What is the meaning of the 'molecular cloud' 'stellar nebula' 'average star' 'massive star' 'red giant' 'red super giant' 'planetary nebula' 'supernova' 'white dwarf' 'neutron star' and 'black hole'?

Answer 1

These are names for some of the stages of stellar evolution.

• molecular cloud, a stellar nursery where cool clouds of molecular gasses begin collapsing at various point to form protostars.
• stellar nebula, a nebular region of collapsing gas in the process of collapse star formation.
• average star, a star of mass not exceeding 3 solar masses that is on the main sequence of the H&R diagram.
• massive star, a star of 3 solar masses or greater.
• red giant, a star beginning to die, it has run out of hydrogen to burn collapsed enough to ignite helium burning, the resulting large increase in energy release has made its outer shell expand enormously.
• red super giant, a very very large red giant.
• planetary nebula, the cloud of hot gasses surrounding an average star in the process of dieing or that has died. they were initially called planetary nebula because astronomers frequently mistook them for planets before realizing they were really nebular clouds of gas.
• supernova, a massive explosion of a dieing massive star (many times larger than a nova) that completely disrupts a dead star. it is driven by a rebound effect as inwardly collapsing matter of the dead star collides with the surface of the newly formed neutron star. the rebound shockwave blows away most of the infalling gas and drives formation of all elements more massive than nickle and iron. if the supernova is powerful enough it causes the neutron star to implode, becoming a black hole.
• white dwarf, the final visible stage of an average star that has died, after this it cools until it becomes an invisible black dwarf.
• neutron star, one of the final stages of a massive star following a supernova. if the massive star was able to eject enough mass as it died so that the mass of the neutron star formed in the supernova does not exceed 3 solar masses the neutron star is stable. neutron stars consist of nothing but neutrons bound by gravity (not the strong force that holds them together in atomic nuclei).
• black hole, the other of the final stages of a massive star following a supernova. this star failed to eject enough mass as it died to be able to form a stable neutron star. it continues collapsing until its escape velocity exceeds the speed of light. as now even light cannot leave the resulting object is called a black hole. however unlike the cold undetectable black dwarf, black holes because of their enormous mass can be detected by their effects on visible matter surrounding them.