Shannonside Astronomy Club

[Simon Kenny]

Stellar Evolution (Article118)

Now and again, strange things happen in the sky. Over time these strange happenings were observed and noted down by astronomers and writers. Star-like objects suddenly appeared in the sky, got brighter until some of them could be seen in daylight and then slowly faded and disappeared. There are two famous examples of these in the past millennium. On July 4th, 1054, Chinese astronomers observed a new ‘guest star’ in the constellation Taurus. It grew brighter until it was the brightest object in the sky after the Moon and could be seen in daylight for three weeks. The astronomer Johannes Kepler observed a similar event in 1604. Kepler wrote about the event in a book entitled ‘De Stella Nova’ (About the New Star), from which derives the name: nova, or supernova. What was observed in both events was the death of a star.

All stars begin their lives in clouds of dust and gas, mostly hydrogen and helium in interstellar space, that collapse into clumps due to gravity. If enough gas is accreted, gravity compresses the gases in the centre of the clump until the compression and temperature ignites the fusion reactions that are the source of every star’s energy.

The amount of gas going into the making of the star determines how long it will live and how it will die. At one extreme is the ‘red dwarf’. Just large enough to sustain a feeble fusion reaction at its core, its energy output is low and shows up in telescopes as a dim red glow as its name implies. These unimpressive stars are nevertheless the longest living due their low energy consumption and can survive for over half the age of the universe itself. Some neighbouring examples are Barnard’s Star about six light-years and Wolf 359 about eight light-years distant.

At the other extreme is the massive star, many times the Sun’s mass, which lives fast and dies young. It burns its vast hydrogen reserves at prodigious rates creating spectacular, unstable outputs of vast quantities of energy, exhausting it in a few million years. Then, depending on its size, it continues to convert helium to carbon, carbon to neon, neon to oxygen, oxygen to silicon, and finally, silicon to iron, at ever increasing temperatures and decreasing times. Finally, the fuel runs out and the star collapses under its gravity, creating pressures and temperatures so high that the dying star is ripped apart in a cataclysmic explosion, called a Type II Supernova. These stars leave interesting legacies. The stars cores are compressed in the explosion creating super-dense objects like neutron stars, as happened in the 1054 supernova, located inside the Crab Nebula in Taurus—see image by Hubble Space telescope. The expanding gases from the explosion seed the cosmos with elements forged during the star’s lifetime, and during its death throes. These elements go on to make new worlds- and new life. The atoms in our bodies were forged deep inside a massive, dying star!