Why is the Sun mostly hydrogen despite being a later generation star?

Context

The user is curious about the composition of the Sun, specifically the high percentage of hydrogen. They understand that the Sun is likely a second or third generation star, meaning it formed from the remnants of previous stars. The confusion arises because previous generations of stars would have consumed their hydrogen fuel before dying. The question is how the Sun can still be composed of 71% hydrogen if its stellar predecessors presumably exhausted their hydrogen supply.

Simple Answer

• Previous stars didn't use up all their hydrogen.
• When stars die, they spread their stuff around.
• New stars form from this recycled stuff.
• The Sun formed from this mix of old star stuff.
• This mix still had lots of hydrogen left.

Detailed Answer

The premise that previous generations of stars completely exhausted their hydrogen fuel before dying is not entirely accurate. While massive stars do burn through their hydrogen relatively quickly, many stars, especially those smaller than our Sun, have lifespans much longer than the age of the universe. These smaller stars burn hydrogen at a very slow rate and haven't had enough time to exhaust their supply. When these stars eventually die, they don't necessarily explode and scatter their contents far and wide. Instead, they may gently shed their outer layers, enriching the surrounding interstellar medium with hydrogen and other elements. Furthermore, even massive stars, despite their rapid hydrogen consumption, don't convert all of their hydrogen into heavier elements before their demise in a supernova. A significant portion of their outer layers, still rich in hydrogen, is expelled into space during the explosion. This ejected material then becomes part of the raw material for future star formation.

The interstellar medium (ISM), the space between stars, is not a homogeneous void. It's a dynamic environment filled with gas and dust, the remnants of previous generations of stars. This material is constantly being churned and mixed by stellar winds, supernova explosions, and gravitational interactions. The composition of the ISM is constantly evolving as new material is ejected from dying stars and mixes with existing gas and dust. When a new star forms, it draws its material from this interstellar soup. The Sun, therefore, inherited its composition from the ISM at the time and location of its birth. The ISM at that time still contained a significant amount of hydrogen, even after multiple generations of star formation and death. This is because the rate of hydrogen production through the Big Bang and subsequent stellar nucleosynthesis, balanced against the rate of hydrogen consumption in stars, still results in a significant hydrogen abundance in the universe.

The process of star formation itself also plays a role in determining the hydrogen content of new stars. When a molecular cloud collapses under its own gravity, it doesn't all coalesce into a single star. Instead, it fragments into multiple smaller clumps, each of which can potentially form a star. The composition of these clumps can vary slightly depending on their location within the cloud and the specific material they draw from. However, since hydrogen is the most abundant element in the universe, it's likely to be a major component of all these clumps. As the clumps collapse further, they heat up and eventually ignite nuclear fusion in their cores. This process is most efficient when the core is primarily composed of hydrogen. A higher concentration of heavier elements would impede the fusion process, making it more difficult for the star to sustain itself. Therefore, stars tend to form with a high hydrogen content, regardless of the composition of the ISM from which they originate.

Another important factor is the way hydrogen is distributed within a star. Nuclear fusion primarily occurs in the core of a star, where temperatures and pressures are high enough to overcome the electrostatic repulsion between hydrogen nuclei. The outer layers of a star, however, remain relatively cool and are not involved in fusion. This means that even if a star has been fusing hydrogen in its core for billions of years, its outer layers will still be composed primarily of hydrogen. When a star dies and sheds its outer layers, this hydrogen-rich material is returned to the ISM, replenishing the supply of hydrogen available for future star formation. The Sun, being a relatively small star, will not explode as a supernova. Instead, it will eventually evolve into a red giant and then shed its outer layers as a planetary nebula, leaving behind a white dwarf. The material expelled in the planetary nebula will be rich in hydrogen, contributing to the overall hydrogen content of the ISM.

In summary, the Sun's high hydrogen content is a result of several factors: previous generations of stars did not completely exhaust their hydrogen fuel, the interstellar medium is constantly being enriched with hydrogen from dying stars, the star formation process favors the formation of hydrogen-rich stars, and hydrogen is preferentially concentrated in the outer layers of stars, which are returned to the ISM when they die. The Sun formed from a region of the ISM that still contained a significant amount of hydrogen, and this hydrogen became the primary fuel for nuclear fusion in its core. This process is expected to continue for billions of years to come, ensuring that the Sun will remain a stable and luminous source of energy for the foreseeable future. Therefore, the abundance of hydrogen in the Sun is a natural consequence of the ongoing cycle of star formation and death in the galaxy.