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"I assume most of us know that Mass is just relative to the amount of Matter the object consists off, but Mass is not representative of the amount of Volume it takes up. However, there are some that do not, so let's not be misleading and clarify a few things. For example, some of the most Massive Objects in the Universe are Black Holes, which have theoretically 0 volume. R136a1 impresses with 265 Solar Masses? The Supermassive Black Hole OJ 287 is 18,000,000,000 Solar Masses. R136a1, for Mass of a Star sets a new standard. As for Volume though, even though it's 1,000,000 times brighter than our Sun, it's Volume is shadowed by that of VY Canis Majoris, STILL.
R136a1 is 265 Solar Masses and 35 times the diameter of our Sun. In terms of Mass vs. VY Canis Majoris, R136a1 wins. However, in terms of Volume, R136a1 is ONLY 35 times the Diameter of our Sun. Our Sun is approximately 865,000 Miles in Diameter. Being 35 times the Diameter, R136a1 is approximately 30,275,000 miles in Diameter, pretty large. If it were to replace our Sun, it would be touching the boundaries of Mercury's Orbit, which is approximately 35,000,000 miles. Earth is approximately 93,000,000 miles away from the Sun. To give you an idea of our Voluminous VY Canis Majoris is, it is 1,800,000,000 miles across. 1,800,000,000 miles (VY Canis Majoris) > 30,275,000 miles (R136a1) by a long shot. To give you an idea of how much space that would take up, Saturn's Orbit is 1,350,000,000 away from the Sun. So, if R136a1's outer edges would singe Mercury if it were in the place of our sun, VY Canis Majoris in it's place would be hiding Mercury, Venus, Earth, Mars, The Asteroid Belt, Jupiter and Saturn within itself, with room to spare. Hoever, R136a1 is just starting it's main sequence, whereas VY Canis Majoris is near the end of it's life. R136a1 may become even larger later on in it's life."