Matter, the substance that makes up everything around us, exists in various forms, characterized by how its constituent particles are arranged and interact. These arrangements, known as states of matter, depend on conditions such as temperature and pressure. On Earth, we commonly encounter familiar states, but the universe operates under vastly different conditions. Exploring these diverse states helps us understand the fundamental nature of cosmic environments and reveals what constitutes the most prevalent state across the cosmos.
Understanding the Basic States of Matter
On Earth, matter is most commonly observed in three fundamental states: solid, liquid, and gas. Solids possess a definite shape and a fixed volume because their particles are tightly packed and vibrate in place, maintaining a rigid structure. For instance, a rock maintains its form regardless of its container.
Liquids, conversely, have a definite volume but conform to the shape of their container. Their particles are more loosely packed than solids, allowing them to move past one another while remaining close. Water, for example, fills the bottom of a glass but retains its volume.
Gases lack both a definite shape and a fixed volume, expanding to fill any container they occupy. The particles in a gas are widely separated and move freely with little attraction between them. Air inside a balloon disperses if the balloon is popped, demonstrating its variable shape and volume.
Plasma: The Universe’s Most Common State
Beyond the familiar solid, liquid, and gas, another state of matter, called plasma, overwhelmingly dominates the universe. Plasma forms when a gas becomes ionized, meaning its atoms lose or gain electrons, resulting in a collection of electrically charged particles—ions and free electrons. This process typically occurs under conditions of very high energy, such as extreme temperatures, which cause vigorous collisions that strip electrons from their atoms.
Plasma is often referred to as the fourth state of matter. Unlike gases, plasma is an excellent electrical conductor because of its free-moving charged particles. These particles also make plasma highly responsive to electric and magnetic fields.
The behavior of plasma is governed by collective electromagnetic forces, not just individual particle collisions as in a gas. This allows for complex phenomena like plasma waves and the ability to generate its own electric and magnetic fields. While gases are primarily influenced by gravity, plasma’s charged nature means electromagnetic forces play a far more significant role in its dynamics.
Despite being uncommon on Earth’s surface under normal conditions, plasma constitutes an estimated 99.9% of all ordinary, or baryonic, matter in the universe. This makes it by far the most abundant state of matter observed in the cosmos.
Where Plasma Dominates the Cosmos
The prevalence of plasma in the universe stems from the extreme conditions found in cosmic environments. High temperatures, often exceeding 10,000 Kelvin, are necessary to ionize gases and create plasma. These conditions are widespread throughout space, particularly in massive celestial objects and the vast stretches between them.
Stars, including our Sun, are almost entirely composed of plasma. The immense gravitational pressure and nuclear fusion reactions within stellar cores generate temperatures high enough to fully ionize hydrogen and helium, creating a dense, hot plasma.
Beyond stars, plasma also fills what might seem like empty space. The interstellar medium (ISM), the material found between star systems within a galaxy, is largely composed of diffuse gas and dust. A significant portion of the ISM exists as ionized gas, or plasma, particularly in hotter regions near massive stars.
Even the space between galaxies is not truly empty. The intergalactic medium (IGM) is a tenuous, hot plasma that permeates the vast expanses between galaxies. Though extremely diffuse, the sheer volume of the IGM means it contains a substantial fraction of the universe’s baryonic matter.