Saw this on the news this morning, and just found an article about it on the Independent website, thought it was pretty interesting that something this size exists, and theres probably even bigger out there! 
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The biggest star found, R136a1 within the R136a cluster, has a current mass of 265 solar masses, and it is thought its birthweight was as much as 320 times that of the Sun.
It is also the most luminous star ever found, close to 10 million times that of the Sun.
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