Reality → Matter → Universe → Stars
Our Sun, a giant hydrogen/helium plasma ball about million times the volume and more than 300,000 times the mass of Earth, is a typical star (often somewhat misleadingly named 'yellow dwarf' [1] ). Masses of stars vary from about 0.1 to more than 100 solar masses. Terrestrial and space telescopes register the full electromagnetic spectrum of stars, galaxies and nebulae at various stages of their development (a look into deep space is always also a look backwards in time). Spectral analysis of the detected radiation and ever sharper photographs, as well as computer simulations based on quantum and particle physics have revealed a fascinating picture of creation and recycling of stellar matter. Stars are created by gravitational collapse of huge gas clouds consisting mainly of hydrogen. As pressure and temperature in the core increase, nuclear fusion of hydrogen into helium is ignited and a new star is born. The star then shines at a stable rate over a period of billions of years [2] , with final phases that differ according to the star’s mass [3] . The fusion processes inside stars create increasingly heavier elements, at a rate and limit depending on a star’s mass (pressure), with supernova explosions of supermassive stars providing the extreme pressure and heat needed for formation of the heaviest elements [4] . The spewed matter, together with matter from planetary nebulae , form the interstellar medium which coagulates into dense molecular clouds that become birthplaces of new stars [5] .
A yellow dwarf is a typical main-sequence star. Like 90 % of all stars, the Sun (whose light is more white than yellow) belongs to this group. The main sequence is a dominant band of stars in the Hertzsprung-Russell diagram which plots luminosity against color for all stars.
The Sun, as a typical star, shines about 10 billion years, with half of this period already past. More massive stars burn their fuel faster, less massive ones slower (based on the mathematical model of stellar evolution, a small star of less than 0.5 but more than 0.1 solar masses could shine 100 billion years or longer). During the long main phase of a star’s life (the star is a main-sequence star during this period), equilibrium is maintained between gravitational pressure and the counter-pressure created by radiation from the fusion of hydrogen into helium.
When the hydrogen fuel nears depletion, gravitational pressure exceeds radiation pressure, causing a hotter interior, along with expansion and cooling of outer layers. The star becomes a red giant with greatly expanded volume (when the Sun becomes a red giant, its radius is estimated to expand to about one astronomical unit, i.e., to the orbit of Earth). After a few million years (i.e., a very short period compared to the main phase), the red giant sheds its outer layers into space and the dense and hot core of a typical star becomes a white dwarf (at that stage, the Sun’s mass will be compressed into a body the size of Earth with a density about 200,000 times that of Earth). In the case of a very massive star, temperatures can raise to a point where the kinetic energy of particles overcomes the pressure and triggers a runaway reaction in which all matter is blown apart in a gigantic supernova explosion. In cases where the star’s mass is insufficient to trigger the explosion, a collapse would occur into a superdense neutron star, or even into a black hole. It is estimated that on average one supernova occurs every 50 years in our galaxy. The event can then be observed as a super bright star over a period of a few weeks.
In small and medium stars the fusion stops at helium. To ignite fusion of helium into heavier elements, the required ignition temperature is only reached in stars more massive than the Sun. Stellar fusion stops at iron, the element with the most stable nucleus and highest binding energy. A different process is needed to cross the iron barrier. In a supernova explosion neutrons are slammed into heavy nuclei, building still heavier ones that through subsequent beta decay of excessive neutrons into protons become the nuclei of elements heavier than iron.
Planetary nebulae are emanating from the transition of red giants into white dwarfs (see Note 3) and are thought to be an important source of new star forming matter. Actual star birth is believed to occur in molecular clouds, with gravitational collapse often triggered by shock waves from supernova explosions.