Greetings, fellow spacefarers. Captain Nova here, reporting from the endless void. Yesterday, we marveled at the largest stars in the universe, those cosmic giants that burn furiously and shape entire galaxies. But today, we take a different journey—to the opposite end of the stellar spectrum.

What happens when a star is too small to shine brightly or too faint to be easily detected? What are the smallest stars in the universe, and how do they compare to their massive counterparts?

Buckle up as we explore the dwarfs of the cosmos, some of which could outlive the universe itself.

How Small Can a Star Be?

A star’s size is determined by the balance between gravity pulling inward and nuclear fusion pushing outward. If a star is too small, it won’t have enough mass to ignite fusion and shine like the Sun.

Scientists define the smallest possible stars based on their ability to sustain hydrogen fusion—the process that powers most stars. The theoretical lower limit for a true star is about 0.08 times the Sun’s mass (or 80 times the mass of Jupiter). Any smaller, and the object becomes a brown dwarf—a “failed star” that glows faintly but never truly ignites.

Now, let’s take a look at some of the tiniest stars that still make the cut.

The Smallest Main Sequence Stars

The smallest stars that can still fuse hydrogen belong to the class of red dwarfs—specifically, the M-dwarf stars at the very bottom of the size spectrum. These stars burn slowly, giving them lifespans that stretch for trillions of years.

1. EBLM J0555-57Ab – The Smallest Known Star

• Size: Barely larger than Saturn
• Mass: About 85 times Jupiter’s mass (just above the fusion limit)
• Location: 600 light-years away in the constellation Pictor
• Type: Red dwarf (M-dwarf)

This tiny star holds the record as the smallest fully-fledged star known. If it were just slightly less massive, it wouldn’t be able to sustain hydrogen fusion and would have been classified as a brown dwarf instead.

Despite being so small, EBLM J0555-57Ab still qualifies as a true star because it has enough pressure and temperature in its core to fuse hydrogen into helium. However, its brightness is so faint that it would be difficult to see even from nearby planets.

2. TRAPPIST-1 – A Tiny Star with Big Discoveries

• Size: Only slightly larger than Jupiter
• Mass: About 84 times Jupiter’s mass
• Location: 39 light-years away, in the constellation Aquarius
• Type: Ultracool red dwarf (M8 V)
• Notable Feature: Hosts 7 Earth-sized planets

TRAPPIST-1 is one of the smallest and coolest known stars, and yet, it has an incredible planetary system. Scientists have discovered seven Earth-sized planets orbiting it, three of which are in the habitable zone, where liquid water could exist.

Because TRAPPIST-1 burns incredibly slowly, it will likely outlive the Sun by trillions of years—giving its planets a much longer potential timespan for life to develop.

3. Proxima Centauri – The Closest Star to Earth

• Size: About 14% of the Sun’s diameter
• Mass: Around 12% of the Sun’s mass
• Location: 4.24 light-years away, in the Alpha Centauri system
• Type: Red dwarf (M5.5 V)

Proxima Centauri is the nearest star to our solar system, and it also happens to be a tiny red dwarf. Despite being so close, it’s too faint to see with the naked eye.

One of its planets, Proxima b, orbits within its habitable zone. However, Proxima Centauri is known for violent stellar flares, which might make life on its planets challenging.

Why Are These Small Stars So Important?

Though less flashy than giant stars, these small, cool stars dominate the universe. Red dwarfs make up about 75% of all stars, meaning they are far more common than stars like our Sun.

Here’s why they matter:

• They Burn for Trillions of Years
  • Unlike massive stars that burn out quickly, these tiny stars live extremely long lives.
  • Some of them will outlive the universe as we currently understand it.
• They Could Host Life for Eons
  • Since they burn slowly and steadily, planets orbiting red dwarfs could remain habitable for much longerthan those around stars like the Sun.
• They Challenge Our Definitions of What a Star Is
  • Stars like EBLM J0555-57Ab push the limits of how small a star can be, blurring the line between stars and brown dwarfs.

How Small Can a Star Get? (Theoretical Limits)

Scientists believe the absolute smallest possible star would have a mass around 80 times that of Jupiter, with a size only slightly larger than Saturn. Anything smaller wouldn’t be able to fuse hydrogen, meaning it would be a brown dwarf instead of a true star.

Brown dwarfs, sometimes called “failed stars,” glow faintly in infrared light but never ignite like true stars.

Some theoretical models even suggest that the smallest stars could be smaller than Jupiter, yet still manage to sustain fusion at a very slow rate.

What Happens to These Small Stars Over Time?

Unlike giant stars that explode in supernovae, small stars fade away peacefully. Here’s their typical life cycle:

1. Long Life as a Red Dwarf
   • These stars burn hydrogen extremely slowly, meaning they last trillions of years.
2. Gradual Cooling into a Blue Dwarf (Future Stage)

1. As they run out of hydrogen, they will slowly cool and shrink, turning into blue dwarfs.
2. The universe is still too young for us to see any blue dwarfs, since even the oldest red dwarfs haven’t had time to evolve into them yet.
3. Final Stage as a White Dwarf

1. After trillions of years, the smallest stars will become white dwarfs—faint, slowly cooling remnants.
2. Eventually, they will turn into black dwarfs, which are completely dark and cold.

Looking Ahead: White Dwarfs – The Ghosts of Dead Stars

Tomorrow, we’ll explore white dwarfs, the final stage of many stars, including the Sun. These strange, Earth-sized remnants are among the densest objects in existence, slowly cooling for eternity.

Until then, keep your eyes on the stars. Even the smallest ones hold some of the biggest mysteries in the universe.

Captain Nova
Odyssey Explorer