The Sun

The Sun
The Sun emitting an M6 Solar Flare (bottom left) – Composite image in the ultaviolet wavelengths 193 and 335 Ångstroms. (Credit: NASA/SDO) – click to enlarge

The Sun is our local star, located at the centre of our solar system. It is around 333,000 times the mass, and around 109 times the diameter, of the Earth.

The Sun is a main-sequence star of spectral class G2V, unofficially categorised as a “yellow dwarf” star.


Warning: you should never look directly at the Sun, even through sunglasses, as this can result in permanent damage to your eyes! Specially designed eclipse glasses must be worn to observe solar eclipses.

The Sun can be observed safely, however, by carefully projecting the image from a telescope or binoculars onto a piece of white paper or card. Never be tempted to look directly through the telescope or binoculars, while doing so, however, as this could result in instant and permanent blindness.

Even projecting the Sun’s image through a small telescope, or binoculars, will reveal any large sunspots which may be present.

Alternatively, special solar filters can be used, although these should always be designed specifically for the model of telescope you are using.

Specially designed solar telescopes are also available that can provide spectacular images of the Sun in the hydrogen-alpha (H-alpha) part of the visible spectrum.

Satellites and space probes also image the Sun in various other wavelengths of the electromagnetic radiation spectrum that do not readily pass through the Earth‘s atmosphere, such as ultraviolet light and x-rays.


The Sun is a ball of plasma – a high temperature state of matter where the electrons are separated from the atomic nuclei.

The Sun is composed of around 73 per cent hydrogen plasma by mass, i.e individual protons surrounded by a sea of electrons. Around 25 per cent of the Sun’s mass is from helium nuclei (two protons mostly paired with two neutrons) and the remaining mass made up of heavier atomic nuclei including oxygen, carbon, neon, and iron.

The Sun’s light and heat are produced by nuclear fusion reactions in its core. The main process for this is the proton–proton chain reaction, whereby hydrogen nuclei combine to form helium, emitting energy as gamma rays – a highly energetic form of electromagnetic radiation.

These gamma ray photons are repeatedly absorbed, re-emitted and scattered by the Sun’s plasma, reducing in energy until they emerge at the Sun’s surface approximating a blackbody radiation spectrum, including visible light and infrared heat radiation.

On average, a proton in the core of the Sun waits around 9 billion years before it fuses with another proton. However, the Sun is so massive that its core fuses about 600 million tons of hydrogen into helium every second. This converts matter into energy – via Einstein’s famous E=mc2 equation (see Relativity Theory) – at a rate of 4 million tons a second.

It can take anything from around 10,000 to 170,000 years for the electromagnetic radiation from the Sun’s core to reach the surface and be emitted into space.


The Sun’s core extends from the center to about 20–25 per cent of the Sun’s radius.

Beyond the core is a layer known as the radiative zone. This is the thickest layer of the Sun at around 45 per cent of the radius.

This, in turn, is surrounded by the convective zone which extends out to near the surface.

As these names suggest, the primary form of energy transfer through the radiative zone is by radiation, whereas convection is the dominant means of energy transfer through the convention zone. This difference is due to the lower relative density and temperature of the plasma in the convective zone.

Between these two zones is a thin boundary layer known as the tachocline. It is believed that the Sun’s strong magnetic field originates from this layer.

The visible surface of the Sun is known as the photosphere. This atmosphere of the Sun above the photosphere allows visible light to pass through, whereas the material just below the photosphere is composed mainly of negatively charged hydrogen ions, which absorb visible light. The photosphere is only slightly less opaque than the Earth’s atmosphere, and is tens to hundreds of kilometers thick.

The Sun’s Atmosphere

The atmosphere of the Sun, above the photosphere comprises four distinct parts: the chromosphere, the transition region, the corona and the heliosphere.

The chromosphere is dominated by a spectrum of emission and absorption lines, analysis of which has allowed us to determine much about the composition of the Sun.

The transition region is a thin turbulent layer that is most easily observed in the ultraviolet part of the spectrum.

Beyond this is the solar corona, a thin atmospheric layer which extends out into space and can be readily observered from Earth during a total solar eclipse.

The heliosphere, is comprised of the solar wind – streams of charged particles that travel outwards from the Sun to the heliopause, which is the theoretical boundary where the Sun’s solar wind is stopped by the interstellar medium. The heliopause is around 50 AU (i.e. 50 times the radius of the Earth’s orbit) from the Sun.

The Sun’s Colour

The visible light from the Sun appears approximately white in colour when seen from space, meaning that it emits light fairly equally over the entire range of the visible spectrum.

However, the Sun’s surface temperature is around 5,800 Kelvin, meaning that the peak of the Sun’s radiation curve is actually in the blue-green part of the visible spectrum.

On Earth, the Sun’s light appears more yellow in colour – and redder at sunrise and sunset – due to scattering of light in the Earth’s atmosphere. Light nearer the blue end of the spectrum is more readily scattered than light from the red end of the spectrum. This scattering of blue light from the Sun is why the Sun appears yellow and the sky appears blue on a sunny day.

When the Sun is lower on the horizon, the sunlight passes at a lower angle through the atmosphere meaning that it travels a greater distance through the atmosphere. This gives more of its yellow and green light a chance to be scattered too, which can cause the Sun to appear much redder near the horizon. There is usually more dust (and pollution) close the Earth’s surface too, which can scatter light at lower wavelengths, causing spectacular red sunrises and sunsets.


Sunspots are visibly darker patches on the face of the Sun that form temporarily where distorted magnetic field lines pass through its surface or photosphere.

The surface temperature in the area where a sunspot appears is lower than its surroundings, meaning that the light output is also diminished. In contrast with the rest of the Sun’s surface, this causes the area to appear as a dark spot, although if the entire surface of the Sun was at the same temperature as a sunspot it would still be much brighter than the full moon.

Sunspots appear as a dark central spot, or cluster of spots, know as the umbra, surrounded by a lighter area known as the penumbra.

Sunspots are active regions of the Sun’s surface that can give rise to prominences and flares.

Prominences and Flares

Solar prominences, also known as filaments when viewed against the surface of the Sun, are eruptions of plasma that extend outwards from the Sun’s surface following magnetic fields lines.

Often these eruptions of plasma form loops that are anchored at the points where the magnetic field lines leave and enter the Sun’s surface.

Solar flares are intense eruption of electromagnetic radiation from the Sun’s surface.

Coronal Mass Ejections

Some solar flares and prominences may give rise to coronal mass ejections, where significant amounts of plasma can be released into space. These “interplanetary” coronal mass ejections, can sometime reach the Earth’s magnetosphere where they can cause what are known as geomagnetic storms that can damage electrical equipment and cause intense aurorae.

Solar Activity Cycles

The number and size of sunspots, and other activity-related phenomena, that appear varies with the solar activity cycle, which peaks roughly every 11 years. The Sun’s magnetic field flips at each solar maximum, and then dies down to a period of minimum activity half way between each maximum.

On top of this 11-year cycle, it has been hypothesised that several longer-period activity cycles occur, varying in length from hundreds to thousands of years.

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Astronomy, Cosmology, Space and Astrophysics