In  the 18 th & 19 th century astronomers studied the Nebulae. The astronomers classify objects according to their appearance. The Nebulae are clouds of gas and dust in the Galaxy-milky-Way.  They saw the Crab nebula for the first time on the 4th of July 1054 in the Far East. The Crab nebula is 6000 light years away from the Earth and its diameter is about 14 light years long. The energy from the Crab nebula sends in one second 25000 times more energy than our sun and is today still visible after 900 years from its explosion. As planetary nebulae occur only late in the life of a star, they are usually absent in open star clusters because these stellar swarms tend to dissolve in times much shorter than that needed for a star to evolve in a planetary nebula: Only few open clusters live longer than a billion years, while planetary nebulae occur only for stars of less than 3 solar masses (the more massive explode as supernovae. Nebulae exist within other galaxies as well as in our own Milky Way galaxy. They are classified as planetary nebulae, supernova remnants, and diffuse nebulae, including reflecting, emission, and dark nebulae.The bright stars inside the clouds charge hydrogen gas with extra energy, nebulae exist in many shapes and sizes and form in many ways.