The Sun is not a solid, liquid, or even a conventional gas, despite its appearance as a burning ball of fire. Our star exists primarily in a unique physical state known as plasma, often referred to as the fourth state of matter. This superheated substance is created by the extreme temperatures found throughout the Sun, which prevent the formation of stable atoms, molecules, or any solid or liquid structures.
The Sun’s Elemental Makeup
The Sun is overwhelmingly composed of light elements, primarily hydrogen and helium. By mass, hydrogen accounts for roughly 73% to 74% of the Sun’s composition, while helium makes up about 24% to 25% of the total mass. Nuclear fusion in the core gradually converts hydrogen into helium, slowly changing this ratio over billions of years.
The remaining fraction, less than 2% of the Sun’s mass, consists of heavier elements, which astronomers collectively call “metals.” These trace elements include oxygen, carbon, neon, and iron, among others. These elements were inherited from the interstellar cloud of gas and dust from which the star formed, confirming the Sun is a second-generation star born from the remnants of previous stellar explosions.
Understanding Plasma as a State of Matter
Plasma is the fourth fundamental state of matter, distinct from solid, liquid, and gas. It is essentially an extremely hot, ionized gas, meaning electrons have been stripped away from their atoms. This process leaves behind a fluid of freely moving, electrically charged particles: positive ions and negative electrons.
Unlike a normal gas, which is electrically neutral, plasma is highly conductive and responsive to electric and magnetic fields. This electrical nature separates plasma from a simple gas. The majority of the visible matter in the universe, including all stars, nebulas, and the space between galaxies, exists in this plasma state.
The transition from a neutral gas to plasma is called ionization and requires a massive input of energy, typically extreme heat. When atoms collide violently due to high temperatures, the force of the impact overcomes the attractive electric force holding the electrons in orbit. This process creates the electrically charged “soup” of particles that defines plasma. Because plasma is composed of charged particles, its motion and structure are dictated by electromagnetic forces.
Temperature, Pressure, and Ionization Within the Sun
The Sun’s immense mass generates gravitational forces that create the necessary conditions for matter to exist as plasma. In the solar core, where nuclear fusion occurs, the temperature reaches approximately 15 million degrees Celsius (27 million degrees Fahrenheit). This heat, combined with an enormous pressure estimated at 265 billion atmospheres, ensures that all matter is completely ionized.
Even in the relatively cooler outer layers, the plasma state is maintained. The visible surface of the Sun, known as the photosphere, has a temperature of about 5,500 degrees Celsius (9,900 degrees Fahrenheit). Although this temperature is much lower than the core, it is still far too hot for stable, neutral atoms of hydrogen and helium to form.
The density of the plasma varies drastically across the Sun’s structure, driven by the varying pressure. The plasma in the core is incredibly dense, roughly 150 times the density of water on Earth. Conversely, the plasma in the outermost layers, like the solar corona, is extremely tenuous, with a density much lower than a high vacuum on Earth. Despite these differences, the matter throughout the Sun’s interior and atmosphere is consistently in the plasma state, dictated by high temperatures and immense gravitational pressure.