Stars are balls of mainly HYDROGEN and HELIUM. Nuclear reactions in the heart of the stars, like those in atom bombs, generate heat and energy. The heart of a star reaches 16 million c. A grain of sand this hot would kill someone 150 km away.
The gas in stars is in a special hot state called PLASMA, which is made of atoms stripped of electrons. In the core of the star, HYDROGEN nuclei fuse to form HELIUM. This nuclear reaction is called as PROTON-PROTON chain.
Stars twinkle because we see them through the wafting of the Earth's Atmosphere.
Astronomers work out how big a star is from its brightness and its temperature. The size and brightness of a star depends on its mass that is, how much gas it is made of. Our Sun is a medium sized star, and no star has more than 100 times the Sun's mass of less than 6-7% of its mass.
The coolest stars, such as Arcturus and Antares, glow reddest, hottest stars are yellow and white. The hottest are blue-white, like Rigel and Zeta Puppis.
The blue super giant Zeta Puppis has a surface temperature of 40,000 c while Rigel's is 10,000c