Hubble uses red giant stars to calculate the distances to different galaxies. When a more massive star runs out of hydrogen at its core, it forms a red supergiant instead, prior to exploding as a supernova. This process can take hundreds of millions of years and applies to intermediate mass stars (with a mass greater than 80% and less than 800% of the Sun’s mass), which then go on to form planetary nebulae. The energy at the star’s surface becomes far more dissipated, causing the star's bloated surface to cool, turning from white or yellow to red. As fusion in this shell begins, the extra heat causes the outer layers of the star to expand dramatically, and the surface extends up to several hundred times beyond the former size of the star. As the core collapses, the shell of plasma surrounding the core becomes hot enough to begin fusing hydrogen itself. Once a star’s core runs out of hydrogen, however, that state of equilibrium is lost and the core begins to collapse. A red giant forms after a star has run out of hydrogen fuel for nuclear fusion, and has begun the process of dying.Ī star maintains its stability through a fine balance between its own gravity, which holds it together, and the outwards pressure from ongoing thermonuclear fusion processes taking place at its core.
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