Simplify the following text for a Grade 4 native English-speaking person.
Original: Currently the sun 's core has a temperature of approximately 15 million K , but when it runs out of hydrogen , the compression of the core will cause the temperature to rise to about 100 million K.
Simplified: The sun 's core is about 15 million K. When it runs out of hydrogen , the smaller core will cause it to rise to about 100 million K.