The Origin of the Elements

The first stars formed from nebulae that consisted entirely of atoms of the five lightest elements (H until B), which were formed during big bang nucleosynthesis. Atoms of heavier elements, such as carbon (C), oxygen (O), silicon (Si) and iron (Fe), were all formed later during the life cycles of stars in a process known as stellar nucleosynthesis. Through a progressive series of fusion reactions, stars continuously assemble heavier elements out of lighter elements.

The specific elements that may be formed during stellar nucleosynthesis depend on the mass and temperature of the stars. Stars with a low mass, like the Sun, burn slowly and are able to produce elements with an atomic number of up to 6 (C). In comparison, stars with a high mass, for instance 10 – 100 times the mass of the Sun, burn quickly and are able to produce elements with an atomic number of up to 26 (Fe). However, in order to form elements heavier than Fe, even more extreme conditions are required than those that are generally found in very massive stars. The heaviest elements are therefore mostly formed during supernova explosions at the end of a stellar life cycle.

Atoms may either be released into space during the lifetime of stars, or upon their collapse. If atoms move fast enough to overcome the gravitational pull of their stars, they may escape in streams of gas known as stellar winds. Alternatively, they are discharged in large gas clouds and supernovae during the death of stars. In space, atoms may subsequently form new nebulae or may be incorporated into existing nebulae. So, from the remnants of dying stars, successive generations of stars with an increasingly diverse elemental composition are born.

Book reference: Marshak, S. (2007). Earth: Portrait of a Planet: Third International Student Edition. WW Norton & Company.

Image: Galactic center of the Milky Way as seen by the Hubble Space Telescope, Spitzer Space Telescope and Chandra X-ray Observatory. Credit: NASA/JPL-Caltech/ESA/CXC/STScl.

The Formation of the First Stars

Approximately 200 million years after its inception, the universe consisted of massive, slowly swirling nebulae with large voids in between. Because of the effects of gravity, denser regions of these nebulae started to attract gases from their surroundings and thereby started to grow in mass. These regions pulled in progressively more matter and as they became more condensed over time, the initial swirling movement of the gases transformed into a progressively faster rotation around an axis in accretion disks. Eventually, the gravitational attraction of these spinning accretion disks grew strong enough to cause complete inward collapse of the surrounding nebulae. Gravity further moulded the inner portions of these accretion disks into dense balls and consequently large amounts of energy were transformed into heat. These hot balls of gas ultimately became the first precursors of stars, so-called protostars.

Protostars continued growing until their cores became very dense and reached a temperature of approximately 10 million degrees. Under these conditions, hydrogen nuclei joined to form helium nuclei in a series of fusion reactions that released tremendous amounts of energy. The bodies of the protostars began to light up, resulting in the formation of the first true stars approximately 400 million years after the universe was born.

Stars of the first generation were generally very massive (for example, 100 times the mass of the Sun) because of the large amounts of matter present in the young nebulae. These stars burned very hot and bright, but consequently their lifetime was also relatively short – only a few million years. When stars exhaust all of their resources, they die in a dramatic explosion and flash of light known as a supernova.

Book reference: Marshak, S. (2007). Earth: Portrait of a Planet: Third International Student Edition. WW Norton & Company.

Image: The Pillars of Creation in the Eagle Nebula as seen from the Hubble Space Telescope. Credit: NASA/ESA/Hubble Heritage Team.