When people ask what is the biggest known star, the answer depends on how astronomers measure “biggest.” The Universe contains red supergiants and hypergiants so enormous that comparing them is surprisingly difficult, and the current record holder can change as measurements improve.
What does “biggest” mean in astronomy?
In everyday language, “biggest” might mean the most massive object.
In astronomy, however, the biggest star usually refers to the one with the largest radius, or the widest physical size from center to outer atmosphere.
That distinction matters because a star can be extremely massive without being the largest in diameter.
Dense blue stars can outweigh cooler red supergiants, while still taking up less space.
- Mass: how much material a star contains
- Radius: how far the star extends from its center
- Luminosity: how much energy it emits
- Volume: the amount of space it occupies
For “biggest known star,” astronomers usually mean the star with the largest measured radius, though that measurement is often uncertain.
So what is the biggest known star?
The answer most often given is Stephenson 2-18, a red supergiant in the Milky Way that has been reported with an estimated radius of more than 2,000 times that of the Sun.
If placed at the center of our solar system, it would extend far beyond the orbit of Jupiter, though exact numbers vary by study.
Stephenson 2-18 is widely cited because its size estimate, while uncertain, places it among the largest known stars.
Earlier candidates included UY Scuti, another red supergiant that is also enormous and remains one of the best-known giants in popular astronomy.
The reason the record is not perfectly settled is that stellar size estimates depend on distance, temperature, dust, and the outer layers of an expanded atmosphere.
These factors make ultra-large stars especially hard to measure.
Why are red supergiants so huge?
Red supergiants are late-stage stars that have exhausted much of the hydrogen fuel in their cores.
As the core changes, the outer layers expand dramatically and cool, giving the star a reddish color and a vast radius.
These stars are rare, short-lived on cosmic timescales, and highly unstable.
Many are massive enough to end their lives in supernova explosions, leaving behind neutron stars or black holes.
Key features of red supergiants
- Cool surface temperatures compared with hotter blue stars
- Immense diameters, often hundreds to thousands of times the Sun’s radius
- Strong stellar winds and mass loss
- Advanced evolutionary stage before core collapse
How does the biggest known star compare with the Sun?
The Sun is a main-sequence star and is tiny compared with the largest known stars.
A red supergiant like Stephenson 2-18 can be thousands of times larger in radius, but not necessarily thousands of times more massive.
That difference is important.
The Sun is far denser and more stable, while giant stars are puffed-up outer envelopes surrounding compact cores.
Their average density can be so low that, in some cases, it would be less dense than Earth’s atmosphere at sea level.
If the Sun were a basketball, the biggest known stars would be massive structures spanning an entire city or larger.
Their outer layers are so extended that they are difficult to define cleanly.
Which stars are also contenders?
Several stars are often mentioned in discussions of the largest known stars.
Because measurements are uncertain, the rankings shift as observational methods improve.
- UY Scuti — long considered one of the largest known stars
- VY Canis Majoris — a famous red hypergiant and one of the most luminous nearby giants
- WOH G64 — a large red supergiant in the Large Magellanic Cloud
- VX Sagittarii — a highly variable supergiant with extreme size estimates
- NML Cygni — a bright, dusty red hypergiant in the Milky Way
These stars are not just large; they are among the most physically extreme objects in the galaxy.
Their atmospheres can be irregular, turbulent, and surrounded by thick dust, which complicates accurate measurement.
How do astronomers measure stellar size?
Astronomers cannot usually measure a star’s edges directly the way they would measure a planet.
Instead, they estimate size using luminosity, temperature, and distance, then apply the Stefan-Boltzmann law to infer radius.
Modern surveys use data from observatories such as the Gaia mission, the Hubble Space Telescope, and large ground-based telescopes.
However, when a star is far away or hidden by interstellar dust, the resulting uncertainty can be large.
Why measurements change over time
- Distance estimates are refined with better parallax data
- Dust can dim a star and distort brightness readings
- Cool, extended atmospheres make a star’s edge hard to define
- Different research teams may use different models
That is why the biggest known star is not a permanently fixed title.
It is a best current estimate based on available evidence.
Is the biggest known star also the most massive?
Not usually.
The largest star by radius is often not the most massive star known.
Very massive stars can be hot, compact, and extremely luminous without being the widest.
For example, R136a1 is one of the most massive known stars, but it is not among the largest in radius.
This illustrates the difference between size, mass, and brightness in stellar astronomy.
In practical terms, astronomers separate these traits because each reveals something different about a star’s structure and evolution.
Why the biggest stars matter in science
Studying giant stars helps researchers understand how massive stars evolve, lose mass, and enrich galaxies with heavy elements.
When these stars die, they can seed the interstellar medium with oxygen, carbon, calcium, and iron needed for planets and life.
They also help astronomers test models of stellar atmospheres, dust formation, and supernova progenitors.
Because these stars are unstable and short-lived, catching them in the act provides valuable clues about late-stage stellar evolution.
- They shape chemical evolution in galaxies
- They may become supernovae or black holes
- They challenge current models of stellar physics
- They reveal how matter behaves under extreme conditions
Why isn’t there a single final answer?
There is no single permanent answer because the largest known star depends on measurement technique, updated distance data, and how astronomers define the star’s boundary.
A star’s outer atmosphere can be diffuse and irregular, so the “surface” is not as simple as it is for rocky planets.
As telescopes improve and catalogs expand, candidates may be reclassified.
That is why the question what is the biggest known star remains both answerable and open-ended at the same time.
What to remember about the biggest known star
Today, Stephenson 2-18 is one of the most widely cited answers to what is the biggest known star, based on estimated radius.
But stars such as UY Scuti, VY Canis Majoris, and NML Cygni remain important contenders in the ongoing search for the largest stellar giants.
The key takeaway is that “biggest” in astronomy usually means largest radius, not greatest mass or brightness.
In a universe full of extremes, the largest stars are among the most fascinating—and most difficult—to measure.