The early universe was vastly different from the one we know today. The big bang left the universe in a state of relative simplicity. Our observations of the cosmic microwave background tell us that after the big bang, the infant universe was rather uneventful, compared to the present.
Clearly, something changed. Our observations of the oldest galaxies and quasars, from about a billion years after the big bang, show us a remarkably active universe. Sometime between the beginning and then, the universe came to life: the first stars were born.
The first stars were very different from the ones we know today, and they sparked a revolution of sorts. After them, the universe was virtually a brand new place with a great deal of complexity. Understanding them is key to understanding the evolution of the universe as a whole. Although we haven’t observed one of these early stars yet, astronomers have put together many models to describe them using clues from the established timeline of the universe and our understanding of present-day stars. Despite what we can deduce, there is still so much about these fascinating ancient stars we have yet to learn.
The Universe Before Stars
In the instant after the big bang, the universe was in an incredibly hot and dense state. As the baby universe expanded, it cooled and allowed for the formation of quarks and electrons. While the former aggregated rather quickly, forming nuclei, electrons did not form orbits around these nuclei to form atoms for approximately 380,000 years. At the time, these atoms were almost entirely hydrogen and helium. The cosmic microwave background tells us a great deal about this era. The measured radiation is distributed quite evenly telling us that the universe was quite smooth and featureless. As the universe continued to expand, the background radiation was red-shifted to longer wavelengths, making it much harder to detect.
The period between the formation of atoms and the birth of the first stars is what cosmologists refer to as the cosmic dark ages. This name comes from the fact that at the time, there were no sources of light and the universe was composed of merely clouds of gas. There is a large gap in our understanding of cosmic evolution owing to the fact that we cannot currently make any direct observations from this period. The universe at the time seems to have been quite dark and sparse.
The Birth of the First Stars
While information from the dark ages continues to elude us, we know of galaxies and quasars appearing at around one billion years after the big bang. It can therefore be deduced that the first stars appeared anywhere from 100-250 million years after the big bang. These stars would have formed very slowly. In the uniformity of the cosmic microwave background radiation, we see evidence of small-scale density fluctuations which would, over a large enough time scale, form gravitationally bound systems. These would then merge with other systems until the first star-forming protogalaxies formed in the nodes of the densest regions of this structure. Multiple protogalaxies would eventually come together to form galaxies which would in turn, in a process that is still presently ongoing, would come together to form galaxy clusters.
These early protogalaxies would have been similar to the present-day molecular clouds in our galaxy where star formation takes place, measuring anywhere from 30-100 lightyears across and having a mass 100,000-one million times more than that of the sun.
In the early years of the universe, the only elements available were hydrogen, helium, and almost negligible amounts of lithium. Consequently, the first stars were almost entirely composed of hydrogen and helium, a far simpler composition than stars today, which incorporate a number of heavier elements. In astronomy, any elements heavier than helium are referred to as “metals”. Based on the composition of stars, they are grouped into three categories: Population I, II, and III.
Population I stars are the newest and are metal-rich. Population II are older stars and are metal-poor. As one might predict, Population III stars are the oldest stars that contain no metals.
Population III stars
Population III stars is the rather counterintuitive name given to the first stars to appear in the universe. The “Population” system of stellar classification comes from Walter Baade’s original classification of stars in the 1940s. Stars found in the disc region of the milky way are primarily Population I or high-metallicity stars while those found in the central bulge region are primarily Population II or low-metallicity stars. Since Population II stars still contain a high metal concentration compared to the Primordial composition of the universe, the existence of a third group of Population III stars was hypothesized.
While no Population III, or Pop III as they are often called, stars have ever been observed, it is fairly easy to computationally model their formation as a result of their simple composition. We can predict that these stars were significantly more massive than the average star today. Since more massive stars tend to die out faster, these stars had a relatively short lifespan of only a few million years.
Given that their masses likely ranged up to about a 100 solar masses, these stars would end their short lifespans by undergoing supernovas, resulting in the formation of the very first metals. Star-formation systems would incorporate these metals during the formation of future generations of stars and this process would repeat, leading to the formation of the over a hundred elements we know of today.
While our theoretical models of Population III stars line up with what we know about the universe, none of it has been confirmed observationally. While scientists continue to search for signs of the Population III stars in the first galaxies, it is commonly accepted that all of them must have died out billions of years ago due to their massive size. New data collected through new generations of telescopes, such as the upcoming James Webb Space Telescope (JWST), and gravitational wave astronomy might just tell us more about these stars that started it all.
The Universe After Population III
Early stars have moulded the universe into what it is today. For one, they seemed to have played an instrumental role in the reionization of the universe, in what is called the Cosmic Renaissance. The early universe consisted of only neutral atoms, but today’s universe is filled with ionized matter.
Since Pop III stars did not contain any metals, they would have had to be significantly hotter than stars today. As a result, light from these stars was primarily ultraviolet radiation, which would therefore ionize the clouds of hydrogen and helium gas surrounding them.
The process of reionization would have proceeded slowly. At first, only small pockets of ionized gas formed around these stars which would soon revert to their atomic forms. Over time, with the formation of more stars and active galactic nuclei, reionization became more prominent leading to today’s fully ionized universe.
Over time, new stars and galaxies would form from the new matter left over from Population III stars. The black holes they left behind from their supernovas may have combined to form supermassive black holes, the likes of which we see at the centre of nearly every galaxy today, including our milky way. The universe would then evolve into the spectacularly complicated one we know today.
Our models of the early universe seem virtually unrecognizable compared to the one we call home now. These changes can almost entirely be traced back to the first stars. Despite the extensive models we have made about these stars, there is still much to be known about exactly how they came about.
The future holds exciting possibilities for the study of Population III stars. As technology develops and our telescopes get stronger, we might one day find more clues as to how exactly these stars worked. Perhaps we might even be able to look back to when they formed and directly observe them, one day. It is difficult to overstate the impact Pop III stars have had on the universe. As time passes, perhaps we will learn more about them and in doing so, move closer to understanding the elusive secrets of how everything we see around us came about. All in all, it is a remarkable time to study the origins of the universe. .
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