Stars are big, no doubt about it. But even among the stars, there are stars so enormous that their size defies imagination (and stretches our understanding of astrophysics in the process). To wrap your mind around just how big the biggest stars are, let’s talk about our own humble Sun.
The Sun is, formally, a G-Type main-sequence star and, informally, a yellow dwarf. It’s not particularly large by cosmic standards, but even then it’s huge compared to Earth. You could pack 1.3 million Earths into the space occupied by our modestly sized sun.
The biggest stars in the universe are known as “supergiants” and the largest of such known stars is a red supergiant called UY Scuti. First observed in 1860, further observations and calculations have established that UY Scuti is astoundingly enormous in volume and is currently the leading candidate for the largest observed star (as well as one of the most luminous red supergiants).
How big is UY Scuti? Its estimated volume is five billion times that of the Sun and if we were to replicate the packing-Earth-inside experiment with UY Scuti, it would hold approximately 6.6 quadrillion Earths. It’s such a large star that if you were to drop it into the same place as our Sun, its photosphere would extend just beyond the orbit of Jupiter.
Source = “https://www.howtogeek.com/trivia/what-classification-is-bestowed-upon-the-largest-stars-in-the-universe/”