It is about 1,000 million km across. Rigel is one of the brightest stars to the naked eye. Although it is only one tenth of the size of Betelgeuse, is still 100 x larger than our sun. Variable stars are stars that change in brightness. These brightness changes can range from a few hundredths to as much as twenty magnitudes over periods of a fraction of a second to years, depending on the type of variable star.
Stars change in brightness either when they are very young , or when they are very old or dying .
Currently there are more than 28,000 known variable stars. Star Nurseries . A good example of a star nursery is once again in the constellation of Orion. If you look at Orion's sword, you will see that the middle star appears misty. This is M42 - the great Nebula in Orion , one of the most famous and most photographed objects in deep space.