Background image: The Carina nebula, an estimated 7,500 light-years away in the southern constellation Carina the Keel (of the old southern constellation Argo Navis, the ship of Jason and the Argonauts, from Greek mythology). See star birth in the extreme >
Apart from the Big Bang era most of the information we have on the universe comes from the stars. Understanding stars and their evolution is key to understanding our universe and its history. However, the process of star formation remains a complex problem to understand and any progress is a major step into progress in cosmology. The sun is a standard star in our Galaxy and the most dominant stellar population is composed of standard suns called main sequence stars. More massive stars get rarer; actually the second dominant stellar population are white dwarfs. The mass of a star is what determines how it is going to evolve, for how long and what chemical elements it is going to synthesise before the end of its life. Astronomers express the mass distribution of all kinds of stars in a quantity called the initial mass function whose spectrum stretches from 0.1 to 100 solar masses, accounting for stellar populations from brown dwarfs, red giants, white dwarfs to supermassive stars. The size of stars can only be determined when they part of a binary system.
Stars are characterised by their absolute luminosity, that is the total energy radiated per unit time in the form of photons. This ranges between 10-6 to 106 as a factor of the solar luminosity. However, luminosity radiated in the form of neutrinos is lost to the energy capital of a star because neutrinos travel through space without interacting with matter (i.e. their detection on Earth is an experimental challenge). The only (serendipitous) detection happened in 1987 during the explosion of the Supernova 1987A in the Large Magellanic Cloud.
The presence or scarcity of metals (i.e. Carbon, Oxygen and heavier elements) segregates two stellar populations: young Population I (rich in heavy metals) and old Population II (poor in heavy metals). Most stars are part of a binary system or a cluster of stars. There are 3 kind of stellar clusters in our Galaxy: galactic or open clusters found in the vicinity of the disc, globular (spherical) clusters found in the halo, and associations found in the spiral arms (hot blue stars such as O-B associations). The sun is a Pop I star; there are speculations that Jupiter (a quasi-star) is its binary companion.
The observational properties of stars come from their surface: particularly their temperature and the spectral lines of their chemical abundances. Hence stars are classified by spectral types O, B, A, F, G, K, M from hot to cooler temperatures (25,000 °K to 2,200 °K) and according to their dominant spectral lines. Each spectral type is organised in sub-classes. The sun is a G2 class.
A star is a chemical factory and its life span is traced with the synthesis of chemical elements – depending of its mass- from Hydrogen to Helium burning, to more advanced stages such as Carbon and Oxygen burning, through to explosive burning stages. Key information on stellar evolutionary stages and stellar nucleosynthesis can be summed up in three diagrammes: 1) the Hertzsprung-Russel diagramme which represents the total luminosity versus surface temperature of stars, 2) the chemical abundances of elements relative to Hydrogen, and 3) the binding energy of the nuclei of chemical elements. Put together they correlate the stages of evolution of a star with the synthesis of a given element through nuclear burning as well as they corroborate the fact that the life of a star is powered by nuclear energy.