Our body is principally made up of carbon and oxygen, while also containing small quantities of nearly 30 other elements. It might come as a surprise to learn that all the atoms that make up our body were once in stars! In fact, all of them, other than the lightest three (hydrogen, helium and lithium), were made by nuclear reactions in stars, and during many supernovae which are exploding stars. In a new collaborative study, scientists from the Indian Institute of Astrophysics (IIA), Bangalore, University of Notre Dame, USA and Universidad de La Laguna, Spain have attempted to trace out how these elements form by analysing the spectra of ancient stars to determine their chemical composition.
Astronomers study the chemical composition of stars by analysing the light originating from these stars. They examine the spectrum of light using a spectrograph—an instrument which splits light into its constituent colours or wavelengths. Each element produces a characteristic feature in this spectrum, as an increase or decrease in the light put out at a specific wavelength. The strength of each feature tells us how much of the element there is.
Previously, scientists believed that all stars have a similar chemical composition as the Sun. However, in the 1940s, astronomers were able to obtain spectra of a large number of stars and found that many of them had minimal quantities of metals compared to the Sun; metals in this context refers to all elements other than hydrogen and helium.
Since metals are created in stars and supernovae, the stars that formed the earliest have smaller quantities of metals compared to stars like the Sun that formed much later from gas that had already been processed by previous generations of stars. Hence, these metal-poor stars are a relic from the early Universe. Determination of their chemical composition sheds light on how they were formed, the nature of the environment in the early Universe when they formed and various other related questions in the theory of galaxy formation.
In this study, published in the Astrophysical Journal, the researchers have used data from the Sloan Digital Sky Survey (SDSS) dataset, which contains data from 15 years of observations made by an automated telescope that scans the sky, to identify exciting candidates that could be metal-poor stars. They then took spectra using the High-Resolution Spectrograph (HESP) at the Indian Astronomical Observatory at Hanle in Ladakh.
“These are the first scientific results from HESP. We have undertaken a study of the most ancient population of stars in the Universe. This could lead us to the knowledge of the physical processes of the early Universe like the births, deaths and evolution of the first stars,” explains Mr. Avrajit Bandyopadhyay, a doctoral student at IIA who is the lead author of the study.
The researchers analysed the spectra of two classes of stars—Carbon enhanced metal poor (CEMP) stars and extremely metal poor (EMP) stars. CEMP stars are low-metallicity stars that have a high carbon content for reasons that are not well understood. EMP stars, on the other hand, are thought to be the immediate successors of the very first stars in the Universe that formed when the Universe was only a few hundred million years old, a hundredth of its current age.
Surprisingly, the researchers found that these two types of stars have a strikingly similar chemical composition. However, the CEMP and EMP stars they observed have different levels of iron-peak elements—elements close to iron in the periodic table that are only produced during supernovae. This evidence allows the researchers to conclude that these two classes of stars must have different types of ancestors.
While this study sheds some light on the question of where the elements come from, many puzzles remain. Mr. Bandopadhyay’s team intends to step up the effort to unravel them. “Nucleosynthesis (formation of elements) in the primordial and subsequent population of stars is also being investigated. We plan to do subsequent studies in the future with additional details,” signs off Prof. Bandyopadhyay.