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What is the largest star known, and how far out would its habitable zone be?

Samuel Heinitz, Edmond, Oklahoma
HabitableZone
The habitable zones of the star with the largest diameter (VY Canis Majoris) and of the most massive star known (R136a1) dwarf that of our Sun. Both of these stars' habitable zones are farther out than our solar system's Kuiper Belt (the region of icy, rocky bodies just beyond Neptune's orbit). In fact, R136a1's would extend into the Oort Cloud — a region no spacecraft has yet explored. Astronomy: Roen Kelly
We think the stars with the largest radii are the red hypergiants. However, which one of these objects is the biggest is unclear because most are too far away for us to precisely measure their sizes. But one possibility, VY Canis Majoris, is so big that if you placed it in the Sun’s position at the center of the solar system, it would extend to Saturn’s orbit!

But what is the star with the largest mass? These tend to be blue hypergiants. Once again, we don’t know exactly which one has the most mass, but R136a1 is a good candidate. Scientists recently announced that this star might be some 265 times the Sun’s mass.

The “habitable zone” is the range of distances from a star where liquid water can survive on a planet’s surface without freezing or boiling. We can estimate a habitable zone based on its parent star’s luminosity. For VY Canis Majoris, the habitable zone is between roughly 600 and 1,200 astronomical units (the Earth-Sun distance, or AU) from the star. For R136a1, the habitable zone is located about 2,500 to 5,100 AU from the star.

But around such giant stars these "habitable zones" might not be so habitable. The biggest stars and the most massive ones don't live very long. These suns emit so much energy that they burn themselves out in about a million years. We don't know exactly how long it took for life to arise on Earth, but it probably took much longer than a million years. Humans certainly didn't appear on Earth for about 4.5 billion years.

These monster stars also have a nasty habit of varying greatly in brightness. So most astronomers think these large stars would be bad places for life, within or outside their habitable zones.

— Marc Kuchner, NASA’s Goddard Space Flight Center, Greenbelt, Maryland
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