What's the second closest star to the Earth?

Alpha Centauri AB, taken by the SDSS
Question posed by Sam

The second closest star to the Earth is Proxima Centauri (the closest, of course, being the Sun). You can read more about that in this post from last month.

So lets move further out!

The third and fourth closest stars to the Sun are in the same area of space as Proxima Centauri and about 0.2 light years further away.

Proxima Centauri is part of (or currently passing through*) the Alpha Centauri star system. Alpha Centauri is actually a binary star system made up of two stars (known imaginatively as Alpha Centauri A and Alpha Centauri B) in orbit around each other and is about 4.24 light years away from us and orbit each other at a distance of somewhere between 11 and 36 AU**. These stars can be seen from Earth with the unaided eye but appear as only one point in the sky due to the distance between them being small compared to their distance from us.

Alpha Centauri A
More luminous and slightly larger than our Sun, Alpha Centauri A is the primary member of the star system. It is a 'main sequence' star much like the Sun with a radius 23% larger and 10% more mass.

Alpha Centauri B
The secondary star in the Alpha Centauri system has a radius 14% smaller than that of our Sun and contains about 90% of the mass.
Comparing the sizes of Alpha Centauri A & B, Proxima Centauri and the Sun

* There's some disagreement about this.
** If Alpha Centauri A was where our Sun is, then Alpha Centauri B would be somewhere between the orbits of Saturn and Pluto.


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