The temperature at the surface of the Sun (top) is about 6,000 kelvins. The Sun's atmosphere is much hotter. The bottom image shows ultraviolet "light" coming from the Sun's atmosphere. The temperature in that part of the Sun's atmosphere is around 70,000 kelvins.
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Images courtesy of SOHO (ESA & NASA).
Kelvin Temperature Scale Used in Astronomy
The Kelvin scale is a temperature scale that is used a lot in astronomy. You probably know about the Celsius (or Centigrade) scale, which is part of the metric system of measures. If you live in the USA, you also know about the Fahrenheit scale, which is used in the English system of measures.
Why do astronomers need another temperature scale? On Earth, the temperatures we feel most often are pretty much where water is liquid. A temperature scale that has "reasonable" numbers for "normal" temperatures makes sense for day-to-day use on Earth. For example, Earth's average temperature is around 15° C (49° F). Fifteen and 49 are pretty easy numbers to deal with. It wouldn't be so good if our temperature scale used really big numbers (like 6,437°) or really small numbers (like 0.052°) or negative numbers (like -147°) for normal temperatures. The Celsius and Fahrenheit scales are set up to have "reasonable" numbers for common temperatures on Earth.
Temperatures in space are often much colder or much hotter than we are used to on Earth. Comets and icy moons have temperatures close to absolute zero. Stars can have temperatures of thousands of degrees or higher. The Kelvin temperature scale is good to use for really hot and cold places in space. That's why astronomers and space scientists use it a lot. Other kinds of scientists sometimes use the Kelvin scale too.
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