The Sun is gigantic, no doubt about it. It represents more than 99 percent of all the mass in the Solar System, which explains its absolute dominance over everything else. However, we also understand that the Sun can be a real dwarf compared to other stars, hypergiants that develop thousands of times its radius. That leads us to an inevitable question: What is the largest star in the universe?
To understand the largest star in the universe, it's not a bad idea to review the smallest ones, better known as red dwarfs. On average, a red dwarf has a hundred times the mass of Jupiter, and it is the most common type of star in the Milky Way. Fuel consumption in red dwarfs occurs at a very slow pace, which means that in the most extreme cases, they could exist for ten trillion years (our trillions).
What is the largest star in the universe?
As explained in the Kurzgesagt video, the key to reaching the largest star in the universe is not adding mass... but killing it. When a star begins to exhaust its hydrogen supply, its core contracts, becoming denser and hotter. This breaks the star's equilibrium, overcoming gravity and causing the star's outer layers to expand. That is the fate that awaits our own Sun, with an expansion that will cover the orbit of Venus, and probably the orbit of Earth.
But hypergiants are on a completely different scale. They are so large that they tear themselves apart, losing mass due to stellar winds. One of the most studied hypergiants is Rho Cassiopeiae, so bright that it can be detected with the naked eye, despite the 10,000 light-years that separate us from it. Its radius is between 450 and 500 times that of the Sun, but it has only 40 solar masses.
Now, who holds the record? Our best measurements point in the direction of Stephenson 2-18, a red hypergiant located some 20,000 light-years from Earth. While a "normal" red hypergiant possesses 1,500 times the Sun's radius, the most optimistic estimate for Stephenson 2-18 reaches 2,150 solar radii. A single orbit at the speed of light would take 8.7 hours, and with the fastest aircraft in the world, it would be a journey of 500 years. In other words, Stephenson 2-18 could fill Saturn's orbit if we placed it at the center of the Solar System.
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