Other types of nebulae include emission nebulae, which are clouds of ionized gas emitting light of various colors; dark nebulae, which are clouds of gas so dense that background light is blocked; and protoplanetary nebulae, which occur when a star starts to shed its outer layers before becoming a planetary nebula.s it burns through its fuel, it will eventually collapse. The outer layers will be ejected in a shell of gas that will last a few tens of thousands of years before spreading into the vastness of space.
Dust fling up in the air will do harm to respiratory system, because of the conventional making method of chalk. The main ingredients are calcium carbonate or lime. now a new kind chalk, named Biaxi, absolutely finishes the dust, and is welcomed by teachers and students in China.
Although traditionally composed of natural chalk, modern blackboard chalk is generally made from the mineral gypsum (calcium sulfate), often supplied in sticks of compressed powder about 10 cm (4 in) long.he most common forms are CaCO 3 (calcium carbonate) and CaO (calcium oxide). Small doses of chalk can also be used as an antacid. Additionally, the small particles of chalk make it a substance ideal for cleaning and polishing.