There are approximately 1 trillion trillion stars in our Universe, a number approximated by European Space Agency (ESA). Stars in the Universe come in all sizes. They can be as small as 20 km in radius and as big as 1 billion Km in radius. One such extreme star is UY Scuti, the beast inside of which 5 billion Suns can fit.
UY Scuti is a red supergiant star is the constellation of Scutum, approximately 9,500 light years away, where 1 light year is 6 trillion miles. In the summer of 2012, astronomers from the Very Large Telescope in the Atacama Desert in Chile measured the parameters of three red supergiants near the Galactic Center region: UY Scuti, AH Scorpii, and KW Sagittarii. They determined that all three stars are over 1,000 times bigger than the Sun and over 100,000 times more luminous than Sun. The stars' sizes were calculated using the Rosseland Radius, the location at which the optical depth is 2/3, with distances adopted from earlier publications. UY Scuti was found to be the largest and the most luminous of the three stars measured, at 1,708 ± 192 R☉ which larger than the orbit of Jupiter.
Stellar Evolution models tell that UY Scuti has already started the fusion of helium in its core. This means it is left with just 10% of its life. Stars like these have a short lifespan of a few million years as compared to trillions of years for red dwarfs. After helium, heavier elements like carbon, neon, oxygen, magnesium, sulphur, silicon, iron and nickel will be created. Nickel-56 would be the last MAJOR fusion product in its core after which, no fusion reaction will be possible and a catastrophic collapse will ensue forming a stellar mass black hole.
The size of UY Scuti can be realised from the fact that even if we build a hypothetical object that travels at speed of light, it will take 7 hours to travel around it, whereas for Sun it will just take 14.5 seconds.
UY Scuti is 340,000 times more luminous than the Sun. This means that the total energy output of this beast is 340,000 times greater than our star. However, the surface temperature of UY Scuti is about 3,300 K, roughly half the value for Sun. The surface temperature can be estimated by using Wein's law in Physics which states that the wavelength of light emitted from a black body is inversely proportional to the 4th power of its absolute temperature. Hence, red colored stars are cooler.
Although the star is very luminous, it isn't visible with the naked eye as it is a ninth magnitude star (See The Concept of Magnitude in Astrophysics).