The life of stars has also been fascinating for all. Even though a given star may live a couple of million or billions of years, depending upon its mass, we know in great detail the phases a star would take from its birth to its death. Mr. Ammar exposed to us this stellar life through all of its cycles using the H-R diagram. All stars are born within a huge molecular cloud a couple of light-years across. A nearby perturbation (a closeby supernova explosion) triggers the collapse of such a cloud. As the cloud collapses further due to gravity, its interior gets hotter and hotter until nuclear fusion starts: a new star is then born. What comes next depends upon the initial mass of the star.

For stars less than about eight times the mass of the sun, the lifetime will be quite long (a couple of billions of years), and the end result after the main sequence phase (burning hydrogen to make helium) and the red giant stage (burning helium to make carbon), is a white dwarf with a size around Earth’s size and a mass limit of about 1.4 solar mass.

For stars with more than eight solar masses, several nuclear fusions will happen inside the core until iron is made. Fusing iron requires an extra amount of energy that even superstars do not have it. What will happen next is extremely violent since the whole star (with a size 10-100 times the size of the sun) will collapse in a fraction of a second. The pressure inside the star will be extremely large that the entire star will explode in a flash called a supernova. The aftermath of the explosion is just a small core. If its mass is less than three solar masses, we will refer to it as a neutron star with a size no larger than the size of a medium city, 10-15 km star. But if the remaining core has a mass larger than three solar masses, it will further collapse unit it disappears from sight: no radiation is then emitted. Such a corpse is termed a black hole. The size of such a black hole depends on its mass if the black hole does not rotate. For one solar mass black hole, its size can be just three kilometers. Just imagine how huge its gravity can be.