This photo was taken in 2010 by the Atmospheric Imaging Assembly (AIA) of NASA's Solar Dynamics Observatory (SDO). Our sun is an incredibly complex ball of gas. The surface of the sun is believed to be about 5,500 degrees C (10,000 degrees F). Astronomers measure the temperature of the sun in a metric unit called the Kelvin (K), according to NASA. One Kelvin equals exactly 1 Celsius degree (1.8 Fahrenheit degree), NASA says, but Kelvin and Celsius start at different points. The temperature of the sun's core is thought to be more than 15 million K.