Question

The hydrogen in our Sun is undergoing fusion and turning into helium. Billions of years from now, the hydrogen will run out and the helium atoms will fuse, forming heavier atoms. Eventually, these heavier atoms will also fuse. Interestingly, when astronomers examine the remnants of burnt-out stars, they find them to be extremely rich in iron. Explain why this is so.

Short answer

In the core of a star, nuclear fusion occurs, forming heavier elements by fusing lighter ones and releasing energy. Fusion proceeds in stages, starting with hydrogen fusing into helium and then forming heavier elements such as carbon, oxygen, and eventually iron. Iron is the most stable element with the highest binding energy per nucleon; hence fusion processes involving heavier elements require energy instead of releasing it. As the star's core accumulates iron, fusion becomes unsustainable, leading to core collapse, supernova, or neutron star/black hole formation. The remnants of such stars are extremely rich in iron due to its stability and role in the final fusion processes.

Step-by-step solution

Understand what happens in the core of a star

In the core of a star, nuclear fusion occurs. This is the process where atomic nuclei combine to form larger and heavier elements, releasing a large amount of energy. Initially, stars like our Sun primarily fuse hydrogen atoms to form helium.

Recognize the stages of fusion in a star's lifetime

After the hydrogen in a star's core is depleted, the star then begins to fuse helium to form heavier elements such as carbon and oxygen. This process continues with heavier and heavier elements being formed in successive stages of fusion, each time requiring a higher temperature and pressure in the core of the star.

Understand the role of iron in fusion

Iron plays a crucial role in the fusion process, as it is the most stable element with the highest binding energy per nucleon. When the core of a star starts producing elements heavier than iron (through additional fusion processes), the reactions no longer release energy but instead require energy to take place.

Identify how iron accumulation leads to a star's death

As the core of a star accumulates iron, the energy required to sustain fusion increases, and the core becomes increasingly unstable. When the energy from fusion is no longer sufficient to counteract the gravitational force pulling the star inward, the core collapses, leading to a supernova or the formation of a neutron star or black hole.

Relate iron-rich remnants to burnt-out stars

When a star's core collapses, the outer layers are expelled, and the remnants of the burnt-out core are left behind. Since iron is the most stable element and was the last to be formed during the star's fusion process, this is why the remnants of these burnt-out stars are found to be extremely rich in iron.

Key concepts

Nuclear Fusion

Nuclear fusion is a powerful process that fuels stars, including our Sun. It's the process where lighter atomic nuclei combine to form a heavier nucleus, releasing enormous amounts of energy in the process.
At the heart of this mechanism, hydrogen nuclei (protons) come together to form helium under extremely high temperatures and pressures.
This fusion reaction powers stars for millions to billions of years. As stars mature and exhaust their hydrogen supply, they begin to fuse helium into heavier elements like carbon and oxygen. This transformation doesn't just stop there.
The fusion process continues, sequentially forming heavier and heavier elements in the star's core, each stage requiring hotter and denser conditions.
These fusion reactions are the stars' primary source of energy, balancing out the gravity trying to collapse them into oblivion.

Iron Production in Stars

Iron stands out as a pivotal element in a star's lifecycle due to its incredible stability. It has the highest binding energy per nucleon of all elements, meaning that producing elements heavier than iron requires an input of energy rather than releasing it.
During the lifecycle of a massive star, successive fusion reactions create heavier elements, inching their way up to iron. However, when iron production is reached, no further energy can be extracted from further fusion processes.
Instead, creating elements heavier than iron through fusion absorbs energy, disrupting the balance of inward gravitational pressure and outward heat from fusion within the star. Thus, the build-up of iron in a star's core signifies the end of energy-providing fusion processes. This critical juncture marks the turning point toward the next dramatic stage in a star's life.

Core Collapse Supernovae

When a star's core becomes saturated with iron, it reaches a critical instability. With no further fusion energy available to push outward, gravity wins, causing the core to collapse in upon itself. This rapid collapse results in a spectacular explosion known as a supernova.
A core-collapse supernova is a dramatic explosive event that marks the death of a massive star. During this explosion, the immense pressures and temperatures can create some elements heavier than iron.
The outer layers of the star are expelled into space, and the remnants of the core are left behind, often becoming a neutron star or, if massive enough, a black hole. This cataclysmic event disperses heavy elements throughout space, contributing to the formation of planets, new stars, and even life itself. It's a cosmic recycling process, where stars give back their material to the universe, enriching it with every collapse.