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.
Since chalk is a very porous material, it tends to absorb water and store it. Use of chalk Chalk is pressed into white sticks which are then used for a variety of purposes from scribbling on the blackboard, outlining designs on fabrics for tailors and making art works on pavements.