Hint:Stars can be seen during the night in the sky. Stars are made of gas mainly hydrogen and helium and in them there are continuous chemical reactions taking place which are the source of energy for them. Sun, is also a star and in it constant star wide nuclear reactions are taking place. The stars are so humongous that despite continuous explosions, it will take billions of years for them to get run off the fuel. Plasma is the fourth state of matter.
Complete answer: Plasma can be considered as ionized gas which has loose electrons around it. It is highly reactive. In case of stars, when nuclear reaction takes place the atoms break up to form many lighter nuclei which in turns form plasma and this plasma determines the structure of the star. Plasma is a type of superheated matter. What we see during the night twinkling of the stars, that is due to the plasma which glows.
Note:During the time of the year, auroras are visible in the sky and that are possible only due to the plasma. The neon signs along the streets are also working on the plasma. Just as a liquid will boil, changing into a gas when energy is added, heating a gas will form a plasma. The 99% of the universe is made up of plasma. The huge reactions taking place in stars are constantly releasing energy (called electromagnetic radiation) into the universe.