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Stars are so immensely distant that only the most powerful telescopes have been able to directly measure the diameters of the largest, closest ones.

The greatest visual observer of the pre-telescopic era, Tycho Brahe (1546-1601), was convinced that he had discerned the brightest stars’ diameters by eye, but he was fooled by an optical illusion: the human eye tends to misinterpret “bright” as “big.”

If there were no upper limit on stellar diameters, it’s a safe bet that the largest star would be well beyond our sight, by virtue of the universe’s vast size. But theory dictates that any star more than two thousand times the sun’s diameter would be unstable, pushing its outermost layers (far from most of the star’s gravitational attraction) off into space.

The star with the largest, well-measured diameter is VY Canis Majoris, a red hypergiant just above the tail of Orion’s big dog. It spans 1,420 suns wide, and emits 270 thousand times more light, but at a distance of nearly four thousand light years, is still (barely) too faint to be seen with the unaided eye.

A few other hypergiants may be larger than VY Canis Majoris, but the parameters used to calculate their size (including distance and surface temperature) are too poorly known to be certain. So if a 2,000-solar-diameter hypergiant exists, we have yet to find it.

The star with the largest apparent size (other than the sun) lies in the southern constellation of Dorado, the Dolphinfish. R Doradus is a red giant star 370 times the sun’s diameter, but because at only 178 light years distant, it spans the largest known angular diameter of 0.057 arcseconds (the size of a dime from 40 miles away).

Next column: Looking ahead to space and astronomy events in 2019.

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Chris Anderson manages the College of Southern Idaho’s Centennial Observatory in Twin Falls. He can be reached at 208-732-6663 or canderson@csi.edu.

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