Hydrogen is still available outside the core, so hydrogen fusion continues in a shell surrounding the core. Deprived of the energy production needed to support it, the core begins to collapse into itself and becomes much hotter. When a star has fused all the hydrogen in its core, nuclear reactions cease. In general, the larger a star, the shorter its life, although all but the most massive stars live for billions of years. Although extreme stars such as these are believed to have been common in the early Universe, today they are extremely rare - the entire Milky Way galaxy contains only a handful of hypergiants. Hypergiants emit hundreds of thousands of times more energy than the Sun, but have lifetimes of only a few million years. On the other hand, the most massive stars, known as hypergiants, may be 100 or more times more massive than the Sun, and have surface temperatures of more than 30,000 K. Despite their diminutive nature, red dwarfs are by far the most numerous stars in the Universe and have lifespans of tens of billions of years. The smallest stars, known as red dwarfs, may contain as little as 10% the mass of the Sun and emit only 0.01% as much energy, glowing feebly at temperatures between 3000-4000K. The outflow of energy from the central regions of the star provides the pressure necessary to keep the star from collapsing under its own weight, and the energy by which it shines.Īs shown in the Hertzsprung-Russell Diagram, Main Sequence stars span a wide range of luminosities and colors, and can be classified according to those characteristics.

Stars are fueled by the nuclear fusion of hydrogen to form helium deep in their interiors.

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