The formation, evolution and death of our Solar System

At Home In The Universe Essays > Science, IT, Nature

Part 11: Just like you and me, our Sun and all the other stars have a life cycle of conception, embryo, birth, childhood, adulthood, old age and death. Crucial is how massive they are.

Our Solar System began forming about 4,6 billion years ago, 9 billion years after the Big Bang. A fragment of a giant molecular cloud, made mostly of hydrogen and traces of other elements, began to collapse, probably triggered by the shock wave of a nearby supernova explosion, forming a large sphere in the center, which would become the Sun, as well as a surrounding disk. The surrounding accretion disk would coalesce into a multitude of smaller objects that would become planets, asteroids, and comets.


The sun is a population I star: it has a higher abundance of elements heavier than hydrogen and helium (“metals” in astronomical parlance) than the older population II stars. Elements heavier than hydrogen and helium are formed in the cores of ancient and exploding stars, so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, whereas stars born later have more. This high metallicity is thought to have been crucial to the Sun’s development of a planetary system, because planets form from the accretion of “metals”.

Rocky planets are more likely to be found orbiting high metallicity and low mass stars.

In astronomy and physical cosmology the metallicity is the fraction of the mass of a star (or any other kind of astronomical object) beyond hydrogen and helium. Most of the physical matter in the universe is in the form of hydrogen and helium, so astronomers conveniently use the blanket term metals to refer to all other elements. For example, stars or nebulae that are relatively rich in carbon, nitrogen, oxygen and neon would be metal-rich in astrophysical terms, even though those elements are non-metals in chemistry. So, this term should not be confused with the usual physical definition of solid metals.

Metallicity within stars and other astronomical objects is an approximate estimation of their chemical abundances, that change over time by the mechanisms of stellar evolution, and therefore provide an indication of their age. In cosmological terms, the universe is also chemically evolving. According to the Big Bang Theory the early universe first consisted of hydrogen and helium, with trace amounts of lithium and beryllium, but no heavier elements. It is through the process of stellar evolution, where stars at the end of their lives discard most of their mass by stellar winds or explode as supernovae, that the metal content of the Galaxy and the universe increases.

It is postulated that older generations of stars generally have lower metallicities than those of younger generations. Observed changes in the chemical abundances of different types of stars, based on the spectral peculiarities that were later attributed to metallicity, led astronomer Walter Baade in 1944 to propose the existence of two different populations of stars. These became commonly known as Population I and Population II stars. A third stellar population was introduced in 1978, known as Population III stars. These extremely metal-poor stars were theorized to have been the first-born stars created in the universe.

Today, 13,7 billion years after the Big Bang, the expansion of the universe and the recycling of star-materials into new stars continues.

The Sun and the planets of our Solar System (distances not to scale)

The Solar system will remain roughly as we know it today, until the hydrogen in the core of the Sun has been entirely converted to helium, which will occur roughly 5 billion years from now. This will mark the end of the Sun’s life. At this time the core of the Sun will collapse and the energy output will be much greater than at present. The outer layers of the Sun will expand to roughly 260 times its current diameter: the Sun will become a Red Giant. The expanding Sun is expected to vaporize Mercury and Venus and render Earth uninhabitable, as the habitable zone moves out to the orbit of Mars. Eventually, the core will be hot enough for helium fusion; the Sun will burn helium for a fraction of the time it burned hydrogen in its core.


The Sun is not massive enough to commence the fusion of heavier elements, so nuclear reactions in the core will dwindle. Its outer layers will move away into space, leaving a white dwarf, an extraordinarily dense object, half the original mass of the Sun, but only the size of Earth. The ejected outer layers will form what is known as a planetary nebula, generously returning some of the material that formed the Sun, but now enriched with heavier elements like carbon, to the interstellar medium.

Van Gogh painting
Credit to Vincent van Gogh, Saint-Rémy, France, June 1889
Museum of Modern Art, New York

Thanks to Wikipedia for their postings of the Solar System and Metallicity, to NASA Origins Art, and special thanks to Philip and Phyllis Morrison for ‘Powers of Ten: About the Relative Size of Things in the Universe’.

ShantiShanti is a regular contributor to Osho News

All articles of this series can be found in: At Home in the Universe


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