hydrogen gas left over from the Big Bang started to collapse due to gravity. In the gas, each hydrogen atom had its own momentum, and when the atoms collected together into larger and larger clumps of gas, the conservation of momentum across all the particles set these clumps of gas spinning. Tey grouped together and eventually the whole cloud of gas starts to spin in a giant circle. Tis massive spinning ball of gas then continued to collapse under its own gravity, spinning faster and faster, just as a figure skater increases her rotation speed by pulling in her arms. Te spinning cloud forms a pancake-shaped disc
because of the rotational force, with a core in the centre where a protostar starts to form. Under its own gravity, the centre collapses. It heats up while gas and dust in the surrounding disc continue to fall in towards the centre. When the protostar is massive enough, about one sixth the size of our sun, the force of gravity holding the core together makes the temperature and pressure at the centre high enough to start a fusion reaction. It ignites and shines like our sun – a star is born. A cool or failed star which lacks the mass to ignite is called a brown dwarf. After the star has developed it is time for planets to
form. In the left-over material in the disc surrounding the star particles of dust collect into larger and larger rocks until planet-sized objects are developed. We will discuss this later, but first let us look at fusion.
9.1.4.1 Creating Elements Through Fusion
Te fusion of nuclei, starting from the initial hydrogen and helium abundance, provides energy and synthesizes new nuclei as a byproduct of the fusion process. Hydrogen atoms fuse into helium, then the fusion of two helium nuclei forms beryllium. Another helium nucleus fuses on to make carbon, and another one fuses on to make oxygen. In really
Figure 9.7: The Smiley image of galaxy cluster SDSS J1038+4849, approximately 4.5 billion light-years from Earth in the Ursa Major constellation, appears to have two eyes and a nose as part of a happy face. The eyes are very bright galaxies. Their gravity distorts the light from a much more distant blue galaxy and is seen as smile lines or arcs. This gravitational lensing effect was predicted by Albert Einstein over 70 years ago, and rings like this are now known as Einstein Rings.
large stars, where temperature and pressure are even higher, further heavier elements are made by the fusion process: silicon, magnesium, argon and calcium, finally up to iron, one of the most common of all elements. Unlike the elements before it, iron
Figure 9.8: Artist’s impression of a protoplanetary disc around the brown dwarf OTS44. A protoplanetary disc is a circumstellar disc, or pancake/ring-shaped accumulation of gas and dust in orbit around a star. Driven by electrostatic and gravitational forces, the matter begins to accrete into larger bodies called planetesimals, and eventually planets.
releases no energy if fused since it has the most stable nucleus of all the elements. Iron thus is the end of the line for fusion. Now, the star’s core begins to fill with iron. Energy production drops, and there is no longer enough energy to stop the pull of gravity. Te star begins to collapse. Te core temperature rises to over 100 billion Kelvin. A supernova explosion is precipitated, and the majority of the star’s mass and elements are blown off into space. Here, these first-generation stars end their lives. Around the heavy-element core of a
dying star, neutrons can be added to iron and lighter isotopes to make bismuth, which then sheds an alpha particle (identical to a helium nucleus) to make lead. In the shock wave from a supernova neutrons are forced onto bismuth and iron nuclei to make thorium, uranium, and other large nuclei. At the end of the star’s life, all these heavy atoms enrich the interstellar gas.
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NASA
NASA/ESA/JPL-Caltech
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