UY Scuti is a fluctuating hypergiant with a radius roughly 1,700 times the sun. To put it in context, a sphere size UY Scuti could hold about 5 billion suns.
UY Scuti was discovered in 1860 by German astronomers at the Bonn Observatory, who named it BD -12 5055. A second observation revealed it dims and brightens over 740 days, classifying it as a variable star.
The star is 9,500 light-years distant from Earth, at the Milky Way‘s core. UY Scuti is a hypergiant star in the constellation Scutum. Hypergiants are uncommon, brilliant stars that are bigger than supergiants and giants. They lose mass to fast-moving stellar winds.
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How to find the biggest star
But stellar sizes are just approximations.
“The problem with stars is that they have hazy edges,” said astronomer Jillian Scudder of Sussex University. “Most stars lack a solid surface where gas meets vacuum, serving as a sharp dividing line and easy marker of the star’s demise.”
Astronomers measure a star’s size using its photosphere. The photosphere is where the star becomes transparent to light, and photons may escape.
The star’s surface is defined by an astronomer as the point where photons may escape the star.
UY Scuti’s photosphere would reach beyond Jupiter’s orbit if it replaced the sun in the solar system‘s core. The gas emitted from the star stretches 400 times the distance between Earth and the sun.
In terms of mass, UY Scuti is not the heaviest star. The winner is R136a1, which has 300 times the sun’s mass but just 30 solar radii. UY Scuti, by contrast, has a mass 30 times that of the sun but a volume significantly bigger.
Size comparisons are difficult since UY Scuti changes size. Scudder noted that the star’s brilliance fluctuates with its radius. The measurement currently has a 192 solar radii margin of error. The fluctuation or margin of error might enable other stars to outgrow UY Scuti. The giant shouldn’t be too comfortable on its throne since up to 30 stars have radii close to or larger than UY Scuti’s.
Which star would replace UY Scuti if its size was changed? Here is a handful that might unseat the monster now measuring 1,700 times the sun’s width:
- WOH G64 was previously considered to be 3,000 times the sun’s width. According to a 2009 Astronomical Journal study, it is now roughly 1,504 suns wide. It’s a red hypergiant in the Large Magellanic Cloud, a Milky Way satellite galaxy. UY Scuti’s brightness fluctuates.
- NASA says Westerlund 1-26 is more than 1500 times the sun’s width.
- According to a 2012 Astronomy and Astrophysics article, NML Cygni is 1,639 times the sun’s width.
- KY Cygni is over 1,033 times the sun’s width, according to a 2020 publication in Astrophysics of Galaxies.
- In a 2012 publication in the journal Solar and Stellar Astrophysics, VY Canis Majoris was estimated to be 1,420 times the sun’s width. New measurements have reduced the size of this red hypergiant star to 1,800-2,200 times the sun’s width. Some publications still call it the biggest star.
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