The Sun is not a solid, a liquid, or an ordinary gas. It is made almost entirely of plasma, a state of matter in which atoms have been stripped of some or all of their electrons by extreme heat. Plasma is sometimes called the fourth state of matter, and it behaves differently enough from the other three that forcing the Sun into the solid-liquid-gas framework misses what is actually happening. The story gets more interesting when you look at how conditions change from the Sun’s core to its visible surface and why, centuries ago, serious scientists debated whether the Sun might be liquid or even have a solid shell.
What Makes the Sun Plasma Rather Than Gas
At temperatures of thousands to millions of degrees, the hydrogen and helium that make up the Sun cannot hold onto their electrons the way atoms do in everyday gas. Electrons get knocked free, creating a soup of positively charged ions and free electrons. This electrically charged state is what distinguishes plasma from a neutral gas. A neon sign, a lightning bolt, and the Sun all share this basic trait: their matter carries electric charge and responds strongly to magnetic and electric fields.
Hydrogen makes up roughly 73% of the Sun’s mass, with helium accounting for most of the rest and heavier elements contributing a small fraction. Throughout the Sun, hydrogen ionization plays a central role in determining how energy moves and how the material behaves. The degree of ionization varies enormously depending on depth: near the visible surface, only about 10% of hydrogen is ionized, while deeper in the interior, essentially all of it is fully stripped of electrons.1The Astrophysical Journal. Simulations of Solar Granulation. I. General Properties At the core, where temperatures reach about 15 million degrees Celsius, atoms of every element are completely ionized. This gradient of ionization from surface to core means the Sun’s plasma properties change dramatically with depth, but plasma it remains at every layer.
Conditions from Core to Surface
The Sun’s core is extraordinarily dense, with matter packed to roughly 150 times the density of water. At that density, the material might sound like it should be solid or at least liquid. But density alone does not determine whether something is solid, liquid, or gas. What matters is how the particles are arranged and how they interact. In the core, the plasma is so hot that the ions are in constant, violent motion, never settling into the kind of ordered lattice that defines a solid or even the close-packed but disordered arrangement of a liquid. The material is best described as a dense, fully ionized plasma, and researchers model it using equations that account for quantum effects and the screening of nuclear charges by the surrounding electron sea.2Contributions to Plasma Physics. Equation of State of Dense Plasma Mixtures: Application to the Sun Center
Moving outward from the core, temperature and density both drop. The radiative zone, which extends from roughly the outer edge of the core to about 70% of the Sun’s radius, is still enormously hot and dense. Energy in this region moves primarily as photons being absorbed and re-emitted, slowly working their way outward. The plasma here is still fully or nearly fully ionized, and it behaves as a thick, opaque fluid.
Beyond the radiative zone lies the convection zone, where the plasma is cooler but still far too hot for neutral atoms to survive intact. Here, energy transport shifts: instead of photons doing most of the work, massive cells of hot plasma physically rise toward the surface, cool off, and sink back down. Hydrogen ionization in this region has a profound effect on the material’s thermodynamic properties, significantly lowering an index that describes how the plasma responds to compression, and this effect extends across nearly the entire convective zone.3arXiv. Hydrogen Ionization Inside the Sun The convection zone is where the Sun behaves most like a boiling pot of fluid, even though that fluid is plasma rather than any liquid you would recognize.
Why the Sun Moves Like a Fluid
One reason people sometimes wonder whether the Sun is liquid is that it moves like one in many respects. The Sun rotates, but not as a rigid body. Its equator completes a rotation faster than its poles, a phenomenon called differential rotation. Helioseismology, which studies oscillations that ripple through the Sun in much the way seismic waves travel through Earth, has revealed that the Sun’s rotation profile is surprisingly complex. The radiative interior rotates roughly as a single unit, while the convection zone shows latitude-dependent rotation rates, with layers of rotational shear at the boundary between the two zones and near the surface.4Annual Review of Astronomy and Astrophysics. The Internal Rotation of the Sun This kind of flow pattern is characteristic of a fluid, not a solid. A solid body would rotate rigidly unless it fractured.
The convective motions at the surface create the granulation pattern visible in high-resolution solar images. Each granule is a cell roughly the size of a large country, where warm plasma rises in the bright center and cooler plasma sinks in the dark lanes between cells. Simulations of this process show that convection is driven by radiative cooling at the surface: as plasma reaches the top, it loses energy, recombines slightly, becomes denser, and sinks back down along narrow, turbulent downdrafts at the edges of the cells.1The Astrophysical Journal. Simulations of Solar Granulation. I. General Properties These flows are remarkably dynamic, with individual granules forming and dissolving on timescales of about ten minutes. The pattern looks a lot like the surface of a pot of boiling water viewed from above, and the physics is analogous, just at enormously higher temperatures and in plasma rather than liquid water.
How Scientists Know What the Interior Looks Like
Nobody has sent a probe into the Sun, and the interior is completely opaque to light. So how do researchers know the core is dense plasma rather than, say, a solid iron ball? The primary tool is helioseismology. The Sun rings like a bell, with millions of acoustic oscillation modes continuously excited by the turbulent convection near the surface. These oscillations travel through the interior, and their frequencies depend on the density and sound speed of the material they pass through.5The Astrophysical Journal. Acoustic Wave Propagation in the Sun: Implications for Wave Field and Time-Distance Helioseismology By measuring the oscillation frequencies from the surface, scientists can infer the internal structure with remarkable precision.
The resulting picture is consistent with a fully gaseous or plasma body from center to surface, with no discontinuities that would indicate a phase boundary between, say, a solid core and a gaseous envelope. If a substantial solid or liquid region existed inside the Sun, it would alter the oscillation frequencies in detectable ways. Nothing of the sort has been found. The helioseismic data also confirmed the rotation profile described above, revealing that the radiative interior rotates nearly uniformly rather than showing the kind of differential rotation seen in the convection zone.4Annual Review of Astronomy and Astrophysics. The Internal Rotation of the Sun
The Historical Debate Over a Liquid or Solid Sun
The question of the Sun’s physical state was genuinely contested for decades in the 19th century and has even attracted fringe attention more recently. Before anyone understood nuclear fusion or plasma physics, the Sun’s nature was an open problem. Some early models treated the Sun as a hot liquid sphere. The idea was not crazy at the time: the Sun clearly radiates like a hot dense body, and liquids were the familiar example of dense, glowing matter. In fact, early calculations of the Sun’s gravitational energy, including work by Helmholtz and Kelvin, explicitly modeled the Sun as a contracting liquid sphere.6New Astronomy Reviews. Why the Kelvin–Helmholtz timescale is not really their timescale
What killed the liquid-Sun idea in mainstream science was thermodynamics. In 1869, Thomas Andrews established that every substance has a critical temperature above which it cannot exist as a liquid, no matter how much pressure is applied. Since the Sun’s interior was already understood to be far hotter than any critical temperature known for ordinary substances, it seemed impossible for any liquid to persist inside it. As one late-19th-century account put it, a liquid interior to the Sun was considered “next to an impossibility” given Andrews’ findings.7Progress in Physics. Liquid Metallic Hydrogen: Building Block of a Liquid Sun The gaseous model became standard and eventually evolved into the plasma model once physicists understood ionization.
An interesting footnote: before the gaseous model fully took hold, researchers placed solid or liquid carbon (graphite) on the Sun’s surface to explain why the photosphere emits radiation resembling a blackbody. Graphite was the best-known source of blackbody radiation in laboratories, so it made sense at the time to propose that something similar coated the Sun.7Progress in Physics. Liquid Metallic Hydrogen: Building Block of a Liquid Sun That idea persisted for about 50 years before being abandoned. Today we understand that any sufficiently hot, dense, opaque material radiates approximately as a blackbody, whether it is solid, liquid, or plasma.
Could Exotic Phases Exist Inside the Sun
A small number of researchers have revisited the liquid-Sun hypothesis by pointing to metallic hydrogen, a phase of hydrogen that can exist at extreme pressures and has critical temperatures in the thousands of degrees rather than the very low temperatures of ordinary molecular hydrogen. The argument is that metallic hydrogen’s elevated critical point could, in principle, allow a liquid phase to survive at solar interior temperatures.7Progress in Physics. Liquid Metallic Hydrogen: Building Block of a Liquid Sun This idea has not gained traction in mainstream astrophysics. The helioseismic evidence fits the standard plasma model extremely well, and no observational anomaly has required invoking a liquid metallic hydrogen interior.
The broader question of phase transitions in hot, dense hydrogen remains an active area of research in high-pressure physics. Laboratory experiments using shock compression and laser-driven techniques have explored what happens to hydrogen and deuterium under the pressures and temperatures found deep inside giant planets and stars. There is evidence for a fluid-to-fluid phase transition in warm dense hydrogen at high pressures, which is relevant to understanding the interiors of gas giant planets.8Physics-Uspekhi. Plasma phase transition For the Sun, though, the temperatures are so far above these transition conditions that the material remains firmly in the plasma state.
When Stars Actually Do Become Solid
The Sun will not stay plasma forever. In about five billion years, after it exhausts its hydrogen fuel, expands into a red giant, and sheds its outer layers, the remaining core will become a white dwarf: an object roughly the size of Earth but with a mass comparable to the Sun’s. White dwarfs are supported not by fusion energy but by the quantum mechanical pressure of tightly packed electrons. As a white dwarf slowly cools over billions of years, it eventually reaches temperatures low enough for its carbon and oxygen ions to settle into a crystalline lattice. At that point, the interior of the star genuinely becomes solid.
Molecular dynamics simulations of dense carbon-oxygen mixtures at white dwarf conditions have mapped out the phase diagram for this crystallization. Observations of white dwarfs in the globular cluster NGC 6397 suggest that the melting temperature of these stellar cores is close to what models predict for pure carbon.9PubMed. Crystallization of carbon-oxygen mixtures in white dwarf stars So while the Sun as it exists today is plasma through and through, its distant descendant will contain a genuine crystal, making it one of the largest solid objects in the galaxy. Some of the oldest white dwarfs already observed are likely crystallized or in the process of crystallizing right now.
What Happens at the Visible Surface
The photosphere, the layer of the Sun you actually see, is the thinnest and coolest part of the Sun’s atmosphere, with temperatures around 5,500 degrees Celsius. Even at this relatively modest temperature (by solar standards), the material is still plasma, though it is much less ionized than the interior. As noted earlier, ascending plasma in the granules carries hydrogen that is only about 10% ionized.1The Astrophysical Journal. Simulations of Solar Granulation. I. General Properties Once the plasma cools and recombines slightly in the dark intergranular lanes, it sinks back down, maintaining the convective cycle.
Above the photosphere, the Sun’s atmosphere becomes even more tenuous and, paradoxically, much hotter. The chromosphere and corona reach temperatures of one to two million degrees, making them even more fully ionized than the surface beneath them. During solar flares, the plasma composition in these outer layers changes rapidly as material is heated and mixed. Researchers track these changes to understand how energy is released during explosive events on the Sun.10The Astrophysical Journal. The Evolution of Plasma Composition during a Solar Flare The corona is so hot and diffuse that its plasma barely interacts with itself, streaming outward as the solar wind and filling interplanetary space. Even at Earth’s distance, spacecraft sit inside the Sun’s extended plasma atmosphere.
Why “Gas” Is Not Quite Wrong but Not Quite Right
Many textbooks and casual descriptions call the Sun a “ball of hot gas,” and this is not entirely wrong. Plasma shares many properties with gas: it fills its container, it has no fixed shape, and its particles move freely. The distinction matters, though, because the electrical charge of plasma gives it behaviors that neutral gas does not have. Plasma can be shaped and confined by magnetic fields, which is why the Sun has sunspots, prominences, and a magnetically structured corona. A ball of neutral gas at the same temperature would not produce any of these features.
For everyday conversation, saying the Sun is made of gas communicates the basic idea well enough. But if you want accuracy, the Sun is a self-gravitating sphere of plasma, with conditions ranging from an extraordinarily dense, fully ionized core to a tenuous, magnetically dominated outer atmosphere. No part of it is solid, no part of it is liquid in any conventional sense, and calling it merely a gas undersells the physics that make it one of the most dynamic objects in our corner of the universe.