In our last article, we talked about the first half of a star’s retirement as it goes through the different stages of hydrogen- and helium-shell burning. The star has still got a long way before its journey comes to an end. So, let’s begin.
Hubble Telescope's image of Eta Carinae shows its hot gases in red, white and blue colours. (Image credit: NASA, ESA, N. Smith (University of Arizona) and J. Morse (BoldlyGo Institute))
So far, our star has spent most of its retirement burning the residual hydrogen and helium shells, and most of the extreme changes are internal. But the star is about to experience some major observable changes. As the star expands and cool, the burning helium shell switches off. Nevertheless, the hydrogen shell keeps producing helium, which gets accumulated. Once this shell reaches a critical mass, it reignites again, creating what’s called a helium shell flash. This causes the outer layers to expand even further, causing them to cool and the hydrogen shell to shut off. The energy generated increases the peak luminosity of the star to around a thousand times of its observed luminosity. The star remains as bright as stars that are ten times its magnitude for several hundred years [1]. As the helium gets depleted, the star contracts again, reigniting the hydrogen shell and causing the cycle to start all over again. Each thermal pulse lasts for around a few hundred years and all the while, the convective layer between the two shells keeps transporting nuclear fusion products to the surface. The extremely powerful solar winds generated by these thermal pulses cause the outer layers to be blown away, causing the formation of a planetary nebula at the end of the AGB. This planetary nebula is what paves the way for the creation of another star. By the end of the AGB, the solar winds completely blow away the outer layers of the star, creating a massive planetary nebula around it which could be as big as roughly one light-year! [2]
The formation of a Planetary Nebula, and a planetary nebula NGC 2022 as captured by Hubble. (Credit: NASA / ESA / Hubble / R. Wade.)
It is in the ABG stage that elements heavier than carbon are synthesised by a series of nuclear reactions. A second- or third-generation star is formed from the remnants of the previous star, which is why there could be nuclei of elements, like iron, in the smaller current star; these elements could have been synthesised in the previous, more massive star. When a neutron with sufficient energy is bombarded onto a seed nucleus, it creates a compound nucleus, which may or may not be stable. This is called the s-process (slow neutron capture process) and is responsible for the nucleosynthesis of approximately half the atomic nuclei heavier than iron. If the compound nucleus is unstable, it would decay, unless another neutron gets captured by it in the small-time frame and the resulting nucleus is stable. This is called the r-process (rapid neutron capture process) [3]. As the star contracts, new layers keep igniting, forming an onion-like structure, with each layer synthesising heavier elements till it reaches iron, after which the binding energy released isn’t sufficient enough to ignite the iron core.
An artist illustration of the different shells in a massive AGB star (Source), and the nuclear landscape. Designer Nuclei - Making Atoms that Barely Exist - Scientific Figure on ResearchGate, accessed on 22 December 2021.
There is also another kind of process, which occurs only in very massive stars, called the p-process (proton capture process). Understanding the synthesis of elements in these end stages of a star’s evolution is a very vast and exotic topic of research today. It can help us create a model for the creation of most of the elements on the nuclear landscape. [4, 5]
At its final stage of evolution, stars of 8 to 10.5 solar masses and lower can synthesise elements up to carbon and oxygen in their cores as they reach the end of AGB. The outer layers are blown away due to stellar winds, leaving it with a hot and stable carbon-oxygen core called white dwarfs (WDs), which are very compact objects that could have a mass of half of the original star but compressed in a small ball as big as an average city. [6]. Stars with very low mass, to begin with, aren’t able to fuse helium into carbon and thus end up with a helium white dwarf [7]. These WDs are stable and get their luminosity from the heat trapped in them, which very gradually keeps dissipating into space over billions of years, after which they turn into cold Black dwarfs. WDs are arguably going to be the last stars that would exist till the end of the universe. [6]
Illustration of a White Dwarf. (Credit: Miriam Nielsen), and a WD in a binary system accreting the outer layers of its partner. Credit: NASA/CXC/M.Weiss.
But not all WDs have such a straightforward end. If a WD in a binary system, is close enough to the other star, it could accrete the material from the outer layers of its partner onto itself, increasing in size and mass. If the mass of a WD exceeds the Chandrasekhar limit, it’s no longer stable and collapses, causing a massive release in energy in a supernova explosion [8]. This could either release all matter into interstellar space, from which a new star could be born, or only half of the mass is lost and what’s left is an object called a Zombie star (ZS) [9]. Detecting ZSs is an exotic field of research as their rate of occurrence is very low, and only thirty supernovae have been identified in this category [10].
An artist illustration of a neutron star. (Credit: NASA's Goddard Space Flight Centre), and an illustration of a black hole with an accreting disk. (Credit: Mark Garlick/Science Photo Library)
In the case of massive stars of more than 8 to 40 solar masses, the process of nucleosynthesis keeps going on till iron, after which the star collapses onto itself and dies in a spectacular supernova explosion; this could be billions of times brighter than our sun. A supernova seen from Earth could easily make the night sky look like a summer afternoon. The result of this spectacle is a neutron star, one of the densest objects in the universe made up completely of tightly packed neutrons [11]. If the final mass of the neutron star is greater than 3 times the solar mass, it would collapse into what’s called a Black Hole (BH) [12], one of the most exotic and mysterious objects in the entire universe. Geniuses like Stephan Hawking have spent their entire lives trying to understand them. Everyday laws of physics that we experience just don’t exist for these objects. These objects deserve a whole article to themselves. Meanwhile, you should check out our article (Detecting Gravitational waves using Pulsars) where we have talked about using neutron stars as GW detectors.
The final stages of a star’s life aren’t as well understood and is an important subject of ongoing research [13]. The story is vastly incomplete, with a lot of potholes in our current knowledge yet to be solved. The thirst of human curiosity to understand the night sky, which began in the early astronomers, has neither quenched nor will it ever be.
References
[1] Marigo, P., Girardi, L., Bressan, A., Groenewegen, M.A., Silva, L. and Granato, G.L., 2008. Evolution of asymptotic giant branch stars-II. Optical to far-infrared isochrones with improved tp-agb models. Astronomy & Astrophysics, 482 (3), pp.883-905.
[2] Osterbrock, D.E. and Ferland, G.J., 2006. Astrophysics of gaseous nebulae and active galactic nuclei, 2nd.
[3] Burbidge, E.M., Burbidge, G.R., Fowler, W.A. and Hoyle, F., 2013. 55. Synthesis of the Elements in Stars. In A Source Book in Astronomy and Astrophysics, 1900–1975 (pp. 374-388). Harvard University Press.
[4] Rauscher, T., Heger, A., Hoffman, R.D. and Woosley, S.E., 2002. Nucleosynthesis in massive stars with improved nuclear and stellar physics. The Astrophysical Journal, 576 (1), p.323.
[5] Hashimoto, M.A., Nomoto, K., Tsujimoto, T. and Thielemann, F.K., 1996. Supernova nucleosynthesis in massive stars. In International Astronomical Union Colloquium (Vol. 145, pp. 157-164). Cambridge University Press.
[6] Richmond, M. "Late stages of evolution for low-mass stars". Lecture notes, Physics 230. Rochester Institute of Technology. Archived. from the original on 4 September 2017. Retrieved 3 May 2007.
[7] Liebert, J., Bergeron, P., Eisenstein, D., Harris, H.C., Kleinman, S.J., Nitta, A. and Krzesinski, J., 2004. A helium white dwarf of extremely low mass. The Astrophysical Journal Letters, 606 (2), p.L147.
[8] Wikipedians, B., Classes of Supernovae. PediaPress.
[9] Hubbard, Amy (6 August 2014). "Hubble sees 'zombie star' lurking in space: What it is, why it matters". Los Angeles Times. latimes.com. Retrieved 30 October 2014.
[10] Feltman, Rachel. "Astronomers may have found a new zombie star". Washington Post. Retrieved 30 October 2014.
[11] Heger, A., Fryer, C.L., Woosley, S.E., Langer, N. and Hartmann, D.H., 2003. How massive single stars end their life. The Astrophysical Journal, 591 (1), p.288.
[12] Wald, R.M., 1999. Gravitational collapse and cosmic censorship. In Black holes, gravitational radiation and the universe (pp. 69-86). Springer, Dordrecht.
[13] Christensen-Dalsgaard, J., 2021. Solar structure and evolution. Living Reviews in Solar Physics, 18 (1), pp.1-189.
Comments