Old at Heart
With myself gradually but persistently sliding into the pre-retirement phase of my life, the above headline was bound to catch my eye. The subheading “Solution to Red Giants' Age Paradox” clearly pointed away from my everyday troubles, but I kept on reading nevertheless. Eventually this announcement by the Max Planck Institute for Solar System Research and its associated literature brought back memories of the somewhat monotonous but catchy tune “Set the Controls for the Heart of the Sun” with which, among other things, we tried to expand our juvenile minds decades ago. The incoherent, mumbled lyrics, apparently influenced by Chinese poetry, were considered absolutely hip. Those were the days… This probably proves that I am old at heart after all. But what about those giants?
It is fairly common knowledge that stars such as our good old sun, currently a “main sequence star”, have a determined life-span, if a pretty long one, leading them through various stages of appearance. Becoming a Red Giant signals that the end is (relatively) nigh – not too untoward at an age of more than 10 billion years. With heavy stars burning their fuel material rather quickly, the mysterious super high mass Red Giants observed by the astronomers posed a paradoxon: they were extremely heavy, technically predisposing them for a shorter life expectancy, whereas the analysis of their building material made them about four billion years older. Enormous plasma currents swill nitrogen, oxygen and carbon to the surface of a star when it becomes a Red Giant. There, these elements can be detected by the astronomers with their fancy instruments, at the same time allowing an indirect view of the contents of the inaccessible core.
As if this would not be complex enough, stars have an additional feature not unlike a heart: they expand and contract. Asteroseismology (in a nutshell: the measuring of these oscillations) also allows conclusions regarding the properties of their innermost structure.  The mechanism behind this movement is entrapment of energy flowing to the surface in so-called opaque layers, which then leads to their expansion. The layers, in turn, become more permeable by their dilation, so when the currents can continue their flow toward the surface, the star “contracts” again. Accordingly these “breathing oscillations” also lead to observable changes in surface temperature and brightness. “Our” sun actually does that too, which has been known since the 1960s.
Coming back to our mysterious Red Giants, a combination of these fascinating techniques made Prof. Saskia Hekker conclude that “some…must have merged with others during or after their transformation into red giants… Their large mass is not an original property and therefore not suitable for age determination. The stars are indeed old.” Quod erat demonstrandum. All this is pretty recent science which has led to a boost in “galactic archeology”. Like the analysis of Roman potsherds, excavated all the time during roadworks in Mainz, gives us insights into the life in our city almost 2000 years ago, galactic archaeology is trying to understand the formation and evolution of our Milky Way and its neighbouring galaxies. They also seem to be expanding all the time. But whereto? And will they ultimately contract or collapse? Then what? Another cycle out of the void?
Chances are that Prof Hekker will be able to give us more answers in and for the future. She has recently been appointed Group Leader, Theory and Observations of Stars (TOS) at the Heidelberg Institute for Theoretical Studies (HITS) and professor of Theoretical Astrophysics at Heidelberg University, Center for Astronomy (ZAH)/Landessternwarte Königstuhl (LSW).  I wish her all the best on her continued search to “Set the Controls for the Heart of the Sun”.
17 October 2020 (online)
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- 1 https://www.mps.mpg.de/Solution-to-Red-Giants-Age-Paradox ; last accessed September 4, 2020
- 2 Waters R. Set the Controls for the Heart of the Sun. On: A Saucerful of Secrets. Magdalene Music & EMI; London: 1968
- 3 Hekker S. Asteroseismology of red giants and galactic archaeology. 6 Nov 2017. arXiv: 1711.02178v1; DOI: 10.1007/978-3-319-59315-9_5
- 4 https://edition.pagesuite.com/html5/reader/production/default.aspx?pubname=&edid=18b98a58-0577-4120-bea8-33ee3dd574b8&pnum=94 ; last accessed September 4, 2020
- 5 Im Herzen der Sterne. https://idw-online.de/de/news753329 ; last accessed September 4, 2020
- 6 https://www.h-its.org/people/prof-dr-saskia-hekker/