Towards the end of a star's life, the temperature near the core rises and this causes the size of the star to expand. This is the fate of the Sun in about 5 billion years. Stars will convert hydrogen to helium to produce light (and other radiation). As time goes on, the heavier helium sinks to the core of the star, with a shell of hydrogen around this helium center core. The hydrogen is depleted so it no longer generates enough energy and pressure to support the outer layers of the star. As the star collapses, the pressure and temperature rise until it is high enough for helium to fuse into carbon, that is when helium burning begins. To radiate the energy produced by the helium burning, the star expands into a Red Giant. A red giant is much larger than a main sequence star. Only low-mass stars with 8 solar masses or less will turn into red giants. As one of these low mass stars runs out of hydrogen fuel in their central core, the energy that was generated from the nuclear reaction begins to slow down, this results in less outward force that is meant to counteract the weight of the outer layers of the star. The star then will begin to compress due to its gravity. From the compression the star starts to heat up until the helium begins to fuse together. This fusing forms carbon in the core. Because the temperature and pressure increased throughout the star, hydrogen could begin fusing out from the core. This layer of hydrogen fusion outside of the core will result in some extra energy that will push against the outer material of the star. This results in the star expanding in size to a red giant, or sometimes even a red super giant. The sun when it turns into a red giant will be 70 times as big as it is now