A nebulas a vast cloud of dust and gases between stars. The word "nebula" comes from the Latin word for "mist" or "cloud". The plural form of nebula is nebulas or nebulae. Nebulas can be studied only through a telescope or with special instruments. Modern high-powered telescopes have shown that some nebulas are really distant galaxies or clusters of stars. Astronomers classify nebulas into three general types- diffuse nebulas, planetary nebulas, and nebulas formed by supernovas.
Diffuse nebulas are the most frequent type. They look like ragged clouds. Some diffuse nebula are very bright. Hot stars within a diffuse nebula make the dust and gases glow. This bright type of diffuse nebula is called an emission nebula.The hot gases in emission nebulas glow with colors, mainly blue and green. The great nebula is and emission nebula.Some diffuse nebulas aren't as bright as emission nebulas. The dust grains in these nebulas shine only because they are reflecting starlight. This type of diffuse nebula is called a reflection nebula. The third type of diffuse nebula is too far from any stars to emit or reflect light. The dust particles block out the light of stars. This type of diffuse nebula is called a dark nebula. The Horsehead Nebula is a dark nebula.
Planetary nebulas are circular in shape. They are called planetary nebula because they look like planets when viewed through a low-powered telescope. It is made to shine by a star at its center. The star is at the end of its life span. Its hot core is exposed. The nebula forms as the star's outer layers of gas spew outward. The Ring Nebula is a planetary nebula.
Supernova ejections produce another kind of nebula. The explosion hurls out threadlike filaments of hot gas, and a nebula forms. The Crab Nebula was formed by a supernova ejection.
(This is a very big picture)