<P> Early in their development, T Tauri stars follow the Hayashi track--they contract and decrease in luminosity while remaining at roughly the same temperature . Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the Henyey track . </P> <P> Most stars are observed to be members of binary star systems, and the properties of those binaries are the result of the conditions in which they formed . A gas cloud must lose its angular momentum in order to collapse and form a star . The fragmentation of the cloud into multiple stars distributes some of that angular momentum . The primordial binaries transfer some angular momentum by gravitational interactions during close encounters with other stars in young stellar clusters . These interactions tend to split apart more widely separated (soft) binaries while causing hard binaries to become more tightly bound . This produces the separation of binaries into their two observed populations distributions . </P> <P> Stars spend about 90% of their existence fusing hydrogen into helium in high - temperature and high - pressure reactions near the core . Such stars are said to be on the main sequence, and are called dwarf stars . Starting at zero - age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity . The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion (4.6 × 10) years ago . </P> <P> Every star generates a stellar wind of particles that causes a continual outflow of gas into space . For most stars, the mass lost is negligible . The Sun loses 10 M every year, or about 0.01% of its total mass over its entire lifespan . However, very massive stars can lose 10 to 10 M each year, significantly affecting their evolution . Stars that begin with more than 50 M can lose over half their total mass while on the main sequence . </P>

What is the true power source for the sun and other main sequence stars