Stars are immense balls of burning plasma. Yet, aside from the Sun, they look like tiny pinpoints of light in the sky. Our Sun is not the biggest or the smallest star in the universe. Technically, it's called a yellow dwarf. It's much bigger than all the planets combined, but not even medium-sized by the standards of all the stars. There are many much more massive and larger than the Sun. Some are larger because they've evolved that way from the time they were formed. Others are larger because they are getting older and expanding as they age.
Star Size: A Moving Target
Figuring out a star's size isn't a simple project. There's no "surface" like we see at planets to give a hard "edge" for measurements. Also, astronomers don't have a convenient "rule" they can hold up to make the measurements. Generally, they can look at a star and measure it's "angular" size, which means its width as measured in degrees or arcminutes or arcseconds. That gives them a general idea, but there are other factors to consider. Some stars are variable, for example. That means they regularly expand and shrink as their brightness changes. So, if astronomers study a star such as V838 Monocerotis, they have to look at it several times over a period of time as it expands and shrinks. Then they can calculate an "average" size. Like virtually all astronomy measurements, there is an inherent bit of error in observations due to equipment error, distance, and other factors. Finally, a listing of stars by size has to take into account that there may be larger ones that haven't been studied (or detected) yet. With that in mind, what stars are the largest ones known to astronomers?
Betelgeuse
Betelgeuse is known to be a whopper and easily seen in Earth's night skies from October through March. It's known to have a radius in excess of a thousand times that of our Sun and is the most well-known of the red supergiants. This is partially due to the fact that at roughly 640 light-years from Earth, Betelgeuse is very close compared to the other stars on this list. Also, it lies in perhaps the most famous of all the constellations, Orion. This massive star is somewhere between 950 and 1,200 solar radii and is expected to go supernova any time.
VY Canis Majoris
This red hypergiant is among the largest known stars in our galaxy. It has an estimated radius between 1,800 and 2,100 times the radius of the Sun. At this size it would reach nearly to the orbit of Saturn if placed in our solar system. 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
This star is located in the direction of the constellation Cepheus, about 6,000 light-years from Earth. It's a red hypergiant star estimated to be around a thousand times the radius of the Sun. It's actually part of a binary star system; its companion is a smaller blue star. The two orbit each other in a complex dance. No planets have been detected at this star. The A in the star's name is assigned to the largest of the pair, and it's now known as one of the largest such stars in the Milky Way.
Mu Cephei
This red supergiant in Cepheus is about 1,650 times the radius of our Sun. It is also one of the brightest stars in the Milky Way galaxy, with more than 38,000 times the Sun's luminosity. It also has the nickname "Herschel's Garnet Star" due to its pretty reddish color.
V838 Monocerotis
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, its actual size is difficult to determine. Also, it pulsates in size, and after its last outburst in 2009, its apparent size was smaller. Therefore a range is typically given of between 380 and 1,970 solar radii.
The Hubble Space Telescope has observed this star several times, documenting the shroud of dust moving away from it.
WOH G64
This red hypergiant located in the constellation Dorado (in the southern hemisphere skies) is about 1,540 times the radius of the Sun. It is actually located outside of the Milky Way Galaxy in the Large Magellanic Cloud. That's a nearby companion galaxy to our own and lies about 170,000 light-years away.
WOH G64 has a thick disk of gas and dust surrounding it. That material was likely expelled from the star as it began its death throes. This star used to be more 25 times the mass of the Sun, but as it gets close to exploding as a supernova, it began to lose mass. Astronomers estimate that it has lost enough material to make between three and nine solar systems.
V354 Cephei
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. It is an irregular variable, which means that it pulsates on a somewhat erratic schedule. Astronomers who are studying this star closely have identified it as being part of a larger group of stars called the Cepheus OB1 stellar association, which contains many hot massive stars, but also a number of cooler supergiants such as this one.
RW Cephei
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, but 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 in place of our Sun, its outer atmosphere would stretch beyond the orbit of Jupiter.
KY Cygni
KY Cygni is at least 1,420 times the radius of the Sun, but some estimates make it more like 2,850 solar radii. It's likely closer to the smaller size. It is located about 5,000 light-years from Earth in the constellation Cygnus. Unfortunately, there is not a good image available for this star at this time.
KW Sagittarii
Representing the constellation Sagittarius, this red supergiant is no slouch at 1,460 times the radius of our Sun. If it was the main star for our solar system, it would stretch out well beyond the orbit of Mars. KW Sagittarii lies about 7,800 light-years away from us. Astronomers have measured its temperature, which is around 3700 K. This is much cooler than the Sun, which is 5778 K at the surface. There is not a good image available for this star at this time.
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