What’s the Closest Star to the Sun? Proxima Centauri or Alpha Centauri?

Proxima Centauri (4.2 light-years away), a red dwarf, is the closest star to the sun. It is a part of Alpha Centauri (4.37 lya), a system of three stars — Alpha Centauri A, Alpha Centauri B & Proxima Centauri.
Proxima Centauri has been the nearest star for about 32,000 years, and it will hold this record for another 33,000 years. After 33,000 years from now, the nearest star will be Ross 248.
Alpha Centauri A and Alpha Centauri B are a binary pair, orbiting a common centre of gravity. Alpha Centauri A is just a little more massive and brighter than the Sun, and Alpha Centauri B is slightly less massive than the Sun.
Alpha Centauri is only visible in the Southern Hemisphere. In the Northern Hemisphere, the closest visible star is Barnard’s Star (second closest), a red dwarf which is too dim just like Proxima Centauri to see with the unaided eye.
The closest star that one can see with the naked eye in the Northern Hemisphere is Sirius (Dog Star). Sirius is the second brightest star when viewed from the earth, the brightest being the sun.

Astronomers use a technique called parallax. They measure the angle to a star when the Earth is on one side of its orbit. Then wait for six months until the Earth has moved to the opposite side of its orbit, and then measure the angle to the star compared to some distant reference object.