The Sun is a giant ball of hot, glowing gas at the center of our Solar System. It's so large that more than a million Earths could fit inside it, and its gravity is what holds every planet, moon, and asteroid in orbit around it.
Deep in the Sun's core, hydrogen atoms are constantly fusing together into helium, releasing enormous amounts of energy in the process. That energy takes thousands of years to travel from the core to the surface, and then just over 8 minutes to reach Earth as sunlight.
The Sun is a middle-aged star — about 4.6 billion years old, roughly halfway through its expected lifetime. In about 5 billion years, it will run out of hydrogen fuel and swell into a red giant, eventually engulfing Mercury and Venus.