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What is the universe’s largest known star? (How about the tiniest?)

When it comes to stars, our sun gets a lot of attention. Not only does life on Earth literally revolves around it, but it dwarfs the rest of the stars in the sky — at least from our point of view. However, when viewed from the far reaches of our galaxy, the sun no longer appears to be such a massive object. In fact, it’s fairly standard in size. So, what is the universe’s largest known star?

According to Phil Massey, an astronomer at the Lowell Observatory in Flagstaff, Arizona, the answer depends on whether you’re talking about a star’s mass or its total volume — that is, how much space it takes up. When it comes to physical size, the heaviest stars are frequently unremarkable, and the most voluminous stars are frequently lightweights. This is due to the fact that as stars age, they tend to expand and shed mass. “It’s like talking about people,” Massey explained, “the tallest people may not be the heaviest.”

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Assume you’re discussing mass. The star R136a1 holds the record there, according to Massey. It’s about 160,000 light-years away from Earth in the Large Magellanic Cloud, a small galaxy orbiting the Milky Way. This star is 30 to 40 times the size of our sun in diameter — imagine a cherry next to a giant yoga ball that is more than 200 times larger. According to Massey, this star is also relatively young — roughly 1 million years old compared to our sun’s 4.5 billion years — and “hasn’t done much cooling off or expanding.”

If the largest star in the universe is the one with the largest diameter, there are several contenders, according to Massey. UY Scuti is at the top of that list. According to a 2013 study published in the journal Astronomy and Astrophysics, the diameter of this red hypergiant is roughly 1,700 times that of the sun. UY Scuti would be a 10-story-high sphere if the sun were a cherry. However, determining the diameter of very distant stars is fraught with uncertainty (UY Scuti is about 9,500 lightyears from Earth, give or take 1,000 lightyears).

To do so, scientists must first determine how much light the star emits, a difficult figure to calculate due to the fact that stars appear dimmer with distance and brighter closeup. Add to that the fact that red hypergiants like UY Scuti are frequently “variable,” meaning their brightness flickers and flares over time, and you get a large margin of error, according to Massey.

The authors of the 2013 paper, for example, reported that UY Scuti could be up to 192 solar radii — a measurement based on the radius of the sun — larger or smaller than previously estimated. “The red hypergiants are very messy, so they’re difficult to model,” Massey explained. “The surfaces are constantly in motion.”

WOH G64, another red supergiant (less than 5 million years old, according to a 2018 article(opens in new tab)) in the Large Magellanic Cloud, and VY Canis Majoris (about 8.2 million years old, according to a 2011 article(opens in new tab)), both have diameters around 1,500 times that of the sun, according to Massey. (And, given the uncertainty, either could outnumber UY Scuti in terms of size.) “In any case, I think it’s fantastic,” Massey said. If any of these stars replaced our sun at the center of our solar system, they would encircle all of the inner planets, including Jupiter. “All of the inner planets, including Earth, would be vaporized,” Massey added.

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But, while our sun is not the largest star in the universe, it is also not the smallest. So, how small is the smallest known star? According to a 2017 study published in the journal Astronomy and Astrophysics, that honor goes to EBLM J0555-57Ab. According to the authors, EBLM J0555-57Ab is smaller than Saturn and barely qualifies for star designation. It would be classified as a brown dwarf — a failed star — if it had a lower mass and could not sustain nuclear fusion at its core.

Of course, the universe is vast, and these are only the stars in our direct cosmic neighborhood. After all, we can’t even measure the size of stars on the other side of the Milky Way, let alone the far reaches of the universe, according to Massey. “There’s too much dust, and there’s too much light interference,” he added. While UY Scuti and EBLM J0555-57Ab approach the upper and lower limits of a star’s possible size, we still don’t know how massive, or heavy, stars can become, according to Massey. “I believe there will be some pleasant surprises.”

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