Iron is the highest mass element that can be produced by fusion, and the chain reaction is suddenly stopped. The star then suddenly looses the outward pressure produced by the fusion heat and can no longer hold up the extraordinary inward pull of gravity. The star violently implodes, pushing the core density higher than nuclear density, which causes the star to bounce back, exploding in a supernova. After the explosion, the core that is left is a neutron star if the mass is under 3 solar masses or a black hole if the mass is over 3 solar masses. If the temperature of a main sequence star is known, then many more characteristic may be found using the HR diagram.
Luminosity is the amount of energy that leaves a star's surface per unit time.
Luminosity increases with increasing radius, surface area, and temperature. Absolute magnitude is the brightness of a star at a distance of 10 pc away from the star. Emission and reflection nebulae are both regions in space composed of very diffuse and cold gas and dust. The gas is mostly hydrogen, some helium, and trace amounts of higher mass elements. Emission nebulae are hot enough that the hydrogen atoms emit light, mostly in the visible red light wavelength range.