Majority of the stars in the universe work as a result of converting hydrogen into helium via nuclear fusion, they are called main sequence stars. As the centuries pass these stars burn through the hydrogen in its core and as a result gravity overrules the outward pressure of fusion and compresses the star smaller and tighter which leads to helium fusing into carbon. Depending on the mass of the star, the helium burning might be gradual or might begin with an explosive flash. Although fusion is no longer taking place in the core, the rise in temperature heats up the shell of hydrogen surrounding the core until it is hot enough to start hydrogen fusion, producing more energy than when it was a main sequence star. The outer core expands and the star reaches a diameter equaling 100 million to 1 billion kilometers.  Because the energy is spread across a larger area, surface temperatures are actually cooler and they start to shine in the redder part of the spectrum, leading to the name red giant. In only a few billion years, our own sun will turn into a red giant star, expand and engulf the inner planets, possibly even Earth.

Here at MathsOne, one of the Best Tuition Centres in Kerala we educate our students in celestial objects and broaden their horizons