Evidence of early megastars was found in an ancient star at the edge of the Milky Way galaxy. Almost as old as the universe itself, this star has an unique chemical composition which identifies it as the offspring of one of the very first stars.
When the big bang first occurred 13.8 billion years ago the universe was flooded with hydrogen and helium. These gasses quickly coalesced into a few megastars – stars up to 200 times larger than the sun. In the world of stars, large often means short-lived. These megastars, with their abundance of fuel, burned hotly and briefly. Through the process of nuclear fusion the hydrogen combined to form helium and the helium combined to form beryllium and so on. Many of the elements that exist in the universe today began in these early stars. However, their short life span meant that the protons did not have time to form much of anything larger than iron. The development of elements is arrested as the star ends its brief span and blows up.
Science predicts that above 100 solar masses, stars would explode in pair-instability supernovas in which there is no physical remnant of the star. The megastar reaches a critical mass where all the fuel is burnt at once and the star rips itself apart. Healthy stars in the prime of life are kept in balance by the opposing forces of inward gravity and outward projection of light and energy. The size of the star determines the end of its life. Giant stars explode in supernovas which release a huge amount of energy and light as much of the physical material condenses at its center. They are theorized to leave behind mysterious black holes or bright, spinning neutron stars, both of which have so much gravity they affect the space around them. Stars the size of the sun will lose their inward pull and expand into red giants. The sun will eventually engulf the entire solar system until it cools and fizzles out, leaving a cool chunk of heavy metals. Both of these events eject much of the material that has been produced by nuclear fusion within the star. Metals and gases are thrown out into the universe to birth new stars.
The pair-instability supernova leaves no trace of itself. Its explosion is so violent it is nearly removed from history. All of its material is scattered across the universe. The entirety of its metals would be thrown out, providing extra, denser matter for new stars. This material condenses into smaller, more stable, longer-lasting stars. The stars made from the material of the first stars of the universe have a slightly different composition than stars with a more complex history. For example, since the early stars did not exist long enough to create heavy metals, their descendants have low iron and none of the common metals such as barium and strontium. However, they have more iron than other low-metal stars because the pair-instability explosion delivered more iron into its surrounding space. Also, since nuclear fusion was happening at such a violent rate, the early stars skipped over the elements with an odd number of protons. The elements found in these stars would go up by twos as atoms smashed into each other, so the stars that subsequently formed would be missing a number of elements.
Just such a low-metallicity star was discovered lurking at the edges of the galaxy. The star was found by a team of astronomers led by Dr. Wako Aoki of the National Observatory of Japan using the Suburu Telescope in Hawaii. They named their find SDSS J0018-0939. Dr. Aoki said it was an unique star with a peculiar chemical composition. The star has 1,000 times less iron than the sun, but still has more iron than other low metallicity stars. The field of stellar archeology investigates the chemical composition of ancient, low-metallicity stars in order to infer the properties of the very first stars of the universe and their explosions. They look at the spectra of these old stars and break down the reflected light into wavelengths to analyze which elements are present. Dr. Aoki states that the unique chemical composition of SDSS J0018-0939 is only possible if it formed from the material ejected from one large exploded star. If it had coalesced from various old stars it would have a much different array of elements; one more typical of other stars.
The goal of stellar archaeology is to map the entire history of star formation. The early universe was an empty, chaotic place. Dr. Volker Bromm from the University of Texas, who wrote a companion to the Aoki’s research, characterizes it thusly: “It was a very featureless, boring Universe. Then the first stars formed and fundamentally transformed it.” The complexity of the resulting universe was very dependent on the early megastars even though they did not last long. They provided the first organization, structure and elegance to the universe. The theory that some of the first stars were super massive, short-lived and exploded in a singular way is fundamental for understanding how the universe evolved.
Evidence of early megastars was found in an ancient star at the edge of the Milky Way galaxy. The star, identified by Dr. Aoki’s team as being a second-generation descendant of a first star, is a milestone in deciphering the universe. Its unique low-metal composition lends credence to the theory of early megastars forming after the big bang and burning just long enough to provide some metals to create more stable stars. A few of those offspring stars are still shining and provide clues to the history of the universe.
By: Rebecca Savastio