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.
Crayola manufactures two types of chalk, extruded and molded. Extruded chalk, such as Crayola Anti-Dust White Chalk, primarily contains calcium carbonate. Molded chalk, such as Crayola Children's Chalk (available in white or colored), is a softer chalk, and is not dustless. This type of chalk is recommended for children's chalkboards as well as construction paper, cardboard boxes and paper bags. Crayola Sidewalk Chalk is a molded chalk that is not intended for use on chalkboard surfaces. It does not include calcium carbonate. This product contains plaster of paris which has a gritty texture and may scratch your chalkboard.