Layers of the Sun – Diagram and Facts


Layers of the Sun Diagram
The four layers of the Sun are the core, radiative zone, convective zone, and atmosphere.

The Sun is a colossal nuclear reactor at the heart of our solar system. Our favorite star is about 109 times the diameter of Earth and over 330,000 times its mass. It generates energy through nuclear fusion at its core, where temperatures and pressures are unimaginably high. This energy radiates into space, providing the light and heat essential for life on Earth. Understanding the Sun’s structure is crucial for comprehending various solar phenomena that affect our planet, such as solar flares and space weather.

The Layers of the Sun

The Sun consists of several distinct layers, each with unique properties and processes. These layers fall into two main sections: the solar atmosphere and the solar interior. Or, since the interior layers are so large, the four layers of the Sun are its atmosphere, convective zone radiative zone, and core.

  • Atmosphere
  • Convective Zone
  • Radiative Zone
  • Core

Solar Atmosphere

The solar atmosphere is the outermost region of the Sun, visible during total solar eclipses. It consists of three primary layers:

  1. Photosphere
    • Thickness/Size: Approximately 500 kilometers.
    • Temperature: Around 5,500°C.
    • Characteristics: The photosphere is the Sun’s visible surface, where light is emitted that we see from Earth. It’s marked by granules and sunspots, which are manifestations of the Sun’s magnetic activity. Below this layer, the Sun is opaque to visible light.
  2. Chromosphere
    • Thickness/Size: Roughly 2,000 kilometers above the photosphere.
    • Temperature: 4,000°C near the bottom, but increases with altitude up to 20,000°C.
    • Characteristics: This layer is visible as a red rim during solar eclipses, caused by hydrogen emissions. The chromosphere is home to spicules and solar prominences, dynamic jets, and arcs of solar material.
  3. Corona
    • Thickness/Size: Extends millions of kilometers into space.
    • Temperature: Surprisingly, it’s much hotter than the layers below, reaching over a million degrees Celsius.
    • Characteristics: The corona is the outermost part of the solar atmosphere, visible as a white halo during eclipses. It’s where solar winds originate and is the source of coronal mass ejections (CMEs).

The transition layer is a thin, irregular layer or boundary that separates the hot corona from the relatively cool chromosphere. Beyond the corona lies the heliosphere. The heliosphere is the outermost layer of the solar atmosphere and is analogous to the Earth’s magnetosphere. It has the shape of a large tailed bubble around the Sun and planets, separating the solar system from the interstellar medium as the star travels through space.

Special Features of the Sun’s Atmosphere

The outer surface of the Sun is home to some interesting phenomena, such as solar prominences, flares, sunspots, and coronal holes:

Solar Prominences

Solar prominences are immense clouds of relatively cooler, dense plasma suspended above the Sun’s surface by the Sun’s magnetic field. They appear as bright, loop-like structures when viewed against the dark backdrop of space, but as dark filaments against the bright solar disk.

Where They Occur:

Prominence are visible within the Sun’s chromosphere, but their roots often extend into the photosphere and they frequently project into the lower corona.

Solar Flares

Solar flares are intense bursts of radiation emanating from the release of magnetic energy associated with sunspots. These appear as bright areas on the Sun and are powerful enough to influence Earth’s ionosphere, affecting communication and navigation systems.

Where They Occur:

Flares typically originate in the Sun’s chromosphere, though their effects are visible in the corona.

Sunspots

Sunspots are temporary phenomena on the photosphere of the Sun that appear as spots darker than the surrounding areas. They are caused by concentrations of magnetic field flux that inhibit convection, resulting in reduced surface temperature compared to the surrounding regions.

Where They Occur:

Sunspots are exclusively features of the Sun’s photosphere.

Coronal Holes

Coronal holes are regions in the Sun’s corona that appear darker and are cooler and less dense than the surrounding areas. These regions are sources of high-speed solar wind particles, a stream of charged particles that extend out into space.

Solar Interior

Beneath the solar atmosphere lies the solar interior, comprising three major layers:

  1. Convective Zone:
    • Thickness/Size: Extends from about 70% of the Sun’s radius to the photosphere.
    • Temperature: Decreases from about 2 million °C to 5,500°C as it approaches the photosphere.
    • Characteristics: In this layer, hot plasma rises, cools as it nears the surface, and sinks back down, creating convection currents. This process produces the granulation seen on the photosphere.
  2. Radiative Zone:
    • Thickness/Size: Stretches from 20% to 70% of the Sun’s radius.
    • Temperature: Ranges from 2 million °C to 7 million °C.
    • Characteristics: Energy generated in the core moves outward through the radiative zone via radiation. Photons (light particles) bounce around in this zone, taking a long time to travel through.
  3. Core:
    • Thickness/Size: Extends to about 20% of the Sun’s radius.
    • Temperature: Around 15 million °C.
    • Characteristics: The core is the powerhouse of the Sun. This is where nuclear fusion occurs, converting hydrogen into helium. This process releases the energy that eventually reaches the surface and radiates into space.

The tachocline is a transition layer that separates the convective and radiative zones.

Fun Facts About the Layers of the Sun

  • Energy Generation in the Core: The core is the hottest part of the Sun, where temperatures reach about 15 million degrees Celsius. Fusion results from self-correcting equilibrium. If fusion occurs at a higher rate, the core heats up and expands slightly. This reduces its density, which then lowers the fusion rate. Meanwhile, if fusion slows, the core shrinks and cools, increasing density and the fusion rate.
  • Radiative Zone’s Role in Energy Transport: In the radiative zone, photons transport energy from the core. This zone doesn’t have the convection currents seen in the outer layers. Instead, radiation transfers energy.
  • Tachocline’s Mystery: The tachocline is a region of sharp change in rotation. The differential rotation of this layer may play a crucial role in generating the Sun’s magnetic field.
  • Granulation on the Photosphere: The photosphere displays a grainy texture known as granulation. These granules are the tops of convection cells where hot plasma rises, cools, and then sinks back down.
  • Chromosphere’s Spicules: The chromosphere, just above the photosphere, features dynamic jet-like structures called spicules. These spicules shoot up to 10,000 kilometers into the Sun’s atmosphere at speeds of up to 20 kilometers per second.
  • Corona’s High Temperature Mystery: The corona, the Sun’s outermost layer, has temperatures exceeding a million degrees Celsius, which is paradoxically much hotter than the surface.
  • Helioseismology for Studying the Sun’s Interior: Scientists use a technique called helioseismology to study the Sun’s internal structure. By analyzing waves and oscillations on the Sun’s surface, they infer details about the interior layers.
  • Solar Cycle and Magnetic Field: The Sun’s magnetic field goes through a cycle, approximately every 11 years, known as the solar cycle. This cycle changes the number and location of sunspots and influences various solar phenomena, including solar flares and coronal mass ejections.

References

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  • Shu, F.H. (1991). The Physics of Astrophysics. Vol. 1. University Science Books. ISBN 978-0-935702-64-4.
  • Solanki, S.K.; Livingston, W.; Ayres, T. (1994). “New Light on the Heart of Darkness of the Solar Chromosphere”. Science. 263 (5143): 64–66. doi:10.1126/science.263.5143.64
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