The constellation Perseus can be found high in the west this time of year. Perseus has star clusters and nebulae inside its borders, but is most famous for the variable star Algol. Algol is labeled in the Perseus picture and can be found just above the ‘A’ in Algol.
Algol is 90 light years away, and has a companion star that orbits every 69 hours. By coincidence, their orbital plane is lined up with the Earth, so every 69 hours one star passes in front of the other, and the total light we see from the two stars is reduced. This type of stellar pair is called an eclipsing binary; the brightness of the two stars varies at regular intervals. You might think that this sort of alignment is rare, but many eclipsing binaries have been discovered.
Algol was known to vary its light output for hundreds of years, as it can be seen to vary without optical aid. Countless studies of Algol and other variable star systems have taught us many things about how binary stars interact with each other, and how they evolve over time. For example: material is sometimes ripped from one star to another in binary systems. This can cause strange changes to the life cycle of a star.
The California nebula is located in the rectangle in the Perseus picture. Refer to the second picture for a close up view. It is named the California nebula because its shape is said to look like the state of California. It is 1,500 light-years away and about 110 light-years across. Made mostly of hydrogen, it glows red from the hydrogen being energized by the incredibly bright nearby star Xi Persei. Xi Persei is the bright star just above center in the image.