Solar Features

Finally we shall place the Sun himself at the centre of the Universe. All this is suggested by the systematic procession of events and the harmony of the whole Universe, if only we face the facts, as they say, "with both eyes open".
Nicolaus Copernicus

The Structure of the Sun

The sun is essentially a giant ball of gas and plasma that gets hotter and denser as you travel from the outer rim to the centre. Temperatures run from a mere 5780K on the outer visible layer (the photosphere) to about 15 MILLION Kelvin in the middle!
(0K = - 273°C)

The Core

The core of the sun is the the real powerhouse. With temperatures at 15 million K and a density of 160,000 Kg/m3, this is the place where the nuclear fusion that generates massive amounts of energy takes place.

The Radiative Zone

Between the core and the convective zone, the radiative zone extends to about 70% of the sun's radius. The energy flowing from the core through the radiative zone, traveling in a very haphazard path, losing energy in the process.

The Convection Zone

The convection zone is a turbulent mass of material through which the radiation cannot pass as the temperature is too low (about 2 million K at the bottom of the convection zone). The energy pouring from the radiative zone gets trapped and cannot escape, so giant convection currents are set up with hot matter rising and the cooler matter sinking. This results in large bubbles of ionised gas rising through the convection zone, reaching the surface in about 10 days. It help to imagine a boiling pot of water with hot rising bubbles and cooler sinking material. The hot bubbles rise quickly to higher levels, cooling and expanding, just like hot air rising in the atmosphere of the earth. When it becomes cooler that its surroundings, the gas sinks to become reheated and rise again. In this way rolling currents of hot and cold gas create a churning motion that carries heat from the bottom to the top. It seems that largest currents of gas and heat generate myriad smaller ones and these manifest themselves as the granulation we can see in white light and H-Alpha.

The Photosphere

Sunlight as we know it - the visible white light, is emitted from the photosphere. The photosphere is one of the coolest regions of the Sun (about 6000 K), and it is here that we can see the granulation caused by the bubbling gas in the convection layer and the sunspots caused by strong magnetic fields.

The Chromosphere

The chromosphere is 2000-3000 km thick and the temperature rises from around 6000k to 20,000K. These high temperatures result in hydrogen emitting a reddish light (H-alpha emission) and can be seen in the exciting prominences that project from the sun and in the thin reddish line that can sometimes be seen as a 'rim' round the dark disk of the moon inside the corona. It is this colour that gives the chromosphere its name (color-sphere). The image to the left of a total solar eclipse demonstrates the colouring of the chromosphere. The chromosphere contains spikes of gas called spicules that rise through it. Spicules are short-lived phenomena, corresponding to rising jets of gas that move upward at about 30km/sec and last only about 10 minutes.

The Corona

This is the outer layer of the sun and is the whitish halo seen around the disc in a total solar eclipse. This can be seen in the picture of the solar eclipse above. Temperatures range from 2 to 3 million °.

The corona can exhibit coronal holes which can be seen in the spectacular x-ray image of the sun (right). It is from these 'holes' that a high velocity solar wind emanates. The wind consists of high speed particles streaming from the sun and can be viewed as an extension of the the corona into interplanetary space.

Coronal Mass Ejections

Coronal mass ejections occur when the confined solar atmosphere can suddenly and violently release bubbles or tongues of gas and magnetic fields. These result from a change in the magnetic field. If one of these coronal mass ejections erupts towards the earth it can effect electromagnetic equipment. Satellites are particularly vulnerable. However, the really exciting effect of a coronal mass ejection coming our way is the auroras that we see usually nearer the poles. In April 2000 a spectacularly big CME brought the aurora down as far as the south of England.

Pictures take at Hampshire Astronomy Group Observatory in April 2000 by Ninian Boyle

Visible Features of the Sun

You can view the sun as a projection, through a white light filter, or through a Hydrogen Alpha telescope or filter.






A full disc image of the sun taken by Jack Newton on a Solarview 50 Telescope, showing the main features that are visible in hydrogen alpha. Sunspots are the only features readily visible in white light, but you can occasionally see faculae and flares.

Sunspots

Sunspots are the dark spots you can see when you look at the sun in white light (through an appropriate filter) or when you project an image of the sun on to a screen. The picture shows sunspots on the surface of the sun as viewed in hydrogen alpha.

These spots are dark because they are cooler than their surroundings (a mere 4000K or so, compared to the 5780K of the surrounding photosphere - K is degrees Kelvin. IK = 1°C and 0K = 273°C, so to convert K to °C just add 273.)

The sun generates very strong magnetic fields, and it is a localised concentration of these magnetic fields that causes the cooling that we see as sunspots. Sunspots usually occur in pairs or groups of opposite magnetic polarity that move in unison across the face of the sun as it rotates. They can last anything from a few hours to a few weeks, or even months for the very biggest. Interestingly sunspot activity exhibits an 11 year cycle in terms of the position and number of spots.

Plages

The glowing region around the sunspot (seen in the picture above) is called Plage (from the French for beach). They always appear with a sunspot but can outlive them. They are bright dense regions of the chromosphere.

Faculae

Faculae are bright areas in the photosphere that are visible near the limb, or edge, of the solar disk. They appear a few hours before a sunspot that arises in the same place and can remain for months after the sunspots have gone. They are also the result of the magnetic fields produced by the sun, being areas where the magnetic field is concentrated in much smaller bundles than in sunspots. While the sunspots tend to make the Sun look darker, the faculae make it look brighter. The word facula comes from the Latin for 'Little Torch'.

Prominences

A Prominence is an elongated structure full of material hundreds of times cooler and denser than the surrounding corona. They are held up and insulated by huge magnetic structures and are seen as prominences at the edge of the sun against the black background of space. They can stay suspended above the photosphere for weeks and even months, but eventually become unstable. Surprisingly, rather then collapse at that point, they actually erupt!

Filaments

Filaments are prominences that are within the disc of the sun as we view it, and so are seen as dark lines of cool matter and against the hotter bright chromosphere behind.

Flares

Solar flares are huge explosions on the surface of the sun, throwing out massive amount of material as matter is heated to millions of degrees in a few minutes. It can last from minutes to hours.

Granulation

A collection of granules - a bright region or cell ( about 1000 km across), which cover the entire Sun except for those areas covered by sunspots. They are formed by hot gases rising to the photosphere, spreading and cooling and falling back again - hence the bright centre and darker edges. Individual granules last for only about 20 minutes so the pattern of granulation is continually changing as old granules are pushed aside by newly emerging ones. The flow within the granules can reach supersonic speeds of more than 7 km/s (15,000 mph) and produce sonic "booms" and other noise. Shown on the right in H-Alpha, granulation can be seen in white light as well.

Coronal Mass Ejection

Often associated with flares and prominences, a coronal mass ejection (CME) is the release of a huge amount of coronal material - measured in billions of tons and traveling at supersonic speeds.