The Sun is currently in the main sequence stage, which is the longest and most stable phase of its life. During this stage, the Sun fuses hydrogen into helium in its core, producing energy and heat. When the Sun exhausts its hydrogen fuel supply, it will begin to evolve into the red giant stage. During the red giant stage, the Sun will expand in size and luminosity, and its surface temperature will decrease. This expansion will cause the Sun to engulf Mercury and Venus, and may also reach Earth's orbit. Eventually, the Sun will shed its outer layers, forming a planetary nebula, and leaving behind a white dwarf—a dense, hot remnant of the Sun.