Active Sun, X-ray image. The Sun is a huge ball of hydrogen gas, ionised into a plasma by the immense temperatures that are generated by nuclear fusion at the core of the Sun. This fusion is ignited by the immense pressure generated by the Sun's mass. An active Sun is more likely to eject plasma from its surface. The activity varies, peaking roughly every 11 years. The Sun does not just emit visible light, but emits a wide range of radiation, including X-rays. The Sun's light and heat directly or indirectly provides almost all of the energy for life on Earth. The Sun is some 150 million kilometres distant from Earth.