Stars begin in large interstellar gas clouds. Collapsing under the weight of gravity, a protostar forms. A star like our sun will spend 30 million years in this life stage. When the temperature reaches 10 million K in the core, hydrogen fusion begins and the star enters its adult life. boojum.as.arizona.edu
A low-mass, Main Sequence star--like our Sun--spends about 10 billion years fusing hydrogen into helium in its core via the proton-proton chain process. members.wolfram.com
When the star has fused all of the hydrogen in its core into helium, the core shrinks and begins to heat up. When the temperature in the area surrounding the inert helium core reaches 10 million K, hydrogen fusion begins in that shell and the star swells into a red giant. The star will spend another billion years in this life stage. fas.org
When the temperature becomes high enough in the core, helium fusion starts quite quickly with what is called a helium flash. The star's core and hydrogen-fusing shell expand with the increased fusion. When the fusion rates slow down with this expansion, gravity again pulls the star's mass inward and it shrinks in size. linus.highpoint.edu
The star is now a double-shell fusion red giant. When it's core has finished fusing all of the hydrogen into carbon, helium fusion takes place in the next shell while hydrogen fusion takes place in the shell outside the first one. When the core is nearly all carbon, thermal pressure can no longer push back against gravity, so the star once again expands. This life stage will only last 30 million years. staff.on.br
With it large size and it's inert core, gravity can no longer help it hold its total mass, and the star ejects its outer layers. As a result, dust and gas clouds now surround the core.Because he core is still very hot, it emits ultraviolet radiation, causing the gas and dust surrounding it to glow. We call the phenomenon a planetary nebula. www.youtube.com YouTube
After the planetary nebula dissipates, the star--now called a white dwarf-- is left alone. It's diameter is similar to that of the Earth. Electron degenerate pressure resists the inward push of gravity and prevents collapse. Essentially the star's inert carbon core, the white dwarf will slowly cool but never be recycled as new stars form. cse.ssl.berkeley.edu

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