Which are the Largest Stars in the Universe?

The Top 10 Stars in the Sky by Diameter

Stars are immense balls of burning plasma. Yet, aside from the Sun in our own solar system, they appear as tiny pinpoints of light in the sky. Our Sun, technically a yellow dwarf, is neither the biggest or the smallest star in the universe. While it's much larger than all the planets combined, it's not even medium-sized in comparison to other more massive stars. Some of these stars are larger because they evolved that way from the time they were formed, while others are bigger due to the fact that they're expanding as they age. 

Star Size: A Moving Target

Figuring out a star's size isn't a simple project. Unlike planets, stars have no distinct surface with which to form an "edge" for measurements, nor do astronomers have a convenient ruler to take such measurements. Generally, astronomers look at a star and measure its angular size, which is its width as measured in degrees or arcminutes or arcseconds. This measurement gives them a general idea of the star's size but there are other factors to consider.

For example, some stars are variable, which means they regularly expand and shrink as their brightness changes. That means when astronomers study a star such as V838 Monocerotis, they must look at it more than once over a period of time as it expands and shrinks in order to can calculate an average size. As with virtually all astronomic measurements, there's also an inherent margin of inaccuracy in observations due to equipment error and distance, among other factors.

Finally, a listing of stars by size must take into account that there may be larger specimens that simply haven't been studied or even detected as yet. With that in mind, the following are the 10 largest stars currently known to astronomers. 

Betelgeuse

Betelgeuse Star
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Betelgeuse, easily seen from October through March in the night sky, is the most well-known of the red supergiants. This is due in part to the fact that at roughly 640 light-years from Earth, Betelgeuse is very close compared to the other stars on this list. It's also part of what is one the most famous of all the constellations, Orion. With a known radius in excess of a thousand times that of our Sun, this massive star is somewhere between 950 and 1,200 solar radii (the unit of distance used by astronomers to express the size of stars equal to the current radius of the Sun) and is expected to go supernova any time.

VY Canis Majoris

Bright star surrounded by clusters of smaller stars
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This red hypergiant is among the largest known stars in our galaxy. It has an estimated radius between 1,800 and 2,100 times that of the Sun. At this size, if placed in our solar system, it would reach nearly to the orbit of Saturn. VY Canis Majoris is located roughly 3,900 light-years from Earth in the direction of the constellation Canis Majoris. It's one of a number of variable stars that appear in the constellation Canis Major.

VV Cephei A

Our Sun compared to the giant star VV Cephei A.

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This red hypergiant star is estimated to be around a thousand times the radius of the Sun and is currently regarded as one of the largest such stars in the Milky Way. Located in the direction of the constellation Cepheus, VV Cephei A is about 6,000 light-years from Earth and is actually part of a binary star system shared with a companion smaller blue star. The "A" in the star's name is assigned to the larger of the two stars in the pair. While they orbit one another in a complex dance, no planets have been detected for VV Cephei A.

Mu Cephei

Mu Cephei

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This red supergiant in Cepheus is about 1,650 times the radius of our Sun. With more than 38,000 times the Sun's ​luminosity, it's also one of the brightest stars in the Milky Way. Thanks to its pretty reddish color, it's been given the nickname "Herschel's Garnet Star" in honor of Sir William Herschel, who observed it in 1783, and is also known by the Arabic name Erakis.

V838 Monocerotis

Variable star V838 Monocerotis in constellation Monoceros
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This red variable star located in the direction of the constellation Monoceros is about 20,000 light-years from Earth. It may be larger than either Mu Cephei or VV Cephei A, but because of its distance from the Sun and the fact that its size pulsates, its actual dimensions are difficult to determine. After its last outburst in 2009, its size appeared to be smaller. Therefore, it's given a range typically between 380 and 1,970 solar radii. The Hubble Space Telescope has documented the shroud of dust moving away from V838 Monocerotis on several occasions.

WOH G64

Artist's concept of a Hypergiant Star.
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This red hypergiant located in the constellation Dorado (in the southern hemisphere skies) is about 1,540 times the radius of the Sun. It's actually located outside of the Milky Way in the Large Magellanic Cloud, a nearby companion galaxy to our own that lies about 170,000 light-years away. 

WOH G64 has a thick disk of gas and dust surrounding it, which was likely expelled as the star it began its death throes. This star was once more than 25 times the mass of the Sun but as it neared exploding as a supernova, began to lose mass. Astronomers estimate that it has lost enough component material to make up between three and nine solar systems. 

V354 Cephei

Artist's conception of a hypergiant.
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Slightly smaller than WOH G64, this red hypergiant is 1,520 solar radii. At a relatively close 9,000 light-years from Earth, V354 Cephei is located in the constellation Cepheus. WOH G64 is an irregular variable, which means that it pulsates on an erratic schedule. Astronomers studying this star closely have identified it as being part of a larger group of stars called the Cepheus OB1 stellar association that contains many hot massive stars, but also a number of cooler supergiants such as this one. 

RW Cephei

Sharpless 140 nebula in Cepheus constellation (infrared)
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Here's another entry from the constellation Cepheus in the northern hemisphere sky. This star may not seem all that large in its own neighborhood, however, there aren't many others in our galaxy or nearby that can rival it. This red supergiant's radius is somewhere around 1,600 solar radii. If it were at the center of our solar system in place of the Sun, its outer atmosphere would stretch beyond the orbit of Jupiter.

KY Cygni

Stars and nebulae in the constellation Cygnus
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While KY Cygni is at least 1,420 times the radius of the Sun, some estimates put it closer to 2,850 solar radii (although it's likely closer to the smaller estimate). KY Cygni is located about 5,000 light-years from Earth in the constellation Cygnus. Unfortunately, there are no viable images available for this star at this time.

KW Sagittarii

The Lagoon Nebula in Sagittarius
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Representing the constellation Sagittarius, this red supergiant is 1,460 times the radius of our Sun. KW Sagittarii lies about 7,800 light-years from Earth. If it were the main star in our solar system, it would stretch out well beyond the orbit of Mars. Astronomers have measured the temperature of KW Sagittarii at around 3,700 K (Kelvin, the base unit of temperature in the International System of Units, having the unit symbol K). This is much cooler than the Sun, which is 5,778 K at the surface. (There are no viable images available for this star at this time.)