The Biggest Star in the Sky

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This is an IR Observation of R Doradus, a variable star in the constellation of Dorado (the Swordfish), located in the far southern sky. R Doradus is a variable star with a period of about 338 days, changing its magnitude from approximately 4.8 at maximum (when it is visible with the unaided eye) to 6.6 at minimum (when it requires a small telescope). R Doradus is approximately 200 light-years away.

The measured size implies that it has a physical diameter of 370 +- 50 times that of the Sun, or well over 250 million km! If R Doradus would be placed at the centre of the Solar System, its surface would be outside of the orbit of Mars. Although even bigger stars are known — Betelgeuse for one — none appears as large in the sky because they are all at greater distances. The very large apparent size of R Doradus is due to the combination of its relative proximity and large physical size.

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