass from a companion star. The Sun is doomed to end its life as a black dwarf. However, intermediate-mass stars are destined to have a much more exciting end and you’ll learn about that in this lesson.

Reading: Ch. 22, section 22.5 and ch. 23, section 23.2 and 23.3

Lesson: Much of the evolution we’ve learned for low-mass stars does apply to intermediate- and high-mass stars. All main sequence stars fuse hydrogen into helium. However, high-mass stars take a slightly different approach to those nuclear processes. A low-mass star will form helium by starting with two hydrogen nuclei and banging them together to form deuterium. We already know that you can take deuterium and fuse it to form helium. Since this process starts with two hydrogen nuclei, which are just protons, this process is often called the proton-proton chain or simply, the pp chain. More massive stars use carbon as a catalyst to the reaction, combining hydrogen with carbon to form nitrogen. Through a series of steps, the helium nucleus is formed and the carbon nucleus is reclaimed so it can be used for another reaction.

Otherwise, the evolution of an intermediate-mass star is very similar to that of a low-mass star. It will exhaust hydrogen fusion in its core, begin shell hydrogen fusion, ignite helium fusion in the core, and ignite helium shell fusion. However, intermediate- and high-mass stars have a strong enough gravity that they can compress the core up to temperatures required to begin carbon fusion. Rather than electron degeneracy setting in, the core continues fusion reactions and continues to live for a while as carbon is fused to form neon. Various phases of fusion occur in the core of the star, as is shown in the following figure:

As you can see, the star goes through many fusion stages before the end. Whenever one fuel source runs out, the core begins to contract while a new form of shell fusion begins. Each successive fusion process lasts for a shorter period of time than the last. Just as the triple-alpha process lasts for a shorter time than the main sequence lifetime, so carbon fusion lasts for a shorter time than the triple-alpha process. For a 25-solar mass star, carbon fusion lasts for only around 600 years, not the millions of years we would expect. Silicon fusion, which creates the iron core, will last for only one day in the core of a 25-solar mass star. We can just guess that this is leading up to a quick and explosive finale. Once the core is composed of iron, it is time for the death of the intermediate-mass star.

We’ve seen this before for low-mass stars. The core can no longer undergo fusion reactions. The core will contract until the point where electron degeneracy begins to become important. However, where the contraction will be halted for a low-mass stellar core, intermediate- and high-mass stars have stellar cores more massive than the 1.4-solar mass Chandrasekhar mass limit, the maximum mass which can be supported by electron degeneracy pressure. So, what happens now? The electrons still vehemently refuse to be in the same energy state but they have nowhere to go. Well, actually, they do have somewhere to go. The electrons in the core of the star can be crushed into the protons. If you look at the masses of electrons, protons, and neutrons, you’ll find that the mass of a neutron is similar to the combined masses of a proton and an electron. When you force an electron and proton together, they form neutrons.

At this point, a lot happens nearly at once. The combining of the protons and electrons makes the core collapse rapidly, taking less than one second to finish this process. The core rapidly becomes composed of neutrons. As it turns out, neutrons follow a similar exclusion principle, so there is now the buildup of neutron degeneracy pressure. For an intermediate-mass star, the neutron degeneracy pressure is strong enough to bring a rapid halt to the core collapse. The resulting shock wave from the core bounce combines with the rapid release of energy from the fusion of the electrons and protons. Neutrinos are produced in this proton-electron reaction. As we know, neutrinos are nearly unreactive, but with so many of them being produced at once, they interact sufficiently with the outer layers of the star that the neutrinos accelerate them outward. The net result is a huge stellar explosion called a Type II supernova. These outer layers of the star are ejected at speeds in excess of 10,000 km/s and with an energy roughly equal to the total amount of energy that the Sun will produce during its entire main sequence lifetime. You can see a nice animation of a supernova explosion by clicking here (or here if you have a broadband connection).

A supernova explosion can be bright enough that it can outshine the entire galaxy in which the star is located.

This is a picture of Supernova 1987A, a supernova which occurred in the Large Magellanic Cloud, a mere 150,000 light-years from us. On average, astronomers believe that supernovae occur in a galaxy once per century and the last one to occur in the Milky Way was roughly 300 years ago, so we are long overdue. When the next one does go off in our galaxy, astronomers are sure to be in for a scientific treat.