Open an imaginary container holding a small amount of hot gas in empty space: the gas will begin to expand. Now picture the Sun. It also lacks a solid shell that would enclose its material. Why does the Sun stay together? And if gravity holds it together, why does it not keep shrinking without limit?
The Sun is a star: a huge body of hot matter, mainly hydrogen and helium, held together by its own gravity. Much of that matter is plasma. Gravity pulls inward, while pressure decreasing outward provides support against compression. The Sun, a main-sequence star, is a useful starting example; later stages of stellar life and stellar remnants need further qualifications. NASA: the Sun’s structure, stellar life stages.
Hydrogen and plasma answer different questions
“Hydrogen” and “helium” name chemical elements: the material making up a star. “Plasma” describes a state of matter. Saying that the Sun is made of plasma instead of hydrogen therefore mixes two kinds of description. Hydrogen can itself be part of a plasma.
A small model is enough to understand this without a separate lesson on atoms. An atom has a positively charged nucleus and negatively charged electrons. In a neutral atom, their charges balance. When some electrons separate, positive ions and free electrons remain. Gas containing such particles can behave as plasma: it conducts electric current and responds collectively to electric and magnetic fields. DOE: what plasma is.
In the hot solar interior, collisions help keep the material strongly ionized. However, “plasma” does not mean that every atom in every layer has lost all its electrons. Partial ionization matters in the Sun’s outer regions. Removing an electron does not turn hydrogen into helium either: the charge state has changed, while the nucleus remains the same. Christensen-Dalsgaard: the equation of state of solar matter.
“Ball of gas” is useful provided we do not imagine ordinary air with the same density everywhere. The Sun has no solid crust, and conditions vary greatly with depth. Its visible “surface”, the photosphere, is the region from which most visible light escapes, rather than a floor to stand on.
Particles move even while a star keeps its size
Return to the gas container. Particles keep moving in different directions. When they strike a wall, they transfer momentum, changing its motion or the force needed to hold it in place. Average force per unit area gives pressure. At fixed volume and particle number, heated gas generally exerts more pressure because its particles’ thermal motion becomes more intense. OpenStax: a molecular model of gas.
A star has no walls between its layers, but neighboring regions of matter still exchange momentum. Pressure therefore exists without a container. At the same time, gravity acts on all the mass and holds the hot material together. In the Sun, thermal particle motion supplies most of the internal pressure; radiation also contributes. This first picture does not include every effect of magnetic fields and flows.
Branch · With formulasOptional · you can skip this
What does “hotter means higher pressure” actually assume?
In the ideal-gas approximation, particle pressure is
P ≈ nk_(B)T.
Here P is pressure in pascals, n is the number of gas particles per cubic meter, T is absolute temperature in kelvins, and k_(B) is Boltzmann’s constant. The count n includes ions, free electrons, and any neutral atoms or molecules present; an electron bound within an atom is not counted as a separate gas particle.
Compare two states with the same n in this approximation:
(P_(2))/(P_(1)) ≈ (T_(2))/(T_(1)).
If the temperature in kelvins doubles, pressure approximately doubles too. This does not mean that a heated star keeps its former size and density. It may expand, changing the number of particles per unit volume; ionization may also change. We have compared local states under a stated condition, rather than calculated the Sun’s future. Accurate models require corrections to the ideal gas. The equation of state and its limits.
A pressure difference supports a layer
Select a small thin layer inside the Sun. Its inner side is closer to the center; its outer side is farther away. Neighboring matter presses on it from both sides. Pressure on the inner side is higher, so the outward force exceeds the opposing force from the outer side. The difference can balance gravity acting on the selected layer itself.
This is hydrostatic equilibrium: on average, the pressure gradient, the change in pressure with distance, balances gravity. What matters is pressure decreasing outward. If the same pressure acted on two faces of equal area, those two forces would cancel, but gravity would remain. Princeton: stars in hydrostatic equilibrium.
In the diagram, “upward” means outward only for this small fragment. Follow the two pressure forces and gravity separately, then mentally add them together.
A small layer in balance
Outward from the center ↑
Toward the center ↓
- Fout
- Force from the lower outer pressure
- Fin
- Force from the higher inner pressure
- Fg
- Gravity acting on the layer
The force from the higher inner pressure balances the force from the outer pressure together with the layer’s weight.
Equilibrium does not stop the particles’ thermal motion. It can also coexist with flows and oscillations around the average structure. The Sun is moving matter, rather than a frozen ball. If support is insufficient in a region, material begins to contract; if the net force points outward, it accelerates outward. Subsequent restructuring changes pressure, density and temperature themselves.
Hot matter glows; lost energy needs replacing
A candle and the Sun both emit light, but their energy sources differ. In an ordinary candle flame, chemical reactions between fuel and oxygen release energy. The Sun needs no such reaction to shine. Its hot matter emits radiation, with energy arriving from deeper regions. Today, nuclear reactions in the solar core turn hydrogen into helium and release energy. NASA: why the Sun is not an ordinary fire.
The distinction between two questions matters. “Why does the material glow?” concerns radiation from a hot medium. “Why does it remain hot for so long?” calls for a source that replaces lost energy. Fusion inside the Sun sustains its long-lasting light and the conditions needed for pressure, but it is not a separate mechanical wall pushing on every layer.
The stage of stable hydrogen fusion in the core is called the main sequence. Later, a star’s core composition and the locations of nuclear reactions can change. A white dwarf, a stellar remnant, can glow while gradually cooling without that fusion. We therefore cannot apply the present-day Sun’s description unchanged to everything called a “star”. NASA: stellar lives and remnants.
Branch · EvidenceOptional · you can skip this
What light shows and what we have to infer
Astronomers spread light into a spectrum and identify characteristic sets of lines from different elements. This is evidence about matter in the regions where those lines form. To obtain an element’s abundance, we must account for temperature, density and the formation of light: the darkest line does not necessarily identify the most abundant element. NASA/ESA: reading a spectrum.
An ordinary photograph of the photosphere does not reveal the opaque interior. Its structure is inferred from physical models and tested against available observations. Oscillations of the solar surface, for example, carry information about internal structure; helioseismology analyzes them. This is neither drilling nor a direct image of the core, but a way to test a model’s consequences. Christensen-Dalsgaard: helioseismology.
Return to the imaginary container. The Sun lacks its solid wall: its own gravity confines the gas. Pressure increasing inward supports a quiet star against unlimited compression. Hydrogen and helium name the main material; plasma names a state with free charged particles; light is energy leaving the star.
Would a large but equal pressure on both faces of our small layer be enough? No. Equal pressure forces would cancel each other, leaving the layer’s weight. This check explains why “pressure against gravity” must retain the pressure difference. You can trace the difference between emitted and reflected light through sunlight and moonlight. Later in the course, we will examine how such a star forms from a cloud of matter.