The size and radius of a star are determined by a delicate balance between the inward gravitational force pulling the star inward and the outward pressure generated by the nuclear fusion reactions occurring in its core. In the case of Betelgeuse, a red supergiant star, its large radius compared to the Sun can be attributed to several factors:
Mass: While Betelgeuse is indeed 15 to 20 times more massive than the Sun, its radius is much larger because of the complex interplay between mass, gravity, and pressure. The greater the mass of a star, the stronger its gravitational pull, leading to a more compact object. However, other factors can counterbalance this effect.
Stage of Evolution: Betelgeuse is in a more advanced stage of stellar evolution compared to the Sun. It has exhausted its nuclear fuel faster due to its higher mass, leading to a different internal structure. As a star consumes its nuclear fuel, it undergoes changes in its core, causing it to expand and enter different stages of stellar evolution.
Red Supergiant Phase: Betelgeuse is currently in the red supergiant phase, which is a relatively short-lived phase in the life cycle of massive stars. During this phase, the star has already burned through its core hydrogen fuel and has started fusing heavier elements in its shell. This leads to the expansion of the outer layers of the star, causing it to swell and increase in radius.
Lower Surface Temperature: Red supergiant stars like Betelgeuse have lower surface temperatures compared to smaller and hotter stars like the Sun. The lower temperature contributes to the star's increased radius. Betelgeuse's surface temperature is around 3,500 Kelvin, while the Sun's surface temperature is approximately 5,500 Kelvin.
In summary, Betelgeuse's larger radius compared to the Sun can be attributed to its higher mass, more advanced stage of evolution, the red supergiant phase it is in, and its lower surface temperature. These factors collectively contribute to the expansion and size of the star.