Image credit: LCOĮventually gravity can no longer contain the outer layers of the red giant and the star ejects these layers into space. Ring nebula image taken using Las Cumbres Observatory. It then becomes a red giant again and remains like this for a few million years with its outer layers continuing to expand. Then a helium fusion shell forms around this core, and the hydrogen fusion shell remains around that. The outer layers then contract and the star's temperature increases a bit.Īfter about 100 million years, the star fuses all its core helium into carbon. This causes the core to expand, which lowers the temperature of the core and reduces the total energy output from what it was during the red giant phase. The helium fusion then heats the core rapidly even more and a helium flash takes place. A hydrogen burning shell forms around the helium core, and the shell contributes more and more helium to the core over time.Įventually the core becomes hotter and denser and reaches a temperature of 100 million K, and helium nuclei begin to fuse into carbon. For a star with the mass of the sun, this expansion takes about a billion years and the star's radius increases 100 times, and its luminosity increases even more. The star's outer layers expand while the core is shrinking and as the expansion continues, the luminosity begins to increase. When hydrogen fusion can no longer happen in the core, gravity begins to collapse the core again. Size comparison of the Sun as a red giant star and the Sun as a main sequence star. Where t is the Sun's main sequence lifetime, a star with a mass 4 times the Sun's would have a lifetime of 1/4 2.5 or 1/32 solar lifetimes. Eventually nuclear fusion exhausts all the hydrogen in the star's core.Ī star's lifetime is proportional to its mass divided by its luminosity t ∝ M/LĪ star's luminosity is roughly proportional to the 3.5 power of its mass so L ∝ M 3.5 The core shrinks and heats up gradually and the star gradually becomes more luminous. Over its lifetime, a low mass star consumes its core hydrogen and converts it into helium. When these field lines line up, the result can be a flare of radiation including X rays. Some of these stars also rotate very quickly which twists their magnetics fields. Some small stars have very deep convection zones. They usually have a convection zone, and the activity of the convection zone determines if the star has activity similar to the sunspot cycle on our Sun. If the star is especially massive, when it explodes it forms a black hole.Low mass stars spend billions of years fusing hydrogen to helium in their cores via the proton-proton chain. These spin rapidly and can give off streams of radiation, known as pulsars. After the dust clears, a very dense neutron star is left behind. This material can collect in nebulae and form the next generation of stars. This scatters materials from inside the star across space. After many thousands of millions of years it will stop glowing and become a black dwarf.Ī massive star experiences a much more energetic and violent end. During these changes it will go through the planetary nebula phase, and white dwarf phase. What happens next depends on how massive the star is.Ī smaller star, like the Sun, will gradually cool down and stop glowing. All stars will expand, cool and change colour to become a red giant. The star then enters the final phases of its lifetime. Smaller stars use up fuel more slowly so will shine for several billion years.Įventually, the hydrogen which powers the nuclear reactions inside a star begins to run out. This means they may only last a few hundred thousand years. Very massive stars use up their fuel quickly. The exact lifetime of a star depends very much on its size. This stage is called the ' main sequence'. Nuclear reactions at the centre (or core) of a star provides energy which makes it shine brightly.
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