The Sun

The Sun is a star made mostly of hydrogen and helium. Nuclear fusion in its core releases energy that eventually reaches space as light and other radiation. It contains most of the mass in the Solar System, making its gravity central to the motion of the planets.^[1]

Core fusion converts hydrogen into helium, releasing energy through a mass difference. Energy transport through the interior involves radiative and convective processes. The visible photosphere is not a solid surface; it is an optical layer within a gaseous, magnetized object whose physical conditions vary strongly with depth.^[1]

The solar wind carries charged particles outward and shapes a large region called the heliosphere. Voyager crossed its outer plasma boundary, the heliopause. That boundary describes the solar wind’s interaction with interstellar material, rather than a sudden end to the Sun’s gravity.^[2]

The heliosphere is structured by the expanding solar wind and its embedded magnetic field interacting with the surrounding interstellar medium. Plasma and field conditions vary with location and solar activity. The heliopause is therefore a dynamical boundary; it should not be equated with the outer extent of bound Solar System objects.^[2]

Solar eruptions can disturb the space environment around Earth. Their effects can reach radio links, navigation systems, satellites, and even power infrastructure on the ground. Space-weather forecasting connects solar observations with models of how disturbances travel and interact with Earth.^[4]

Space-weather impacts involve different mechanisms: electromagnetic radiation can alter the ionosphere, energetic particles can affect electronics and biological exposure, and geomagnetic disturbances can induce currents. Navigation and communications errors depend on the event and the receiving system. A single solar-activity label does not specify all downstream effects.^[4]

Scientists observe the Sun remotely and sample particles and fields with spacecraft. These measurements answer different questions. An image shows where something happened; a detector in space measures what passed its location. Connecting them helps explain how the Sun influences the rest of the system.^[3]

Heliophysics combines remote sensing, in-situ measurements, and modelling across multiple spatial scales. Establishing a causal connection between a solar event and a distant measurement requires propagation modelling and timing analysis. Multi-spacecraft observations help separate spatial structure from temporal variation, although coverage remains incomplete.^[3]