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UY Scuti Is 1,700 Times the Radius of the Sun — And It Would Swallow Jupiter's Orbit
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UY Scuti Is 1,700 Times the Radius of the Sun — And It Would Swallow Jupiter's Orbit

March 28, 2026 · 4 min read

The Fact

The largest known star, UY Scuti, is about 1,700 times the radius of the Sun.

The Sun is immense by any human standard: 1.4 million kilometers in diameter, containing 99.86% of all the mass in our solar system, with a surface area more than 10,000 times that of Earth. It is also, by cosmic standards, a rather modest star. In the constellation Scutum, roughly 9,500 light-years from Earth, there exists a red hypergiant star called UY Scuti with a radius approximately 1,700 times that of the Sun. If UY Scuti were placed at the center of our solar system, its photosphere — its visible surface — would extend approximately 5 astronomical units from that center, engulfing Mercury, Venus, Earth, Mars, and the asteroid belt, reaching out past Jupiter's orbit.

The volume comparison is staggering. Since volume scales with the cube of radius, a sphere 1,700 times larger in radius has a volume approximately 4.9 billion times larger. Roughly five billion Suns, by volume, could fit inside UY Scuti. Despite this extraordinary size, UY Scuti has only about 7 to 10 times the mass of the Sun — a density so low that it is closer to a laboratory vacuum than to anything we would recognize as a solid or even a dense gas.

How Stars Become Giants

Stars spend most of their lives fusing hydrogen into helium in their cores — a stage called the main sequence. Our Sun has been doing this for about 4.6 billion years and will continue for another 5 billion. During this phase, radiation pressure from the fusion reactions pushes outward against gravity, maintaining a stable equilibrium that determines the star's size.

When the hydrogen in the core runs out, the star's behavior changes dramatically. The core contracts and heats, while the outer layers expand enormously. This expansion creates a red giant (for lower-mass stars like the Sun) or a red supergiant or hypergiant (for the most massive stars). In the most extreme cases, stars with initial masses of 20 to 40 times the Sun's mass can expand to radii of hundreds or even thousands of solar radii during their giant phases.

UY Scuti is classified as a red hypergiant, a designation reserved for the very largest and most luminous stars. Its luminosity is approximately 340,000 times that of the Sun — a figure that makes it visible to the naked eye despite its distance of 9,500 light-years, though barely so. Red hypergiants are intrinsically rare objects because the giant phase of a massive star's life is relatively brief: a star of UY Scuti's mass will likely end its life in a supernova within the next few hundred thousand to few million years.

The Measurement Challenge

Measuring a star's angular size is genuinely difficult at distances of thousands of light-years, and the stated radius of UY Scuti comes with significant uncertainty. The star's size has been revised multiple times as measurement techniques have improved. A 2012 study using the Very Large Telescope in Chile measured its angular diameter using optical interferometry and derived the 1,700 solar radii figure, though other studies have placed the value slightly lower.

One complicating factor is that UY Scuti, like many red hypergiants, has an extended and complex atmosphere that makes defining its "surface" somewhat ambiguous. The star pulsates — varying in size and brightness over a period of roughly 740 days — so its radius is not even a fixed quantity but a range of values that change on timescales of years. The 1,700 solar radii figure represents an average or maximum value in the pulsation cycle rather than a constant.

A Star Living on Borrowed Time

Stars as massive as UY Scuti are living fast and dying young by stellar standards. Their enormous mass drives extremely high core temperatures and luminosities, consuming nuclear fuel at rates that cannot be sustained for billions of years. While the Sun will shine for about 10 billion years total, a 40-solar-mass star exhausts its nuclear reserves in a few million years — a thousand times shorter.

When UY Scuti eventually exhausts its fuel and its core collapses, the result will be one of the universe's most spectacular events: a core-collapse supernova, potentially visible from Earth in daylight despite the star's distance, followed by either a neutron star or a black hole at its center. The largest objects in the universe contain, in their structure, some of the most dramatic endpoints.

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FactOTD Editorial Team

Published March 28, 2026 · 4 min read

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