As a kid, I used to look at the stars at night and wonder about those little dots, the Moon, the Sun, and everything I saw. Our Sun seemed huge to me, but I learned it’s pretty average in the universe compared with many stars in the universe. Some stars apparently grow so large that if we placed them at the center of our solar system, their outer layers could extend beyond the orbit of Jupiter. But is there a limit to their size and if there is, what is the largest star in the universe?
Well, there is a limit but finding the largest star in the universe is not as simple as naming one star like you’d name Burj Khalifa as the tallest building. We have leading candidates among the ones we know, like WOH G64. WOH is a leading candidate for the largest star in the universe among stars with well characterized measurements. But there is more than this. Giant stars like Stephenson 2, UY Scuti and VY Canis Majoris are also contenders. But it’s not easy to measure these distant stars. That’s where the problems start.
What is the Largest Known Star?
There is no universally agreed answer to which star is the largest known star. It’s really hard to estimate this and say something concrete. Astronomers estimate a star’s radius using observations of its light, temperature, distance, and in some cases, by looking at its size in the sky. As you may anticipate, these are only estimates and can be uncertain. This especially gets trickier if the star is surrounded by thick clouds of gas and dust.
A little bit of background on star sizes and star classification: our Sun has a radius of 695,700km (referred to as solar radius) and it’s a yellow star. The big ones that I mentioned (UY Scuti, WOH G64, etc) are called red supergiants. They can be up to 3,700 solar radius.
WOH G63 is a leading candidate for the largest known star in the universe among with Stephenson 2 18, UY Scuti, and Westerlund 1-26. WOH G63 is a red supergiant in the Large Magellanic Cloud. It’s a small galaxy that orbits the Milky Way Galaxy. According to the European Southern Observatory, it is roughly 2,000 times the size of the Sun but there are many estimates of its physical radius. UY Scuti, on the other hand, is thought to be at around 1,700 times the Sun’s radius. Stephenson 2 18 is estimated at 2,150 times.
WOH G64: Leading Candidate for the Largest Known Star
WOH G64 takes the stage in these conversations as the largest known star for many reasons. It is surrounded by a thick gas and dust that the star has shed into space. This makes it interesting to study but also harder to estimate its size. In November 2024, European Southern Observatory’s Very Large Telescope Interferometer took a close up picture of WOH G64. This was the first time we imagined a star outside our own galaxy in such a detail. We saw an egg shaped cocoon around the star and its shape had changed in the previous decade. We know its measurements better than many other stars outside our galaxy.
UY Scuti: Very Famous Red Supergiant
For some reason, in the astronomy community, UY Scuti is a famous star, maybe because it’s in the Milky Way Galaxy or maybe truly because of its size. It sits at the constellation Scutum. Earlier estimates put UY Scuit at around 1,700 times bigger radius than the Sun. At this estimated size, if we were to put Scuti at the center of our Solar System, it would extend beyond Jupiter’s orbit. Also, UY Scuti is a variable star. This means that its brightness changes over time and like other red supergiants, it loses material through powerful stellar winds. Similar to WOH G64, the numbers of UY Scuti are also rough estimates and not exact measurements.
VY Canis Majoris
VY Canis Majoris is one of the most studies examples of an extremely large star and it is surrounded by material that it has lost over time. We study the light and gas around it to understand how massive stars shed their outer layers. So these big red supergiants are actually amazing source of understanding star composition and how they act throughout their lives. According to older estimations, VY Canis Majoris’s radius was about a thousand times more than the Sun’s radius. Compared to the other two we talked about, it’s estimated to be slightly smaller. One of the reasons why these estimates are also important is because we can understand the stellar limits. For examples, Stephenson 2 18 may even be bigger than WOH but that is supposed to go beyond the stellar limits, so some thing may be off.
Other Contenders for the Largest Star
The ones I mentioned so far are not the only contenders to be the largest star in the universe, there are more. Other red supergiants like NML Cygni, Stephenson 2-18 are also possibly largest stars. Estimates change heavily between sources and when the measurements were made. Here’s a short list of the contenders:
- Stephenson 2-18 (estimated 2,150 times the radius of the Sun)
- WOH G64 (estimated 1,540 to 2,500+ radius)
- Westerlund 1-26 (estimated 1,530 to 2,550 radius)
- UY Scuti (estimated 900 to 1,708 radius)
- VY Canis Majoris (estimated 1,420 radius)
Why Is it Difficult to Measure a Star’s Size?
Stars are extremely far away, so most appear as points of light even when they are enormous. So naturally, you can’t just go very close to it and measure it, it’s not like they are in our solar system. The ones in our solar system and even closer to us in the Galaxy, we can be quite sure of their size. But farther they go, lesser we are certain. That’s why astronomers often cannot measure a star’s physical diameter directly. Instead, they combine observations with physical models to estimate its radius.
The challenge becomes even greater for red supergiants. Their outer atmospheres can be extended and uneven, and gas and dust around them can affect the light that reaches Earth. One of the most important things when you are looking to estimate star’s size is it’s light that reaches Earth. From it’s distance, how long it takes, what kind, and how it looks like, we can infer a lot of information. A change in the estimated distance or temperature can also change the calculated radius.
How Do Astronomers Determine A Star’s Size?
Astronomers use several methods to estimate the radii of stars. The most useful approach depends on the star’s distance, brightness, temperature, and whether it can be resolved as a disk.
- Luminosity and temperature: Astronomers estimate a star’s luminosity, which is the total energy it radiates, and its surface temperature. These values can be combined using the Stefan Boltzmann law to estimate its radius. The estimate depends on how accurately the luminosity and temperature are known.
- Interferometry: Some observatories combine light collected by multiple telescopes. This technique can measure the apparent angular diameter of a star more precisely than a single telescope could. When astronomers also know the star’s distance, they can estimate its physical radius.
- Eclipsing binary stars: Some stars orbit a companion and periodically pass in front of it from our point of view. By studying the changes in brightness and the orbital motion, astronomers can estimate the stars’ sizes and other properties.
- Distance measurements: A star’s distance is essential for turning its observed brightness or angular diameter into physical measurements. If the distance estimate changes, the calculated luminosity and radius may change too.
- Spectroscopy: Astronomers spread a star’s light into a spectrum to study its temperature, chemical composition, and motion. Spectroscopy does not usually measure radius directly, but it provides important information needed for models that estimate a star’s size.
Why Can Estimates Differ?
We have different methods to look at different things. Each different method can produce a different estimate based on various measurements. For example, a dusty cocoon surrounding a star can make the star appear larger or change the spectrum used to estimate its temperature or a star’s distance may also be uncertain, especially when it lies far beyond the Milky Way.
Some giant stars also have extended atmospheres that do not end at a sharply defined surface. As astronomers gather better observations and improve their models, we usually revise our earlier estimates. For example, especially at UY Scuti, our earlier estimations said it should be about 1,700 radii (of the Sun) but the later, newer estimates puts it at around 900 to 1,200. This is why claims about the largest known star should include the uncertainty rather than present a single radius as an unquestionable fact.
The Largest Star Is Not Necessarily the Most Massive
When people ask about the biggest star, they usually mean the star with the largest radius. But radius is not the same as mass, and neither one is the same as brightness. These properties describe different aspects of a star. For example, neutron stars can be extremely small (way smaller than the Sun) but they can be as dense as a couple of Suns or even more. So when we say “largest star in the universe” we mean the largest in radius, not mass or brightness. That’s a whole different conversation.
Radius vs. Mass
A star’s radius tells us how far it extends from its center, while its mass tells us how much matter it contains. Red supergiants can have enormous radii because their outer layers expand during later stages of stellar evolution. That does not make them the most massive stars known. Some stars are much more massive than the Sun but have smaller radii than the largest red supergiants.
Biggest vs Brightest Stars
If we are looking at the brightest stars, then we look at their luminosity, not their radii or mass. Luminosity measures the total energy a star emits each second. A star can be extremely luminous because it is very hot, very large, or both. A cool red supergiant may have a huge radius, while a much hotter star can emit far more energy from a smaller surface.
For example, R136a1 is known for its extreme mass and luminosity (the most massive star known), but that does not automatically make it the star with the largest radius. The biggest, most massive, and brightest stars are not necessarily the same objects.
FAQ
What is the largest star in the universe?
There is no universally agreed winner. WOH G64 is a leading candidate among stars with well characterized measurements, while Stephenson 2 18, UY Scuti, and VY Canis Majoris are also famous for their enormous estimated radii. Their rankings depend on measurements that still carry uncertainty.
How do scientists measure the size of stars?
Scientists use several methods to find out how big stars are. They combine light from many telescopes for detailed images, compare a star’s true brightness to what we see from Earth, and study how light interacts with matter. This helps them learn about temperature, composition, and motion.
What is the difference between a star’s radius and mass?
A star’s radius is its size from center to surface. Its mass is how much matter it has. Radius helps us see how big a star is. Mass tells us about its gravity and inner workings.