What is the Sun?

Solar flare
Credit: M. Druckmüller

The Sun is our reliable source of light and heat – and therefore energy. It’s also a violent world of nuclear fusion, turbulent gases, twisted magnetic fields and explosive outbursts.

Solar flare in different wavelengths
Solar flare: In February 2014, NASA's Solar Dynamics Observatory captured a giant solar flare at various ultraviolet and X-ray wavelengths.
Credit: NASA
Solar spectrum
Spectral signature: The Sun's spectrum (here chopped into dozens of horizontal strips) contains dark lines, produced when gases in the atmosphere absorb speciific wavelengths of light.
Credit: NOAO

At 1.4 million kilometres across, the Sun is by far the largest body in the Solar System. Like any other star, it is a huge sphere of glowing gas, mainly hydrogen and helium. Nuclear fusion in the Sun’s 15-million-degree core generates the light and heat emitted from its 5500°C surface. The Sun’s rotation period is about 27 days as seen from the Earth, but the equatorial parts rotate faster than the polar regions. As a result, magnetic fields get twisted, leading to dark sunspots, bright flares, huge prominences and massive ejections of gas. Despite all this violence, the Sun has sustained life on Earth for over 3 billion years.

Sunspots
Spot on: Sunspots, which can be larger thatn the Earth, are cooler regions on the Sun, caused by magnetic fields that prevent hot gas from below rising up to the surface.
Credit: NASA/SDO
Coronal vision
Coronal vision: The Sun’s hot, tenuous corona can best be studied during a total solar eclipse, when the Moon covers the blindingly bright solar disc.
Credit: P. Horálek/J. Sládećek/M. Druckmüller
The Earth compared to a solar flare
Explosive power: The Earth is dwarfed by a giant explosion on the Sun that ejects huge amounts of hot gas into space. Here the sizes of the Earth and the Sun are shown to scale, but not the distance between them.
Credit: NASA

The Sun is a star. Like most other stars, it has cooler, dark spots on its surface, as well as giant explosions. Luckily, the Earth is at a safe distance — 150 million kilometres!