In our previous article The Life of a Star, we talked about the different stages of star formation, right from the beginning of a star’s life as a small perturbation in a molecular gas cloud to its retirement from fusing hydrogen in its core and exiting the main-sequence. There are an uncountable number of stars in the entire universe, but neither do we see old stars just wandering around forever nor are there any retirement homes for them. Even the most magnificent celestial beings, like stars, eventually die. Although the events in the lives of all stars are rather straightforward and similar, not every star dies out the same way. In fact, sometimes, the end of a star’s life is so spectacular that it may leave an even bigger impact on the universe than it ever did during its life! So, let’s talk about the different ways a star may end its long journey and perhaps eventually be reborn again!
Our Sun, a main-sequence star (Credit: NASA), and the Hertzsprung-Russell Diagram. (Credits: NSO)
We will track the evolutionary stages of a star using the HR-Diagram (Hertzsprung–Russell diagram), and this journey of ours begins after the turn off point. A main-sequence star processes hydrogen into helium via nuclear fusion. As the star’s core is depleted of its hydrogen fuel, the helium produced sinks into the centre of the star and accumulates, forming a helium core with a thick hydrogen shell around it. The lack of nuclear energy to counter gravitational collapse causes the star to contract and the hydrogen shell to ignite. Nuclear energy is produced in this shell by the CNO (Carbon-nitrogen-oxygen) cycle and it pushes the outer envelopes, causing the star to expand and the temperature to decrease while the luminosity remains fairly constant [1]. This can be seen in the HR diagram as an almost straight horizontal line, called the subgiant branch, over the main sequence.
As the hydrogen shell continues to produce helium, the mass and temperature of the core increase, which in turn increases the rate of hydrogen burning. Stars become more luminous, larger, and somewhat cooler, and it is now said to be part of the Red Giant Branch (RGB).
A red giant star (Source), and an illustration showing the track of a star entering the Red Giant Branch. (Credits: Roen Kelly)
As the star enters the RGB, the convective layers of the star grow deeper and carry the elements forged in the core to the surface, increasing the abundance of carbon, oxygen, nitrogen, and hydrogen on the surface where they appear in the spectrum of the star. This is called the first dredge-up [2]. Here, the star gets cooler while the surface luminosity of the star increases.
The core remains stable and in thermal equilibrium as long as it’s under the Schönberg–Chandrasekhar limit [3], which is the maximum mass a non-fusing isothermal core can have that can support an enclosing envelope. Whether or not the core will be isothermally stable depends on the initial mass of the star. For a star of mass between 1.5 to 6 solar masses, helium keeps accumulating in the core; once its mass gets over the limit, the core contracts rapidly. As the density increases, the temperature approaches 100 million kelvins, which is sufficient to cause helium fusion or helium-burning, generating enormous amounts of nuclear energy and rapidly increasing the temperature, which further increases the rate of helium burning [4]. This process is called thermal runaway nuclear fusion, a process that is accelerated by increased temperature causing subsequent energy releases that further increase the temperature. The star is now out of the RGB and enters the Horizontal branch (HB), where temperatures increase while the radius decreases, thus maintaining constant luminosity.
The helium core of a star at the end of the RGB. (https://sites.ualberta.ca/~pogosyan/teaching/ASTRO_122/lect17/lecture17.html) and a Diagram of the Triple Alpha Process. (Source: https://commons.wikimedia.org/wiki/File:Triple-Alpha_Process.png)
The primary source of nuclear energy generated by an HB star comes from the triple-alpha reactions in its helium core. In the right conditions, three Helium nuclei fuse together to form one carbon nucleus, releasing loads of nuclear energy [5]. Two helium nuclei fuse together to produce beryllium, which is highly unstable with a half-life of 8.19 x 10-17 seconds. This means that it’ll decay in about one-millionth of one-billionth of a second, which leaves an extremely small window for a third helium nucleus to fuse into it, creating one carbon-12 nucleus. [6]. Both the helium core and the hydrogen shell actively produce massive amounts of nuclear energy. The temperature and size of an HB star depend on the mass of the hydrogen shell.
A star tries to maintain equilibrium by expanding and contracting its outer layers, which cause changes in its luminosity. This phenomenon is called stellar pulsation and can be detected by the Doppler effect observed in a star’s spectrum [7]. These stars are used as “standard candles” as the period of pulsation helps astronomers figure out its absolute magnitude, which can be used to calculate the star’s distance from us [8]. Stars with regular pulsations are called Cepheid variables and they pulsate radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude [9].
The Evolution of a 5 solar mass star can be tracked on an HR diagram. (Credit: By Lithopsian - Own work, https://commons.wikimedia.org/w/index.php?curid=48496135), and an illustration of stellar pulsations. (Credits: NASA.)
The products of helium combustion form an inert core of carbon and oxygen. Helium continues to fuse in an envelope around this core, causing thermal pulses, which in some cases cause the star to temporarily increase its temperature. This phenomenon can be seen in the HR diagram as a kind of “loop”. Astronomers named it the “Blue Loop” as increasing temperature causes the star to emit a rather bluish light. Multiple thermal pulses occurring in the helium shell causes the star to go through multiple blue loops [10].
The star now has a carbon core enveloped by a helium-burning shell, which itself is covered with a hydrogen-burning shell. As the helium starts to get depleted, the outer layers expand again and the star gets cooler and more luminous. It follows the same stages of evolution as the RGB; hence, this new branch is called the Asymptotic giant branch (AGB), where the star is more luminous than an RGB star and moves up the HR diagram [11]. It is around this time that the second dredge-up occurs where products from helium combustion are carried to the surface, increasing the abundance of helium and nitrogen in its atmosphere [12].
So far, most of the changes in the star have been internal, with minor changes happening on the surface. But that is about to change soon. In the next article, we’ll see how strong thermal pulses from the interior of the star blow away the outer layers of the star, exposing the hot and solid core and creating a spectacular planetary nebula in the process. We’ll also talk about some of the most exotic celestial objects in the universe.
To be continued…
References
[1] Catelan, M., 2007, September. Structure and evolution of low‐mass stars: An overview and some open problems. In AIP Conference Proceedings (Vol. 930, No. 1, pp. 39-90). American Institute of Physics.
[2] Lambert, D.L. (1992). "Observational effects of nucleosynthesis in evolved stars". In Edmunds, Mike G.; Terlevich, Roberto J. (eds.). Elements and the Cosmos. University of Cambridge. pp. 92–109.
[3] Beech, M., 1988. The Schoenberg-Chandrasekhar limit: a polytropic approximation. Astrophysics and space science, 147(2), pp.219-227.
[4] The evolution of high-mass stars. Archived 2007-10-13 at the Wayback Machine, lecture notes, Vik Dhillon, Physics 213, University of Sheffield. Accessed on line April 27, 2007.
[5] Carroll, B.W., Ostlie, D.A. and Black, A., 2007. An introduction to modern galactic astrophysics and cosmology. Pearson Addison Wesley.
[6] Audi, G., Kondev, F.G., Wang, M., Huang, W.J. and Naimi, S., 2017. The NUBASE2016 evaluation of nuclear properties. Chin. Phys. C, 41(3), p.030001.
[7] Koupelis, T., 2010. In Quest of the Stars and Galaxies. Jones & Bartlett Publishers.
[8] Fernie, J.D., 1969. The period-luminosity relation: A historical review. Publications of the Astronomical Society of the Pacific, 81(483), pp.707-731.
[9] Pigott, E., 1785. VII. Observations of a new variable star. In a letter from Edward Pigott, Esq. to Sir. HC Englefield, Bart. FRS and AS. Philosophical Transactions of the Royal Society of London, (75), pp.127-136.
[10] Groenewegen, M.A.T. and Jurkovic, M.I., 2017. Luminosities and infrared excess in Type II and anomalous Cepheids in the Large and Small Magellanic Clouds. Astronomy & Astrophysics, 603, p.A70.
[11] Vassiliadis, E. and Wood, P.R., 1993. Evolution of low-and intermediate-mass stars to the end of the asymptotic giant branch with mass loss. The Astrophysical Journal, 413, pp.641-657.
[12] Kwok, S., 2001. The Origin and Evolution of Planetary Nebulae. Cambridge University Press, New York.
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