The Universe is about 13.8 billion years old. This is already an astonishing figure, but it pales in comparison to the potential lifespan of certain stars. While blue giants blaze brightly and die out in a few million years, another family of stars can achieve immortality. How is this possible? The answer lies in the very engine of stars: nuclear fusion.
A star lives as long as it converts hydrogen into helium in its core. The mass of the star dictates the pace of this combustion. The more massive a star, the greater its internal pressure, and the faster and more violent the fusion. Conversely, the lightest stars, red dwarfs, consume their fuel with extreme frugality. Their lifespan can thus reach thousands of billions of years, a thousand times the current age of the Universe.
Astrophysical models of stellar structure, based on the work of Subrahmanyan Chandrasekhar (1910-1995) and refined by simulation codes like MESA, estimate that a red dwarf with 0.1 solar mass can burn peacefully for more than 6,000 billion years. This result comes from a simple but formidable relationship: the lifespan of a star (t) is proportional to its fuel reserves (its mass M) divided by the rate at which it consumes them (its luminosity L): \( t \sim \frac{M}{L} \)
The luminosity of a star increases very rapidly with its mass, according to the empirical relation \( L \propto M^4 \). Combining these, we get: \( t \propto \frac{M}{M^4} = M^{-3} \)
A star ten times less massive than the Sun would therefore live about \( 10^3 = 1,000 \) times longer, already placing its lifespan in the thousands of billions of years. Adding the fact that red dwarfs are fully convective, thus using almost all of their hydrogen reserves—whereas the Sun consumes only a fraction—we easily reach factors of 1,000 to 10,000 compared to the Sun's lifespan.
| Stellar type | Mass (M☉) | Lifespan (years) | Example star | Particularity |
|---|---|---|---|---|
| Blue giant | ~ 20 | ~ 8 million | Rigel (β Orionis) | One of the brightest stars in the sky, though distant. |
| Blue giant | ~ 15 | ~ 15 million | Spica (α Virginis) | Massive binary star whose primary component is very hot. |
| Blue giant | ~ 10 | ~ 30 million | Alnilam (ε Orionis) | Central star of Orion's Belt, it will end as a supernova. |
| Massive star | ~ 8 | ~ 100 million | Naos (ζ Puppis) | O-type star, extremely hot and luminous. |
| Intermediate star | ~ 3 | ~ 500 million | Mintaka (δ Orionis) | Multiple star, whose mass is still sufficient for a relatively short life. |
| Yellow-white star | ~ 1.8 | ~ 2 billion | Sirius A (α Canis Majoris) | The brightest star in the night sky, more massive than the Sun. |
| Yellow star | ~ 1 | ~ 10 billion | Sun | Our star, the reference for all comparisons. |
| Orange dwarf | ~ 0.7 | ~ 25 billion | Alpha Centauri B | Companion of the Sun's nearest neighbor, with a longer life than our star. |
| Red dwarf | ~ 0.4 | ~ 200 billion | Gliese 581 | Famous for its planetary system, including Gliese 581c. |
| Red dwarf | ~ 0.12 | ~ 6,000 billion | Proxima Centauri (GJ 551) | The closest star to the Sun (4.22 ly), very active despite its low mass. |
N.B.: Below the threshold (0.08 M☉), the object does not have sufficient internal pressure and temperature to sustain hydrogen fusion into helium. It is then a brown dwarf, sometimes called a "failed star."
What will become of these frugal stars at the end of their lives, in a future so distant that all other stars will have long since gone out? Their end will be uneventful: they will slowly contract, without explosion, without outburst.
As their hydrogen is depleted, they will transform into helium white dwarfs, stellar corpses that will no longer produce any energy through fusion. These stellar corpses will radiate their residual heat into the void, cooling until they become black dwarfs: cold, dense, and invisible objects, whose existence remains purely theoretical to this day, as the Universe is not yet old enough to have produced a single one.
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