Glossary S

Satellite: A body that revolves around a planet, otherwise known as a moon. An artificial satellite is an object deliberately placed in orbit around the Earth or other Solar System body.

Solar Cycle: A cyclic variation in solar activity (for example, the production of sunspots and flares), which reaches a maximum at intervals of about 11 years.

Solar Flare: A violent release of energy  - in the form of electromagnetic radiation from a site located just above the surface of the Sun.

Solar Mass: A unit of mass equal to the mass of the Sun, which provides a convenient standard for comparing the masses of stars.

Solar System: The Sun together with everything that revolves around it.

Solar Wind: A stream of fast-moving charged particles that escapes from the Sun and flows outwards through the Solar System like a wind.

Spiral Arm: A spiral-shaped structure extending outward from the central bulge of s spiral or barred spiral galaxy.

Spiral Galaxy: A galaxy that consists of a spheroidal central concentration of stars surrounded by a flattened disc composed of stars, gas and dust, within which the major visible features are  clumped together into a pattern of spiral arms.

Star: A self-luminous body of hot plasma that generates energy by means of nuclear fusion reactions.

Stellar Wind: An outflow of charged particles from the atmosphere of a star.

Sunspot: A patch on the surface of the Sun that appears dark because it is cooler than its surroundings.

Supergiant: An exceptionally large star with a very large diameter.

Superior Planet: A planet that travels around the Sun on an orbit outside the orbit of the Earth.

Supernova: A catastrophic event that destroys a star and causes its brightness to increase, temporarily, by a factor of around 1 million.