Iron (Fe) is a metal that exists in surprising abundance, forming the majority of our planet’s interior and circulating within our bodies. This element, atomic number 26, is a fundamental building block found ubiquitously across the universe. Iron serves as a physical and biological agent, playing a role in the death of stars, the protection of Earth, and the transfer of life-giving oxygen in every vertebrate.
The Element Created in Dying Stars
Iron marks a definitive endpoint in the process of stellar nucleosynthesis, the fusion reactions that power massive stars. Throughout their lives, these stars fuse lighter elements into progressively heavier ones. This sequence of fusion releases immense energy, which counteracts the star’s powerful inward gravitational force.
The stellar fusion process culminates with the formation of iron-56, which possesses the highest nuclear binding energy per nucleon of any element. Attempting to fuse iron into a heavier element does not release energy but instead consumes it. Iron, therefore, acts as a cosmic “ash” that cannot provide the necessary outward pressure to support the star’s structure. Once the core accumulates a significant mass of iron, energy production abruptly ceases, leading to a catastrophic event. Without energy to resist gravity, the core collapses in milliseconds, triggering a Type II supernova explosion. This explosive event is responsible for forging all the elements heavier than iron, scattering them across the galaxy.
Iron’s Role in Protecting Earth
The importance of iron is not limited to the cosmos, as it is the primary component responsible for Earth’s protective magnetic shield. The planet’s core is composed mostly of iron and nickel, existing as a solid inner core surrounded by molten metal in the outer core. This liquid iron is electrically conductive, a property crucial for generating a magnetic field.
Heat from the core causes the molten iron to undergo thermal and compositional convection, creating massive currents within the outer core. The rotation of the Earth, combined with the movement of this electrically charged fluid, drives a self-sustaining process known as the geodynamo effect. This generates the planet’s powerful magnetic field, or magnetosphere. The magnetosphere extends far into space, forming a barrier that deflects harmful solar wind and cosmic radiation away from the surface. Without the electrically conducting iron in the outer core, Earth would lack this magnetic shield, leaving the atmosphere vulnerable to erosion and making the surface uninhabitable for complex life.
The Metal That Colors Our Blood
Iron is also central to life’s chemistry, serving as the functional core of the hemoglobin protein found in red blood cells. Each hemoglobin molecule contains four subunits, and each subunit holds a heme group with a single iron atom at its core. This iron atom is the specific site where oxygen molecules bind during respiration.
When oxygen is inhaled, it attaches to the iron atoms in the hemoglobin, forming a temporary, reversible bond that transports it from the lungs to the tissues. The presence of oxygen bound to the iron changes the way the molecule absorbs and reflects light, giving oxygenated blood its characteristic bright red color. Deoxygenated blood, carrying carbon dioxide back to the lungs, is a darker, duller red. The body requires a careful balance of this metal; too little iron can lead to anemia, impairing oxygen transport and causing fatigue, while an excess can be toxic, accumulating in organs and causing damage. The ability of iron to switch between oxidation states makes it an efficient carrier, but it necessitates regulation of its storage and use.