The universe is home to some of the most mind-blowing celestial objects, and among them are the largest stars ever discovered. Two of the biggest contenders for the title of the largest known star are UY Scuti and Stephenson 2-18. These stars are classified as red supergiants, meaning they have reached the late stages of their evolution and have expanded to massive sizes.
But which one is truly the largest? How do they compare in size, brightness, and overall significance in astronomy? This topic explores the fascinating characteristics of UY Scuti and Stephenson 2-18, their differences, and their place in the cosmos.
What Are Red Supergiant Stars?
Before diving into the comparison, it’s essential to understand what red supergiants are. These are among the most massive and luminous stars in the universe, having expanded to enormous sizes as they near the end of their life cycle. Unlike main-sequence stars like the Sun, red supergiants burn through their fuel rapidly, eventually leading to a supernova explosion.
UY Scuti and Stephenson 2-18 are both extreme examples of this stellar evolution, making them fascinating subjects for astronomers.
UY Scuti: The Former Largest Star
1. Basic Characteristics
UY Scuti was once considered the largest known star based on stellar radius estimates. It is located in the constellation Scutum, approximately 9,500 light-years away from Earth.
- Type: Red Supergiant
- Radius: ~1,700 times the Sun’s radius
- Distance: ~9,500 light-years
- Luminosity: ~340,000 times that of the Sun
- Mass: Estimated between 7-10 times the Sun’s mass
2. How Big Is UY Scuti?
To put its size into perspective, if UY Scuti replaced the Sun in our solar system, its outer layers would extend beyond the orbit of Jupiter. That’s how massive this star is!
However, despite its immense size, UY Scuti is not the most massive star. It has low density, meaning its outer layers are extremely spread out, making it appear larger but not necessarily heavier than other supergiants.
Stephenson 2-18: The New Largest Star
1. Basic Characteristics
Stephenson 2-18 (St2-18) is the current record-holder for the largest known star by radius. It is part of the Stephenson 2 cluster, located about 19,500 light-years from Earth in the constellation Scutum.
- Type: Red Supergiant
- Radius: ~2,150 times the Sun’s radius
- Distance: ~19,500 light-years
- Luminosity: ~440,000 times that of the Sun
- Mass: Estimated around 40 times the Sun’s mass
2. How Big Is Stephenson 2-18?
If Stephenson 2-18 replaced the Sun, its surface would extend beyond the orbit of Saturn or even Uranus! This makes it significantly larger than UY Scuti.
In addition to its enormous radius, Stephenson 2-18 is also extremely luminous, producing vast amounts of energy. Scientists believe it is in its final stages before going supernova, which will mark a dramatic end to its existence.
Comparing UY Scuti and Stephenson 2-18
1. Size Comparison
| Star | Radius (times the Sun) | Estimated Surface Reach (in our Solar System) |
|---|---|---|
| UY Scuti | ~1,700 | Beyond Jupiter’s orbit |
| Stephenson 2-18 | ~2,150 | Beyond Saturn/Uranus’ orbit |
Winner: Stephenson 2-18 is the largest known star by radius.
2. Mass Comparison
Although UY Scuti appears massive, it has a lower mass than Stephenson 2-18. This is because red supergiants lose a lot of mass due to stellar winds and radiation pressure.
| Star | Estimated Mass (compared to the Sun) |
|---|---|
| UY Scuti | ~7-10 times the Sun’s mass |
| Stephenson 2-18 | ~40 times the Sun’s mass |
Winner: Stephenson 2-18 has significantly more mass.
3. Luminosity Comparison
Both stars are extremely bright, far exceeding the Sun’s luminosity. However, Stephenson 2-18 is even brighter, making it more energetic.
| Star | Luminosity (compared to the Sun) |
|---|---|
| UY Scuti | ~340,000 times the Sun’s luminosity |
| Stephenson 2-18 | ~440,000 times the Sun’s luminosity |
Winner: Stephenson 2-18 is more luminous.
4. Distance from Earth
While both stars are in the Milky Way, Stephenson 2-18 is about twice as far as UY Scuti.
| Star | Distance from Earth |
|---|---|
| UY Scuti | ~9,500 light-years |
| Stephenson 2-18 | ~19,500 light-years |
Winner: UY Scuti is closer to Earth.
What Happens Next? The Future of These Giants
Both UY Scuti and Stephenson 2-18 are nearing the end of their life cycles. As red supergiants, they will eventually explode in a supernova, leaving behind either a neutron star or a black hole.
- UY Scuti’s Supernova – Could happen within the next few hundred thousand years.
- Stephenson 2-18’s Supernova – May occur sooner due to its higher mass.
When they explode, these stars will scatter heavy elements like oxygen, carbon, and iron into space, contributing to the formation of new stars and planets.
Fun Facts About UY Scuti and Stephenson 2-18
- If light could travel around Stephenson 2-18’s circumference, it would take over 9 hours to complete one trip!
- UY Scuti is not visible to the naked eye, even though it’s incredibly large, because it’s located in a dense part of the Milky Way.
- Both stars could eventually become black holes if they retain enough mass after their supernova explosions.
Which Star is the Largest?
While UY Scuti was once considered the largest star, newer observations have confirmed that Stephenson 2-18 is even bigger.
| Category | UY Scuti | Stephenson 2-18 | Winner |
|---|---|---|---|
| Size (Radius) | ~1,700 × Sun | ~2,150 × Sun | Stephenson 2-18 |
| Mass | 7-10 × Sun | ~40 × Sun | Stephenson 2-18 |
| Luminosity | ~340,000 × Sun | ~440,000 × Sun | Stephenson 2-18 |
| Distance from Earth | ~9,500 light-years | ~19,500 light-years | UY Scuti (closer) |
With its greater size, mass, and brightness, Stephenson 2-18 holds the title of the largest known star. However, as technology improves, astronomers may discover even larger stars in the future!
