A low mass star changes throughout it's life cycle. Stars begin to form
from clouds of gas in space. The cold temperatures and high densitiesof
these clouds allow gravity to overcome thermal pressure and start the
gravitational collapse that will form a star. As the cloud collapses, is
begins to spin and by the time a protostar is formed, the cloud
flattens and there is a protostellar disk spinning around the protostar.
It then forms into a protostar. A protostar is a star that is still
forming. It looks like a star but its core is not yet hot enough for
fusion to take place. A protostar becomes a main sequence star when its
core temperature exceeds 10 million K. This is the temperature needed
for hydrogen fusion to operate efficiently.The length of time all of
this takes depends on the mass of the star. The more massive the star,
the faster everything happens. Collapse into a star like our Sun takes
about 50 million years. The collapse of a very high mass protostar might
take only a million years. Smaller stars can take more than a hundred
million years to form.
Once the protostar becomes a main sequence star they spend billions of years fusing hydrogen to helium in their cores using the proton-proton chain. Over its lifetime, a low mass star consumes its core hydrogen and converts it into helium. The core shrinks and heats up gradually and the star gradually becomes more luminous. Eventually nuclear fusion exhausts all the hydrogen in the star's core.The star has now reached the red-giant phase.
It is red because it is cooler than it was in the main sequence star stage and it is a giant because the outer shell has expanded outward. When hydrogen fusion can no longer happen in the core, gravity begins to collapse the core again. The star's outer layers expand while the core is shrinking and as the expansion continues, the luminosity begins to increase. A hydrogen burning shell forms around the helium core, and she shell contributes more and more helium to the core over time. Eventually the core becomes hotter and denser and helium nuclei begin to fuse into carbon. The helium fusion then heats the core rapidly even more 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. After about 100 million years, the star fuses all its core helium into carbon. Then a helium fusion shell forms around this core, and the hydrogen fusion shell remains around that. It then becomes a red giant again and remains like this for a few million years with its outer layers continuing to expand. Eventually gravity can no longer contain the outer layers of the red giant and the star ejects these layers into space. The remaining carbon core is still very hot and radiation that ionizes the gas in the expanding shell and makes it glow brightly. Thus starts the final phases of the stars life, the planetary nebula and then the white dwarf phase.
As the core cools, the glowing gas fades and disperses and the planetary nebula dissapears. The cooling carbon core is all that is left. The cooling core is called a White Dwarf. The matter in white dwarfs is very dense. A teaspoon of it, if brought to earth, would weigh several tons.
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