Betelgeuse

Orion with Betelgeuse at top left and Rigel at lower right

Betelgeuse is once of everyone's favourite stars, at least to say.  This red supergiant forms the left shoulder (from our point of view) of the constellation Orion, the hunter.  Betelgeuse was discovered by Herschel to vary in brightness; sometimes it is actually brighter than Rigel, the blue supergiant at the lower right of the constellation.  Betelgeuse along with the three blue stars of Orion's belt all probably escaped from the Orion OB association that lies in the molecular clouds along Orion's sword.   Unlike in most constellations many of the bright stars of Orion are actually associated with each other.

Betelgeuse is a relatively close yet physically large star extended to a radius of a few AU (Mars's orbit), so the stellar disk of Betelgeuse has actually been resolved at visible wavelengths (like Altair) , and it is also a radio source (like Arcturus), that I used as an example to understand the radio emission from stars in the Galactic bulge and what might be seen with the Square Kilometer Array.

The photosphere of Betelgeuse appears to shift and pulsate, so it has been difficult to measure its distance using parallax.   The theories of such giant stars is that the extended photosphere of such giants is convective with only a few cells (the Sun has thousands).   The heat transport through the extended, tenuous photosphere is extremely inefficient, and the density is so low that it makes sense to call its atmosphere a hot vacuum.   It has recently been discovered that this convective atmosphere efficiently generates a magnetic field (http://adsabs.harvard.edu/abs/2010A%26A...516L...2A).