Today we discuss two giant stars that are easily observed right now and relatively close in the sky. Aldebaran is an orange giant star, while Betelgeuse is a red super giant.
All active stars are undergoing a battle between pressure from the nuclear reactions pushing the star outwards and gravity pulling the star inwards. When the stars hydrogen is exhausted and helium becomes the primary fuel, the star expands rapidly and becomes immense...a giant star. Eventually, gravity will win and collapse the star, but for now, we focus on giant stars.
Aldebaran is in the constellation Taurus and is about halfway between the Pleiades star cluster and Orion's Belt. In fact, Aldebaran means 'The Follower' in Arabic, probably because it follows the Pleiades as the Earth rotates. Aldebaran's radius is 45 times larger than the Sun's radius. If Aldebaran was located where our Sun is, it would envelope the planet Mercury.
The star in Orion's right shoulder is Betelgeuse. It is noticeably redder than the orange giant Aldebaran, especially if viewed in binoculars or a telescope. If you think that Aldebaran is immense, consider this. The radius of Betelgeuse is estimated to be at least 1,000 times larger than radius of the Sun. If it were where our Sun is, it would extend beyond the orbit of Mars, perhaps even beyond Jupiter!
Yet, Betelgeuse is not the largest known star. In fact, it is not even in the top 10! Although there is much uncertainty in estimating the size of stars, the present top 10 are all red supergiants that are 1500 to 2000 times larger than our Sun.