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As Hydrogen burning occurs, a new core of Helium ash is formed within the H envelope. As more He is formed, more energy is released and the temperature increases. When it is hot enough, He-burning can begin, which forms a new core of Carbon. As more Carbon is formed, this core expands also. These processes cause the star to swell.
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Is it true that an average star spends 90 percent of its life as a helium burning main sequence star?
no, as a hydrogen burning main sequence star.
Why is the presence of large amounts of helium in the spectrum of a star generally considered a sign of age?
Stars begintheir lives burning hydrogen and converting it to helium. If there are large amounts of helium in a stellar spectrum, it would indicate that the star has been making helium for a long time - hence age.
Do hydrogen and helium make the sun?
Yes. Well, actually, it isn't entirely made of hydrogen and helium. It's also made of carbon-based substances and methane gas. The hydrogen burning into helium is what makes the star bright. This is also known as luminosity. Which is how bright something is.
Is a red giant a main sequence star?
No, a red giant is a star that has just left the hydrogen burning main sequence and begun the next step, burning helium. As helium undergoes fusion at a much higher temperature than hydrogen undergoes fusion, the star expands dramatically and as it expands its outer layers cool to red heat.
What gases make the main sequence star?
Hydrogen and Helium
What inevitably forces a star like the Sun to evolve away from being a main sequence star?
Helium builds up in the core, while the hydrogen burning shell expands.
What makes a star stop being a sequence star?
Basically, the stars on the "Main Sequence" are the ones that fuse hydrogen-1 into helium-4. Once a star burns up its hydrogen-1 fuel, or doesn't have much left, it will start burning helium-4 into heavier elements. It is then that the star leaves the "Main Sequence".
What is a carbon fusion?
Carbon fusion is a stage towards the end of a star's life. See para below and link Carbon burning starts when helium burning ends. During helium fusion, stars build up an inert core rich in carbon and oxygen. Once the helium density drops below a level at which He burning can be sustained, the core collapses due to gravitation. This decrease in volume raises temperature and density of the core up to the carbon ignition temperature. This will raise the star's temperature around the core allowing it to burn helium in a shell around the core. The star increases in size and becomes a red supergiant.
Why would an old main sequence star have more helium?
Stars make Helium as they shine - so more with age.
Why must a star convert helium after it has converted all of its hydrogen to helium?
Helium is the star's source of energy and if it has no energy the star would die.
Why is the depletion of hydrogen in the core of a star such an important event?
Hydrogen is the original fuel that keeps a star "burning" by nuclear fusion. No hydrogen and the star will die unless it can start use helium to produce energy. The small mass "red dwarf" stars can't use helium, but more massive stars can.
What two elements make up the most matter in a star?
Hydrogen and Helium
