The Immortals of the Cosmos: When the Universe Goes Dark, They Will Still Shine

Illustration of a red dwarf, a frugal star with an almost eternal lifespan — The low luminosity of a red dwarf gives it an almost eternal lifespan. It reminds us that we live in the first age of stars, an age when most of them still shine. In trillions of years, only these frugal immortals will remain, silent witnesses to a Universe in which they will have seen all other lights born, radiate, and die.
Image source: astronoo.com (new window)

Scientific Summary

Red dwarfs, very low-mass stars (0.1 to 0.4 solar masses), are the "immortals" of the cosmos. Their hydrogen consumption is extremely slow because their luminosity scales as the fourth power of their mass (L ∝ M⁴). A red dwarf of 0.1 solar mass can thus live for over 6,000 billion years, a thousand times the current age of the Universe. Fully convective, they exploit nearly all of their hydrogen reserves. Their end will be unspectacular: a slow contraction into a white dwarf, then into a theoretical black dwarf.

What are these immortal stars that will outlive all others?

They are red dwarfs, very low-mass stars (about 0.1 to 0.4 solar masses). Their hydrogen consumption is extremely slow because their luminosity increases as the 4th power of their mass (L ∝ M⁴). A red dwarf of 0.1 solar mass can live for more than 6,000 billion years – a thousand times the current age of the Universe. They will still shine long after all other stars have gone out.

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Why Will Some Stars Still Be Alive in a Trillion Years?

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 (low-mass stars, less than 0.5 solar masses), 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.

How Long Do Stars Really Live?

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 classification: from the most massive to the most frugal
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 Next? The Fate of the Almost Eternal Stars

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 (degenerate star composed mainly of helium, the final stage of red 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.

FAQ: Everything about immortal stars (red dwarfs)

Why does a less massive star live much longer?

A star's lifespan is proportional to its mass divided by its luminosity (t ∼ M/L). However, luminosity increases very rapidly with mass, following the relation L ∝ M⁴. A star ten times less massive than the Sun therefore lives about 10³ = 1,000 times longer. Moreover, red dwarfs are fully convective, thus using almost all of their hydrogen reserves.

What is the difference between a red dwarf and a brown dwarf?

The threshold is around 0.08 solar masses (8% of the Sun's mass). Below this limit, the object does not reach sufficient internal pressure and temperature to initiate and sustain hydrogen fusion into helium. It is then a brown dwarf, sometimes called a "failed star," which is not a true star.

What happens to red dwarfs at the end of their lives?

Their end is unspectacular: they slowly contract and transform into helium white dwarfs, stellar corpses that no longer produce energy through fusion. They then radiate their residual heat until becoming black dwarfs, cold, dense, and invisible objects. However, the Universe is not yet old enough (13.8 billion years) for a single black dwarf to have formed.