Such stars are extremely rare and short-lived. Limits on the upper mass of stars is thought to be somewhere between 150 and 200 solar masses based on theoretical modeling. Gliese 229B, the first brown dwarf, discovered in 1995. Mathematically this relation is expressed by: In general the more massive a star is, the further up the main sequence it is found and the more luminous it is. ![]() If we know where on the main sequence a star is we can infer its mass. This is an incredibly useful relationship, called the mass-luminosity relation. Stellar MassĪs was apparent from the evolutionary Hayashi tracks on the previous page, a star's position on the main sequence its actually a function of its mass. The radiation pressure combined with the gas pressure balances the inward pull of gravity preventing further collapse. This energy exerts an outwards radiation pressure due to the action of the photons on the extremely dense matter in the core. Once the core temperature has reached about 10 million K, fusion of hydrogen occurs, releasing energy. In stars this gas pressure alone is not sufficient to withstand the gravitational collapse. As you try and compress a gas it exerts a gas pressure back, it resists the compression. The more massive the star, the greater its gravitational pull inwards. Main sequence stars essentially have a fixed size that is a function of their mass. Apart from the extremely hot but tenuous corona, the pressure and temperature of stars basically increases as you approach the core. The inward acting force, gravity, is balanced by outward acting forces of gas pressure and the radiation pressure. The simple model of any main sequence star is of a dense gas/fluid in a state of hydrostatic equilibrium. Any model of such stars must be able to account for their stability. Stars on the main sequence also appear to be unchanging for long periods of time. ![]() Mass is the key factor in determining the lifespan of a main sequence star, its size and its luminosity. The rate at which they do this and the amount of fuel available depends upon the mass of the star. They are all undergoing fusion of hydrogen into helium within their cores. Main sequence stars are characterised by the source of their energy. Our understanding of the processes involved and characteristics of this key group of stars has progressed in parallel with our understanding of nuclear physics. The Sun's relative longevity and stability have provided the conditions necessary for life to evolve here on Earth. The majority of stars in the galaxy, including our Sun, Sirius and Alpha Centauri A and B are all main sequence stars.
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