Here's why:

* Closest: Our Sun is the closest star to Earth.

* Second Closest: Alpha Centauri A and B are a binary star system, and they are considered the second closest star system to Earth. However, Proxima Centauri is a red dwarf star that is part of the same system, and it is slightly closer than the Alpha Centauri pair.

So, while Alpha Centauri A and B are technically the second closest, Proxima Centauri is the third closest star to Earth.