What is a red-giant?

When a star runs out of hydrogen, it expands and turns red. Image: Shutterstock

Near the end of a star’s life, the temperature in its core rises and causes the star to expand into a red-giant.

Most of the stars generate energy by converting hydrogen to helium in the core. Over time, the heavier helium sinks to the centre of the star, with a shell of hydrogen surrounding it. Hydrogen is depleted and no longer generates enough energy or pressure to support the weight of the star. As the star collapses, both the pressure and temperature rise until helium fuses into carbon, and helium burning begins. When that happens, the star expands greatly to accommodate the energy produced by the burning. The star’s temperature falls and it changes colour, thereby becoming a red-giant. Eventually, the star will burn out, and most of its matter will end up as blurred dust. This is the eventual fate of our Sun, but no need to panic — estimates put the date some 5 billion years from now.

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