Star Structure

FIELDS OF STUDY: Stellar Astronomy; Astronomy; Cosmology

ABSTRACT: The structure of a star is dependent on its stage of life. Stars are formed from compressed clouds of gas in outer space. Early stars, called protostars, are simpler than their older counterparts. Most stars contain a hydrogen or helium core, surrounded by a radiative zone and a convective zone. Above the convective zone are the photosphere, or surface of the star; the chromosphere, or the star’s atmosphere; and the corona, an area of plasma found around the outside of the star.

Young Stars

Stars begin their lives as molecular clouds, or giant interstellar clouds of dust and gas, usually helium or hydrogen. The cloud, which accumulates ever more matter, eventually becomes too dense and massive to resist its own strong gravity. At that point, the cloud collapses and breaks into a number of smaller, though still massive, clumps. These clumps continue to collapse, becoming even smaller and denser. As their densities increase, they heat up, becoming protostars.

Protostars are one of the simplest and most common types of star. They are relatively small and normally glow shades of red. Protostars are surrounded by disks of gas and other material pulled in by their strong gravity; this influx of matter causes the fledgling stars to grow. Unlike most stars, protostars do not undergo nuclear reactions in their core. Instead, they use convection to move heat from the core to the photosphere, the surface of the star. Some protostars never progress beyond this stage. If they fail to accumulate enough matter to begin a nuclear reaction in their core, they become brown dwarfs, a type of failed star. Brown dwarfs do not produce energy on their own. Instead, they slowly radiate away their energy as light and heat. When all of their energy has been expended, they become dead stars. However, sometimes protostars manage to generate so much heat that they begin fusing the hydrogen atoms in their core into helium. This creates a main-sequence star, which has a layered internal structure.

The Main Sequence

The main sequence is one of the most stable periods of a star’s life. Main-sequence stars are perfectly balanced in a very important way. The gravity generated by their mass does not cause them to collapse because it is countered by thermal energy produced by the nuclear reactions in their cores. In a main-sequence star, these two forces—gravity and thermal energy—are very close to equal. This balance allows main-sequence stars to stay the same size for as long as their cores continue to burn. Because smaller main-sequence stars do not burn through their fuel as fast as larger ones, the smaller stars survive longer. Earth’s sun is a relatively average main-sequence star.

The structure of a main-sequence star is more complex than that of a protostar. All main-sequence stars undergo nuclear reactions in their cores, constantly fusing hydrogen into helium to generate thermal energy. Beyond the core, the star’s interior structure varies depending on its mass. If the star is between about 0.5 and 1.5 solar masses (that is, 0.5–1.5 times the mass of the sun), the core is surrounded by a tightly packed layer known as the radiative zone, in which thermal energy is transported toward the surface via radiative diffusion (the absorption and reemission of photons). That layer is surrounded by a less dense layer called the convective zone, in which thermal energy is transported via convection. If the star is less than 0.5 solar masses, however, it has no radiative zone; its interior is entirely convective. If it is more than 1.5 solar masses, the two layers are reversed, with the convective zone inside the radiative zone.

Next is the star’s outer layer, the photosphere, where the thermal energy from the core is emitted in the form of light. The photosphere is considered the lowest layer of the star’s atmosphere. A stellar atmosphere, by definition, comprises the layers from which photons (that is, light) can emerge. The next atmospheric layer is the chromosphere, followed by the corona, a distant, volatile region full of dramatic solar winds.

Despite being able to study Earth’s sun as an example, astronomers still do not fully understand some aspects of main-sequence stars. For example, the temperature of the sun’s photosphere is roughly 5,500 kelvins (5,227 degrees Celsius, or 9,440 degrees Fahrenheit). However, even though it is farther away from the extremely hot core, the corona has a temperature of more than one million kelvins. Astronomers are unsure why the corona is so much hotter than the photosphere.

Divergent Evolutionary Tracks

Eventually, all main-sequence stars convert all their hydrogen to helium. First they burn up all the hydrogen in the core. When this is exhausted, their cores contract and their outer layers expand. They start burning hydrogen in these outer layers instead. As each layer burns, the star collapses a little more. Every time it contracts, gravitational potential energy is converted into thermal energy, causing the temperature in the core to increase.

The next stage in a star’s evolution depends on its mass. In theory, very low-mass stars continue to collapse until they are almost entirely helium. Then they shed their outer layers, leaving only their white-hot cores. These cores slowly radiate away all the heat from the star’s time in the main sequence. During this time, the star is called a white dwarf. After all the remaining heat is gone, the star stops glowing and becomes a black dwarf. However, because low-mass stars have such long life spans, this process has never been directly observed. It would take many billions of years for such a star to use up all its hydrogen and become a white dwarf—longer than the estimated 13.8 billion years since the universe began—and trillions more for it to radiate away all its heat.

Larger main-sequence stars take a different path. Their greater mass means that more gravitational energy is converted into thermal energy, raising the temperature of the core to reach more than 100 million kelvins. This allows the core to begin fusing helium into carbon, stabilizing the star. Because helium fusion releases more energy than hydrogen fusion, the star expands far beyond its original size. Such stars are called red giants or red supergiants, depending on their luminosity.

When smaller red giants run out of helium to burn, they shed their outer layers and become white dwarfs. Billions of years from now, Earth’s sun will take this path. However, larger red giants have enough mass that their cores eventually heat up to more than 600 million kelvins. At this temperature, they begin fusing carbon, a multistep conversion process that ends with the production of iron. The star gradually forms onion-like layers of progressively heavier elements, ranging from the outer hydrogen and helium layers to the iron core. Other layered elements include neon, oxygen, and silicon.

Once all of the carbon in a large red giant has been converted, there is nothing left for it to burn. While hydrogen, helium, and carbon fusion all generate energy, iron fusion consumes it. At this point, the star finally succumbs to its own gravity and begins its last collapse. What happens next depends on the mass of the core.

If the core is less than about three solar masses, the core contracts to just a fraction of its original size—typically only about twenty kilometers (twelve miles) in diameter. With such a massive amount of matter compressed into such a small space, the core becomes so dense that its atoms are crushed together, something not normally possible. The protons and electrons in these atoms are smashed together to create neutrons. Eventually, due to a phenomenon known as neutron degeneracy pressure, the core can contract no further. The collapsing matter rebounds, creating a massive shock wave that blows away the red giant’s outer layers. This explosion is called a supernova. The remaining core is called a neutron star.

Because of their extreme density, neutron stars are incredibly heavy. On Earth, one teaspoon of neutron star matter would weigh roughly ten million tons. Because neutron stars have no fuel to burn, they do not glow like conventional stars. Instead, their highly compressed nature gives them an incredibly powerful magnetic field. Neutron stars also rotate extremely quickly. In some neutron stars, known as pulsars, this causes their magnetic fields to produce powerful beams of radiation that, if properly oriented, can be detected from Earth.

If the core of the collapsing red giant is more than three solar masses or so, even neutron degeneracy pressure is not sufficient to halt its collapse. In this case, the core continues to contract, compressing more and more matter into a smaller and smaller space, until its gravitational pull is so strong that even light cannot escape it. At this point, the star has become a black hole.

Theoretical Exotic Stars

Astronomers and physicists theorize that several more types of stars may exist. However, none of these stars have ever been observed. These rare stellar structures are called exotic stars.

One of the most commonly mentioned types of exotic star is the quark star. Quarks are elementary particles that combine to make up hadrons, a class of particles that includes protons and neutrons. They are among the smallest particles in existence. In theory, a quark star is created when a neutron star is compressed beyond the neutron degeneracy limit, but not so much that it becomes a black hole. This would cause some or all of the neutrons break down into their constituent quarks. Though scientists are unsure whether the laws of physics allow for quark stars, many astronomers believe they do. Several known ultradense neutron stars have been proposed as potential quark stars, including XTE J1739-285, in the Ophiuchus constellation, and the pulsar 3C 58.

Another theoretical type of exotic star is the electroweak star. Scientists have proposed that as a neutron star is about to collapse into a black hole, the extreme temperatures generated by the collapse may be hot enough to merge two fundamental forces of the universe, the electromagnetic force and the weak nuclear force, into a single force known as the electroweak force. This force would convert the quarks of the neutron star into leptons, elementary particles that have considerably less mass than quarks. The mass difference would be converted into energy, which could halt the star’s collapse for ten million years or more. Such stars, if they exist, would only emit a small amount of visible light. The majority of their energy emissions would be in the form of neutrinos, a kind of lepton, which are very difficult to detect.

PRINCIPAL TERMS

• chromosphere: the middle layer of a star’s atmosphere.
• convection: the transfer of heat via the movement of molecules in a fluid.
• core: the dense, hot center of a star, where heat is normally generated.
• corona: the outermost layer of a star’s atmosphere, a zone of extremely low-density plasma that is several million degrees hotter than the surface of the star.
• main sequence: in a Hertzsprung-Russell diagram, which plots the temperature/color of stars against their luminosity/brightness, the band of stars that crosses from the upper left to the bottom right, representing the evolution of a "normal" star over the course of its lifetime.
• photosphere: the effective surface of a star and the innermost layer of its atmosphere.
• radiative zone: the layer of a star’s interior in which heat is transferred from the core toward the surface via radiative diffusion, or the absorption and reemission of photons.

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