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Stars make Helium as they shine - so more with age.
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Why would an older main sequence star composed of a higher percentage of helium than a young main-sequence star?
Im no astrophysicist but an older star would consume more of its hydrogen over time in comparison to helium through its fusion process. Over time concetrations would change resulting in a higher helium to hydrogen ratio.
What is the name for a star that fuses hydrogen into helium?
Into heavier elements like carbon and oxygen.
Main sequence stars are also called stars?
A "main sequence star" is not really a type of star but a period in a stars life. When a star is in "main sequence" it is converting hydrogen into helium. It is then usually called a dwarf star. This is the longest period in a stars lifetime. Our Sun is a yellow dwarf in "main sequence".
How is the evolution of a main-sequence star with less than 0.4 M fundamentally different from that of a main-sequence star with more than 0.4 M?
Main-sequence stars with mass less than 0.4M convert all of their mass into helium and then stop fusing. Their lifetimes last hundreds of billions of years, so none of these stars has yet left the main sequence. Core hydrogen fusion ceases when hydrogen in the core of a main-sequence star with more than 0.4M is gone, leaving a core of nearly pure helium surrounded by a shell where hydrogen fusion continues. Hydrogen shell fusion adds more helium to the star's core, which contracts and becomes hotter. The outer atmosphere expands considerably, and the star becomes a giant. Comments: I guess 0.4M means 0.4 solar masses. Usually the "M" has an extra little symbol next to it when it means this. Also I'm not sure that there's an exact number you can put on the division between these two types of star. I could argue with the details in the answer, but I have not got the time. Anyway the basic idea seems correct.
How long would main sequence star last if twice as massive as the sun?
A star with a mass of 2solar masses, 2times the mass of the sun, would have a main-sequence stage of half the life of a star with the mass of our sun. More massive stars die faster, less massive stars live longer and therefore have longer main-sequence stages.
Why would an older main sequence star composed of a higher percentage of helium than a young main-sequence star?
Im no astrophysicist but an older star would consume more of its hydrogen over time in comparison to helium through its fusion process. Over time concetrations would change resulting in a higher helium to hydrogen ratio.
What is the name for a star that fuses hydrogen into helium?
Into heavier elements like carbon and oxygen.
Do main sequence stars have a shorter life cycle than other stars do?
All stars that are converting hydrogen into helium via nuclear fusion are said to be on "main-sequence". See related question for more information
What kind of stars are in a band down the the middle of a Hertsprung-Russell diagram?
That's more or less the description of the so-called "main sequence". Those are the stars that get their energy by fusing hydrogen into helium.
What is the main predictor of where a star will spend most of the time on the main sequence and why?
The star's mass. More mass will make the star hotter, and will increase the pressure in the center; this will make the star burn its fuel faster.When a star is on the "main sequence", it burns hydrogen-1, converting it into helium-4.
How do we know that the Sun is made mostly of hydrogen and helium?
If the star plots on the main sequence of the Hertzsprung-Russell (HR) diagram its main energy source is hydrogen fusion. As it converts to helium fusion the core collapses a bit and the temperature of the core goes up. This causes the star's outer layers to expand and the star's surface appears cooler (more red). At this point it will no longer plot on the main sequence of the HR diagram.
Main sequence stars are also called stars?
A "main sequence star" is not really a type of star but a period in a stars life. When a star is in "main sequence" it is converting hydrogen into helium. It is then usually called a dwarf star. This is the longest period in a stars lifetime. Our Sun is a yellow dwarf in "main sequence".
How is the evolution of a main-sequence star with less than 0.4 M fundamentally different from that of a main-sequence star with more than 0.4 M?
Main-sequence stars with mass less than 0.4M convert all of their mass into helium and then stop fusing. Their lifetimes last hundreds of billions of years, so none of these stars has yet left the main sequence. Core hydrogen fusion ceases when hydrogen in the core of a main-sequence star with more than 0.4M is gone, leaving a core of nearly pure helium surrounded by a shell where hydrogen fusion continues. Hydrogen shell fusion adds more helium to the star's core, which contracts and becomes hotter. The outer atmosphere expands considerably, and the star becomes a giant. Comments: I guess 0.4M means 0.4 solar masses. Usually the "M" has an extra little symbol next to it when it means this. Also I'm not sure that there's an exact number you can put on the division between these two types of star. I could argue with the details in the answer, but I have not got the time. Anyway the basic idea seems correct.
What is the main sequence if a black hole?
In astronomy the term main sequence is understood to apply to stellar evolution; since black holes are not themselves considered stars so much as "stellar remnants" they would not fall on this sequence. It would be appropriate to say they are most commonly created at the end of life (once the fuel is exhausted) of a larger star and thus would be more likely to pertain to the most massive stars of the upper main sequence.
How long would main sequence star last if twice as massive as the sun?
A star with a mass of 2solar masses, 2times the mass of the sun, would have a main-sequence stage of half the life of a star with the mass of our sun. More massive stars die faster, less massive stars live longer and therefore have longer main-sequence stages.
What is the suns next stage after 5 billion years?
Theoretically, in 5 billion years, the Sun will still be in the main sequence. 5.4 billion years (estimated) marks the end of main sequence, where it will be in the intermediary point between a main-sequence star and a post-main-sequence red giant, where it will slowly reach over 200 times it's current mass over the course of about a billion more years, at which point it would be classified as an official red giant and be fully burning helium-4 as a main source of fuel as well as burning a hydrogen shell it built up during the main-sequence. After that point it will eject most of its mass into a planetary nebula and slowly cool as a stellar remnant over the course of a few quadrillion years until it reaches 3K.
Why does an old main sequence star have more helium than a young main sequence star?
Stars convert hydrogen into helium over time.However, please note that they do so at very different rates; a massive star burns its fuel (hydrogen) much faster than a less massive star - so you can well have a massive star, which is only a few million years old, but which already burnt much of its fuel.
