Types of Star

The type of star is due mainly to it's age and mass.

  1. A brown dwarf is a very low mass star. (less than about 0.08 solar masses). It does not have a core temperature high enough for nuclear reactions to start in the core, and the heat it generates is due mainly to gravitational contraction.

  2. A main sequence star is has nuclear reactions happening in the core turning hydrogen into helium. The more massive a star, the faster it burns the heium fuel and the shorter it's lifespan. A star the mass of the Sun has a lifetime of about 9 billion years, but the most massive stars, about 200 times the mass of the Sun have a lifetime of only about a million years.

After the hydrogen in the core has been bunt up it will become a Red Giant. If it is large enough, helium in the core may be burnt to make heavier elements and hydrogen burning will move to a shell around the core. The star will expand massive, but cool hence the name Red Giant. Depending on the mass, when all the fuel in the core has burnt up, the star will explode to become a

  1. White Dwarf. These are small and very hot. The outer layers of the star may be blown off to form a shell of matter around the star. No energy production is taking place, and the star gradually cools.

  2. Neutron star. If the star is large enough, then when all the fuel in the core has been burnt up, it will undergo a supernova explosion. The star will be blown apart in a fraction of a second, shining as bright as a whole galaxy. Where the star used to be may be a neutron star – just a big ball of neutrons – possibly spinning like a lighthouse, sweeping an intense magnetic field around in space, in which case we call it a 'pulsar'.

  3. If the remnant of the star left behind is big enough, then the star will collapse into a black hole. Nothing will escape from it – not even light. The star will be invisible against the blackness of space, but can be detected, if for example, it perturbs the motion of nearby stars, or if a gas disk forms around it, radiating intensely as it spirals the black hole prior to falling in, or if the hole defects the light from objects beyond the hole is it passes in front of them relative to us.

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