The Sun, a G-type main-sequence star, has been converting hydrogen into helium in its core for approximately 4.6 billion years. This nuclear fusion creates the outward pressure that counteracts the star’s immense inward gravity, maintaining its stable structure. It has about 5 billion years remaining in this phase before its demise begins.
Becoming a Red Giant
The Sun’s final transformation begins with the exhaustion of hydrogen fuel in its central core. Without this energy source, the inert helium core contracts. This gravitational collapse causes its temperature and density to rise.
The intense heat ignites the surrounding hydrogen layer, starting hydrogen shell burning. This shell-burning produces massive energy, pushing the Sun’s outer layers outward with tremendous force.
The Sun’s outer envelope will swell up to 200 times its current radius, extending nearly to Earth’s orbit. As the surface area expands, the energy spreads thin, causing the surface temperature to drop. The color will shift from yellow-white to a cooler, reddish-orange hue, officially becoming a Red Giant. This transition will span about a billion years.
The Sun’s outer atmosphere will engulf and vaporize both Mercury and Venus as it grows. The Earth’s fate is less certain, as the Sun will lose a significant fraction of its mass through intense stellar winds. This mass loss will cause surviving planetary orbits to drift outward. Even if not swallowed, Earth’s surface will be uninhabitable due to the increased heat long before the Sun’s physical edge reaches its orbit.
The core continues to contract until it reaches 100 million Kelvin, igniting helium burning. This rapid ignition, called the “helium flash,” temporarily stabilizes the star. It causes the star to shrink and become less luminous for about 100 million years. This helium fuel is quickly consumed, leading to the next, more unstable phase.
Ejecting the Outer Layers
Following core helium burning, the Sun enters its most volatile period of mass loss, the Asymptotic Giant Branch (AGB) phase. The star develops an inert core of carbon and oxygen, surrounded by unstable helium and hydrogen burning shells. This structure causes the star to expand once more to an even larger size.
The instability manifests as periodic bursts of energy called thermal pulses. These pulses are driven by fluctuating temperature and pressure in the fusion shells. They cause the outer envelope to swell and contract, increasing mass loss via powerful stellar winds.
The Sun will shed its entire outer envelope into space at a rapid rate, with mass loss reaching up to \(10^{-4}\) of the Sun’s mass per year. Over a few tens of thousands of years, this intense stellar wind will expel 50% to 70% of the Sun’s total mass.
The expelled material forms an expanding, colorful shell of gas and dust called a Planetary Nebula. This is a historical misnomer because early telescopes made them appear round. The nebula glows brightly because the intensely hot, newly exposed stellar core emits ultraviolet radiation.
This radiation ionizes the surrounding gas, causing elements like oxygen and nitrogen to fluoresce. The Planetary Nebula is a fleeting event, lasting perhaps 20,000 years before the gas expands and disperses into the interstellar medium.
The End Products: White and Black Dwarfs
Once the outer envelope is ejected and the Planetary Nebula fades, the super-dense, exposed core remains: a White Dwarf. The White Dwarf is a sphere roughly the size of Earth, containing about half of the Sun’s original mass, making it incredibly dense.
The matter is primarily carbon and oxygen, the “ash” left over from earlier fusion stages. Because no fusion is occurring, the White Dwarf is prevented from collapsing by electron degeneracy pressure. This pressure arises because no two electrons can occupy the same quantum state, stiffening the core against gravity.
The White Dwarf is initially extremely hot, potentially exceeding 100,000 Kelvin. It shines brightly, not due to nuclear fusion, but because of immense residual thermal energy radiating away. Over many billions of years, the White Dwarf will slowly cool and fade.
The final, theoretical state of this stellar corpse is known as a Black Dwarf. The time required for a White Dwarf to cool completely is calculated to be many trillions of years—far longer than the current age of the universe—meaning no Black Dwarfs are thought to exist yet.