This might seem a silly question to ask and answer on this site but when the site's visitor logs are checked, they do ask this question. Our Solar System has only one star, that is the Sun. If you were to include the theoretical nemesis star in the solar system, our star is still the largest star in the Solar System. The answer to the question is therefore the Sun.
If you want to extend the question to what is the largest star in the Milky Way, the answer is UY Scuti. Its the largest star that we have so far discovered. It is estimated to be at 1,708 times the size of the Sun.
UY Scuti might not be the largest in the universe but it is the largest that we know of at the moment. To compare UY Scuti with our Solar System, let theoretically move it to the centre of our Solar System. Once we have moved it to the centre of our Solar System, it would extend out to past Jupiter and maybe Saturn. UY Scuti is humongous. Our planet would be inside its furness.
There might be an even bigger star out there somewhere in the cosmos but we have not discovered it yet. We will never kinow for certain what is the largest star in the universe because there are so many stars and galaxies to faint to measure all the individual stars out there. Given we are so far away from UY Scuti, 1,708 is only an estimated size, it could be smaller or larger than that but its the best calculation that we've come up with.
In the early universe, there were quasi-stars, they were supermassive large blue stars which would have made UY Scuti seem insignificant. It is believed that these Quasi-stars were so big that when they collapsed, they created the Supermassive black holes found in the centres of galaxies. As none of the Quasi-stars are believed to exist now, they are just seen as hypothetical.
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