Arcturus

 

Last month the constellation Bootes was prominent in the western sky after sunset. I've coloured it in the photograph. The circled star at the bottom is Arcturus, which is the brightest star in our northern skies. It is a red giant so has expanded giving a much larger surface area and hence a high luminosity since Stefan's Law says that luminosity is directly proportional to luminosity. It is also very near, being only 37 light years away. It is a K class star. The classes are OBAFGKM (which I remember as Oh Be A Fine Guy, Kiss Me). K stars are much cooler.