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Our Star - the Sun


The sun with sunspots, photographed through a Baader solar filter


Quick Facts

Most of the information can be found in the following websites: and ,

Diameter of the sun: 1.4 million km, or 109 x Earth's diameter
Distance from Earth: 150 million km, = 1 Astronomical Unit (AU)
Solar mass = 1.989 1030 kg  = 333,000 Earth masses
Density at the surface: 1/6000 that of air
Density at the core: 10 x density of lead

The Sun is composed of mostly hydrogen (~94%), with some helium (6%) and other heavier elements (<1%). It is Held together by its own massive gravity, but the enormous internal pressure keeps it inflated like a balloon.

Parts of the sun

Core -- extends ¼ of the way to the surface from the centre. The temperature of the core is about 15 000 000K . In the core, the tremendous heat and pressure force hydrogen nuclei to fuse into helium, releasing enormous amounts of energy and radiation.

Radiative zone  - from 25% to 70% of the distance to the surface. This zone is roughly 7 000 000 K (near the core) to 2 000 000 K (nearer the surface). Heat moves from the core through this zone. The radiative zone is "transparent" -- meaning that photons can pass from atom to atom without being absorbed, but it is so dense that it takes about 1 million years for the light to bounce its way through the layer to the surface.

Convective zone -- this zone heats up at the bottom, causing it to churn like boiling water, delivering heat to the surface layer.

Photosphere - Surface layer, about 100 km thick. 6 000 K It is this layer that we can see with a telescope. Sunspots are in this layer. Sunspots are zones of magnetic field where the surface is cooler -- but still 3 000 K!

Chromosphere -- thin, hot layer above the photosphere. Temp: 20 000°. This is where prominences are seen.

Corona -- the sun's thin outer atmosphere -- extends well beyond the surface of the sun. Gasses in the corona are superheated to 1 000 000°.

Solar wind -- gasses are flung away from the sun at 400 km/s.  At this speed, it takes only a few days for the gasses to reach the earth.


Solar Flares

Solar flares arise from areas of intense magnetic fields called Active Regions. These are associated with sunspots. The magnetic fields cool the surface regions around them to about half of the surrounding temperature, but at the same time can launch plumes of plasma out into the chromosphere.

As solar storms hit the Earth's magnetic field, the shape change of the magnetic field can release enormous energy, which can disrupt satellites, cause power outages, and produce spectacular auroral displays. (see some images here:


A related phenomenon involves magnetic fields in the corona, which can store up energy and release a large amount of plasma from the corona, called a Coronal Mass Ejection (CME). CMEs may be triggered by solar flare activity, but sometimes they occur separately. A large CME may eject 500 million tons of superheated, charged plasma. Any large solar flare or CME that launches material outward towards the Earth is termed a Solar Storm.

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