redgiant

Red giant is a stage in the life cycle of a star, characterized by a significant increase in size and luminosity. It occurs when a star like our Sun exhausts the hydrogen fuel in its core, causing it to expand and cool.

This process begins when a star's core hydrogen is depleted, typically after about 10 billion years. Without the energy source from hydrogen fusion, the star's core begins to contract under gravity, which heats up the core. This increase in core temperature causes the outer layers of the star to expand, leading to a significant increase in size and a corresponding decrease in surface temperature.

As the star swells into a red giant, its outer layers expand to encompass a large amount of space, potentially engulfing nearby planets, including Mercury and Venus in the case of our Sun. The surface temperature of a red giant is relatively low, around 3,000-4000 K, which is much cooler than the surface temperature of its main-sequence stage.

Red giants are not visible from Earth, as they are usually found in the later stages of a star's life, and are often obscured by gas and dust in the interstellar medium. However, the remnants of red giants can become white dwarfs, which are small, cool, and extremely dense stars that are a common endpoint for many stars.

Red giants play a crucial role in our understanding of stellar evolution and the life cycle of stars. They are also of interest to astronomers studying the conditions for life on other planets, as they can potentially harbor habitable worlds that may one day be engulfed by their star.