Here's what that means:

* Fusion: The sun is currently fusing hydrogen into helium in its core, releasing energy in the process. This is what makes it shine.

* Stability: The outward pressure from the fusion reaction balances the inward pressure from gravity, keeping the sun stable in size and temperature.

* Duration: The main sequence stage is the longest stage in a star's life. For our sun, it's expected to last about 10 billion years. We're about halfway through it.

After the main sequence, the sun will enter the red giant phase. This is when it will expand greatly and cool down, eventually becoming a white dwarf.