15. Nebulae: It refers to a giant cloud of dust and gas in space. Some nebulae come from the gas and dust withdrawn out by the explosion of a dying star, like a supernova. Other regions of nebulae are the regions where new stars are starting to produce.
16. In a milky way, the most abundant gas in the emission nebulae is hydrogen.
17. Supernova refers to a start that suddenly enhances greatly in brightness due to a catastrophic explosion, which ejects the majority of its mass.
18. All the stars, in spite of its size, ultimately runs out of fuel and collapse.
19. The sun is a main sequence star. The main sequence stars fuse hydrogen atoms to produce helium atoms in the cores. About 90 percent of the stars in the universe, like the sun, is the main sequence stars.