Flexi Says: The Sun's core is plasma. It has a temperature of around 15 million degrees Celsius (°C). Nuclear fusion reactions create the immense temperature. In these reactions, hydrogen atoms fuse to form helium. This releases vast amounts of energy. The energy moves toward the outer layers of the Sun. Human technology cannot withstand the conditions inside the Sun. The Parker Solar Probe, launched in 2018, is the closest spacecraft to the Sun. It will reach its closest distance to the Sun in 2025, at about 3.8 million miles (6.1 million km) from the Sun's surface.