These objects are extremely big, thousand to millions of times the volume of our Sun, and extremely luminous. These stars also have extended atmospheres and photospheres, and are often shrouded in dust (Like Stephenson 2-18, WOH G64 , UY Scuti and VY Canis Majoris). This makes their true size uncertain. Many of these stars vary in size and brightness (like Betelguese, Antares, UY Scuti and VY Canis Majoris).