The Universe sprang into being about 13.8 billion years ago, expanding exponentially from a point of infinite density and infinite temperature. Such a point is called a singularity, and this event is popularly known as the ‘Big Bang’. Before this point, time and space did not exist, so the word ‘before’ itself ceases to hold meaning. Within the space of a second, the first elementary particles were formed, and the universe shifted from a state of pure energy to one containing matter as well as energy. Within 20 minutes, the universe had cooled down enough to allow protons and neutrons to combine through nuclear fusion, and the nuclei of the first atoms graced the universe. These were the nuclei of hydrogen (simply a proton), helium, and a tiny bit of lithium. The first true atoms, with positively charged nuclei surrounded by negatively charged electrons, would not arise till about 300,000 years later.
The first stars were created about 100 – 200 million years after the big bang, when areas of large matter densities began to cool and undergo gravitational collapse. These first stars were composed primarily of hydrogen and helium, the first elements in the universe, with perhaps a trace of lithium. These stars were probably massive, many hundreds of times the mass of our sun, significantly hotter, and relatively-short lived. Higher molecular weight elements (rather confusingly called ‘metals’ by astronomers) were formed within these stars as a result of thermonuclear reactions, and when these stars exploded as supernovae, these metals were flung far and wide, finding their way into clouds from which the next generation of stars took birth. As a result, newer stars (our own sun being one of these) are much more metal-rich than those formed during the dawn of the universe.
Our galaxy, the Milky Way, was formed sometime within the first billion years of the big bang, and is rather typical, as galaxies go. It has several spiral arms that form a disk with a prominent central ‘bulge‘. The bulge, only 30,000 – 40,000 light years in diameter, has many times the stellar density of the spiral arms (which stretch for about 100,000 – 120,000 light years end to end). The disk and bulge are surrounded by a spheroidal ‘stellar halo‘, which has ancient globular clusters and old stars occupying a region about 100,000 light years from the galactic center, and a ‘gaseous halo’ made up of hot gas extending for hundreds of thousands of light years in all directions.
The oldest known stars in the universe have been found in the stellar halo of the Milky Way. Ages of some of these stars were determined using the radioactive decay of heavy elements, including a star called HE 1523-0901, which has an iron content about 1/900th that of the sun, and is about 13.2 billion years old. Another star, named HD 140283, informally called the Methuselah star and located in the constellation of Libra only 190 light years away from us, is estimated to be 14.46 billion years old, give or take about 0.8 billion years. This uncertainty of about a billion years saves it from conflicting with the estimated age of the universe (about 13.77 billion years). In 2014, Australian Astronomers discovered SMSS J031300.36-670839.3, the most metal-poor star ever found – the upper limit for its iron content is about ten million times less than the Sun. The age of SMSS J031300.36-670839.3 is estimated to be around 13.6 billion years, placing it within 200 million years of the Big Bang.
Even though metal-poor stars have primarily been found in the galactic halo, it is much more likely that the remnants of the first stars would be found in the highly dense central region of galaxies. In a letter published this week in Nature, astronomers led by Martin Asplund of the Australian National University claim to have found some of the most metal-poor stars in the galaxy hiding in the bulge of the Milky Way. To accurately measure metal content, they relied on the spectral signatures of stars, which basically means deducing elemental composition from the observed colors of objects (colors simply represent the wavelength of radiated light). Using the SkyMapper Telescope in the Australian National University, the astronomers first selected roughly 14000 metal-poor stars located in the galactic bulge. They then observed these stars using the Anglo-Australian Telescope in Siding Spring Observatory, Australia, aided by an AAOmega spectrograph, an instrument of frightening complexity. This technique allowed them to identify more than 500 stars whose iron abundance was less than 1% of our Sun. 23 of the most metal-poor stars were selected on the basis of this study, and these stars were then observed using the 6.5m Magellan Clay Telescope, located at the Las Campanas Observatory in Chile.
One of these stars, SMSS J181609.62−333218.7 was found to have an iron abundance less than 1/10000th of the solar value. In the halo, low metal abundance generally means a high content of carbon. Surprisingly, no carbon could be detected in SMSS J181609.62−333218.7 – in fact none of the 23 stars selected in this study were found to have significant carbon enhancement. While some of these stars were found to be halo stars only passing temporarily through the bulge, SMSS J181609.62−333218.7 appeared to have an orbit restricted almost entirely within the bulge. Even though direct measurement of the ages of these stars is currently not possible, there is a high probability that SMSS J181609.62−333218.7 may be a descendant of the oldest stars in our galaxy, formed less than 200 million years after the big bang. In fact, such stars could be older than the galaxy itself, as the galaxy would probably have slowly condensed around such clusters. As the astronomers write in their paper, “Their low metallicities, orbits and binding energies make these stars prime candidates for being direct descendants of the very first stars, probing a cosmic epoch otherwise completely inaccessible currently”.
Are these truly the oldest stars that there is? Is this the closest we will get to knowing about the beginning of the universe? Answer to both questions is likely no, given the level of our current ignorance, and the fact that very little of the observable world has indeed been mapped in detail. If there exists a single oldest star, the probability of finding it conveniently situated in our own galaxy, which is one amongst about a 100 billion, is miniscule. Whatever be the case, recognizing these ancient remnants of the early universe cannot help but inspire awe, and forcefully bring home to us the incredible youth of our existence.
Howes LM, Casey AR, Asplund M, Keller SC, Yong D, Nataf DM, Poleski R, Lind K, Kobayashi C, Owen CI, Ness M, Bessell MS, Costa GS Da, Schmidt BP, Tisserand P, Udalski A, Szymański MK, Soszyński I, Pietrzyński G et al. Extremely metal-poor stars from the cosmic dawn in the bulge of the Milky Way. Nature 2015;527:484–487.