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What is the biggest star ever observed?

The biggest star is UY Scuti, about 1,700 times larger than the sun.
The biggest star is UY Scuti, about 1,700 times larger than the sun. (Image credit: Philip Park (CC BY-SA 3.0))

Our sun is enormous: More than a million Earths could fit inside of it. But on a stellar scale, it could be swallowed up by about half of all stars observed so far — especially stars like UY Scuti. 

The largest known star in the universe, UY Scuti is a variable hypergiant with a radius around 1,700 times larger than the radius of the sun. To put that in perspective, the volume of almost 5 billion suns could fit inside a sphere the size of UY Scuti.  

About UY Scuti

In 1860, German astronomers at the Bonn Observatory first cataloged UY Scuti, at the time naming it BD -12 5055, reported Astronomy Magazine (opens in new tab). During a second observation, astronomers realized it grows brighter and dimmer over a 740-day period, leading to its classification as a variable star

The star lies near the center of the Milky Way, roughly 9,500 light-years away from Earth. Located within the constellation Scutum, UY Scuti is a hypergiant star. Hypergiants — larger than supergiants and giants — are rare stars that shine very brightly. They lose much of their mass through fast-moving stellar winds.

Related: Hypergiant star's weight loss secrets revealed

How to identify the largest star

However, all stellar sizes are estimates.

"The complication with stars is that they have diffuse edges," astronomer Jillian Scudder of the University of Sussex wrote for The Conversation (opens in new tab). "Most stars don't have a rigid surface where the gas ends and vacuum begins, which would have served as a harsh dividing line and easy marker of the end of the star."

Instead, astronomers rely on a star's photosphere to determine its size. The photosphere is where the star becomes transparent to light and the particles of light, or photons, can escape the star.

"As far as an astrophysicist is concerned, this is the surface of the star, as this is the point at which photons can leave the star," Scudder wrote.

If UY Scuti replaced the sun in the center of the solar system, its photosphere would extend just beyond the orbit of Jupiter. The nebula of gas ejected from the star extends far beyond the orbit of Pluto, to 400 times the distance between the Earth and the sun.

Other contenders

NASA's Hubble Space Telescope reveals the supercluster Westerlund 1, home of one of the largest known stars. Westerlund 1-26, a red supergiant, has a radius more than 1,500 times that of the sun. (Image credit: ESA/Hubble & NASA)

UY Scuti's large radius does not make it the most massive, or heaviest, star. That honor goes to R136a1, which weighs in at about 300 times the mass of the sun but only about 30 solar radii. UY Scuti, in comparison, is only about 30 times the mass of the sun, but far greater in volume.

Size comparisons are still more complicated because UY Scuti doesn't remain stagnant. Scudder pointed out that the star varies in brightness as it varies in radius. And the measurement we have now has a margin of error of about 192 solar radii. The variation or margin of error each could allow other stars to beat out UY Scuti in the race for size. In fact, there are as many as 30 stars whose radii approach or surpass UY Scuti's smallest estimated size, so the behemoth shouldn't sit too securely on its throne.

Which star would take UY Scuti's place if its size were reevaluated? Here are a few that could take the crown from the giant currently measured at 1,700 times the width of the sun:

Additional resources and reading

Bibliography: 

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Nola Taylor Tillman is a contributing writer for Space.com. She loves all things space and astronomy-related, and enjoys the opportunity to learn more. She has a Bachelor’s degree in English and Astrophysics from Agnes Scott college and served as an intern at Sky & Telescope magazine. In her free time, she homeschools her four children. Follow her on Twitter at @NolaTRedd

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