Stars appear as fixed points of light, but they are immense, dynamic spheres of matter. Modern science reveals that a star is not a solid object or simple gas, but a colossal, self-regulating furnace. The composition of these stellar bodies is constantly altered by the intense conditions within their cores. Understanding what stars are made of is the first step in comprehending their life cycles and immense power.
The Primary Chemical Ingredients
The chemical makeup of almost every star is overwhelmingly dominated by the two lightest elements. Stars are primarily constructed from hydrogen and helium, the raw materials left over from the Big Bang. This pairing accounts for nearly 98% of a star’s total mass.
Hydrogen is the most abundant element, making up about 73% of a star’s mass. Helium constitutes approximately 25% of the total stellar material. The remaining 2% or less consists of all other elements on the periodic table, which astronomers collectively refer to as “metals.”
These heavier elements, such as oxygen, carbon, neon, and iron, are present only in trace amounts. The precise ratio of hydrogen to helium and the percentage of these heavier elements vary depending on a star’s age and formation location. However, the fundamental two-part chemical structure remains consistent across most stars.
Matter in the Extreme: The Plasma State
The immense heat and pressure inside a star prevent its material from existing in the familiar states of solid, liquid, or gas. Instead, the elements are superheated into plasma, often called the fourth state of matter. Plasma is a gas that has become so hot that electrons are stripped away from their atomic nuclei.
This process, called ionization, results in free-floating, electrically charged particles: positive ions and negative electrons. The stellar interior is a dense environment where these charged particles move at tremendous speeds. The entire star, from its core to its outer atmosphere, is composed of this highly energetic, conductive plasma.
The plasma state is necessary for the energy-generating process that defines a star. The extreme temperatures, which reach millions of degrees, provide the energy needed to overcome the repulsion between positively charged atomic nuclei. Without the plasma state, the core reactions that power the star could not occur.
The Engine of Change: Nuclear Fusion
The composition of a star is not static; it is constantly changed in the core through nuclear fusion. For main-sequence stars like the Sun, the primary reaction is the proton-proton (p-p) chain, which converts hydrogen into helium. This process begins when four hydrogen nuclei, which are single protons, collide and fuse together.
Through a series of steps, these four protons are converted into a single helium-4 nucleus. During this conversion, a small amount of mass (approximately 0.7%) is transformed directly into energy according to mass-energy equivalence. This released energy provides the outward pressure that keeps the star from collapsing under its own gravity.
The long-term effect is that the core slowly becomes enriched with helium as hydrogen is consumed. In more massive stars, fusion can occur through the carbon-nitrogen-oxygen (CNO) cycle, which uses these heavier elements as catalysts to convert hydrogen to helium.
When a star reaches the end of its life, it begins fusing helium into carbon and oxygen. Eventually, elements as heavy as iron can be created in the core. The creation of elements heavier than iron, such as gold or uranium, requires the energy release of a supernova explosion.
Reading the Stellar Fingerprint
Scientists cannot sample a distant star, so they rely on the information carried by the star’s light to determine its makeup. This is achieved using spectroscopy, the study of how light is absorbed and emitted by matter. A spectroscope breaks the star’s light into its component colors, much like a prism separates white light into a rainbow.
When the light from a star passes through the cooler gases of its outer atmosphere, specific wavelengths are absorbed by the atoms present. This selective absorption creates a pattern of dark lines, known as absorption lines, across the continuous spectrum.
Each chemical element, such as hydrogen or iron, absorbs light at a unique set of wavelengths. This pattern of lines serves as a chemical “fingerprint” for every element. By analyzing the position of these lines, astronomers identify the elements in the stellar atmosphere. The darkness and width of the absorption lines reveal the precise abundance of each element, allowing scientists to quantify the star’s composition.