Dating the oldest stars is one of the main ways the age of the Universe can be estimated. Let’s see how.
It is one of the most fundamental questions in Astrophysics, also because of the effect it has on our models of the evolution of the universe: what is the age of the Universe? It can be estimated directly by measuring the expansion rate of the universe and extrapolating backwards to the Big Bang. Alternatively, space satellites (like the “Planck” satellite) observing in detail the cosmic microwave background (CMB, a sort of electromagnetic”echo” of the Big Bang) can derive fundamental cosmological parameters. In both cases, astronomers obtained the currently accepted value of around 13.8 billion years. But there is also another method, whose key concept is determining the age of the Universe by dating the oldest stars we can find, as clearly the universe must be older than this. This is a perfectly analogous way to the method geologist use to determine the age of our planet, except in this case they look at rocks instead of stars.
In order to find these ancient stars, astronomers look for stars with a very low amount of metals, as there was no elements other than hydrogen, helium and small amounts of lithium when they were formed. Among the theories explaining why we have never observed one of these stars, the most popular one is that the cast majority of the first stars were extremely high mass, several hundred times the mass of the Sun. Some studies even theorise of supermassive stars with masses more than 10 thousands times that of the Sun, which are thought to be the progenitors of the earliest quasars in the Universe. Stars with higher masses have much shorter lifetimes and all these stars would have died long ago. If this is the case, finding a “first-generation” star (a class of stars known as population III stars) would be almost impossible.
Developing new, bigger telescopes
On 31st October 2021, the James Webb Space Telescope will be launched, succeeding the Hubble Space Telescope. With its 6.5 metres diameter mirror, almost three times larger than Hubble, it will observe some of the most distant events and objects in the Universe with improved resolution and sensitivity over Hubble. This improved accessibility of the most distant objects, will be translated into looking much further back in cosmic time, capturing the light of astrophysical sources that traveled billion years before reaching the Earth.
Another major instrument which will be completed and operational in the near future is the European Extremely Large Telescope (E-ELT), which will be the largest telescope on Earth once it is completed, hopefully in 2025. With a main mirror size of 39 meters it will be able to gather more than 200 times more light than the Hubble Space telescope. The dome it will be housed in will be of a almost comparable size to the Old Trafford stadium in Manchester. This majestic instrument, is being built on the top of a mountain in Chile, a country hosting many other very important observatories like ALMA, Cerro Tololo and Paranal for a very good reason. In facts, Chile enjoys more than 300 clear days per year, with little or no light pollution and an extremely dry climate in locations like the Atacama desert, offering the best possible conditions for observing the sky.
The large mirrors of E-ELT will allow to more clearly see dimmer objects, such as far away smaller stars or exoplanets, generating discoveries that we can’t yet imagine. Once found, these stars then need to be dated to check how old they are. One of the most common way, is the radiometric dating. This method works by calculating the relative abundance of a radioactive nuclei and its daughter nuclei using spectroscopy data, and comparing it to the ratio when the star was formed. The largest the difference due to the decay of the parent nucleus over time, the older the star would be.
Cover image from spacedocumentary.com
References:
- Haemmerle, L. et al. The evolution of supermassive Population III stars. Monthly Notices of the Royal Astronomical Society, v. 474, n. 2, p. 2757-2773, Feb 2018. ISSN 0035-8711.
- Ade, P. A. R. et al. Planck 2015 results XIII. Cosmological parameters. Astronomy & Astrophysics, v. 594, Oct 2016. ISSN 1432-0746.
- Dauphus, N. The U/Th production ratio and the age of the Milky Way from meteorites and Galactic halo stars. nature, v. 435, p. 1203-1205, 2005.
