Annular eclipse A solar eclipse that occurs when the Sun and the Moon are exactly in line, but when the Moon—as seen from Earth—is smaller than the Sun. The apparent size of the Sun and Moon changes slightly through the year since there are times when Earth is slightly nearer or farther away from them. As a result, the Sun appears as a bright ring, called an annulus, around the black disc of the Moon.

Chromosphere A thin layer of the Sun’s atmosphere around 2000 km (1200 miles) deep, just above the photosphere and beneath the corona. The chromosphere can reach temperatures of up to 20,000°C (36,000°F).

Convective zone The layer of the Sun lying beneath the photosphere. Hot gas bubbles up to the surface of the convective zone before sinking down to be reheated again.

Corona The Sun’s hot outer atmosphere. The corona extends for millions of kilometres into space. It is visible from Earth only during a total solar eclipse.

Electromagnetic radiation Heat, infrared, light, radio waves and other forms of energy which together make up the electromagnetic spectrum. Electromagnetic radiation travels as streams of particles called photons, which often behave as waves. 

Flare A sudden flash of increased brightness on the Sun, usually observed close to sunspots on the surface. A flare is a release of enormous amounts of heat, light and other forms of ​​​​​energy.

Granules Patterns on the Sun’s photosphere, formed by rising material from the convective zone. Similar patterns form on the surface of soup heated on a hob, for the same reason.

Heliosphere A spherical “envelope” that is centred on the Sun and extends to the edge of the Solar System. The heliosphere is the region in which the solar wind "blows". The heliosphere's boundary is called the heliopause. 

Infrared radiation A form of electromagnetic energy. Some wavelengths of Infrared radiation are what we feel as heat. More than half of the energy the Sun gives off is in the form of infrared radiation.

Nuclear fusion The process by which the Sun generates energy. Inside the Sun’s core, nuclei of hydrogen atoms fuse together to form helium nuclei. The energy released through the process of nuclear fusion is the source of the Sun’s light and heat as well as other forms of radiation.

Photosphere The visible surface of the Sun. It is only about 500 km (300 miles) thick and, at 5500°C (9900°F), is much “cooler” than the Sun’s core. It is in a state of constant motion, like hot soup on a hob.

Plasma Gas that is heated to such a high temperature that its atoms separate into ions and electrons, all of which are electrically charged. All parts of the Sun are in a plasmic state.

Prominence A flaming arch of gas that leaps off the photosphere and out into the corona. Prominences are held up by the Sun’s magnetic field.

Radiative zone One of the inner layers of the Sun. Energy radiates out through this layer from the Sun’s core.

Solar core The innermost part of the Sun. The core is a region of high pressure (200 billion times that on the Earth’s surface) and intense heat (about 15 million°C / 25 million°F). This is where the energy that keeps the Sun shining is produced by nuclear fusion. Energy flows out from the core through the radiative zone to the convective zone.

Solar eclipse The disappearance of some or all of the Sun’s disc as seen from some parts of the Earth, caused by the passage of the Moon between the Sun and the Earth. 

Solar wind A stream of subatomic particles that flows steadily away from the Sun. This occurs because gases in the corona are so hot that some of the ions and electrons of which they are composed have too much kinetic (motion) energy to be held in, even by the Sun’s strong gravitational field.

Spicule A bright, narrow jet of plasma that leaps up to 10,000 km (6000 miles) within the photosphere, then fades away.

Sunspot A darker, cooler area that appears temporarily on the Sun’s photosphere. Sunspots are places where lines of magnetic force pass through the photosphere. They usually occur in small groups. The dark inside of a sunspot is called the umbra and the lighter outside is called the penumbra.

Consultant: Mike Goldsmith