How Long Until Our Sun Runs Out of Fuel and Becomes a Red Giant?

The Sun, our parent star and the source of all life on Earth, has five billion more years of prosperity to enjoy. During this period, it will continue to burn hydrogen through nuclear fusion, but eventually, the hydrogen will be depleted. This marks the beginning of its journey into a red giant, a phase that will dramatically alter our solar system. Let's explore the timeline and implications of this remarkable transformation.

The Timeline of the Sun's Evolution

The Sun is currently in the prime of its existence, having been on the main sequence for about 4.6 billion years. This phase, where hydrogen is converted into helium, represents about 90% of the Sun's life. However, as the hydrogen supply dwindles, the Sun enters a new phase that will have far-reaching consequences for our solar system.

The Transition to a Red Giant

The transition begins around 5 billion years from now when the hydrogen supply in the Sun's core becomes exhausted. The Sun will then start to burn helium in its core, marking the start of its evolution into a red giant. This phase is characterized by significant expansion and increased luminosity. As the Sun expands, it will engulf the inner planets, including Mercury and Venus, and possibly even Earth.

The Helium Flash and Core Collapse

Within the Sun, the helium core piles up and becomes electron degenerate. The temperature and pressure in the core increase dramatically, reaching 10 times their current values. Approximately 1.2 billion years after leaving the main sequence, the core's temperature rises to the point where helium fusion begins. This process is vastly accelerated due to the electron degenerate state of the core, leading to a phenomenon known as the helium flash.

During the helium flash, the Sun's core rapidly burns helium, releasing an immense amount of energy. This event is so powerful that the Sun's luminosity temporarily equals that of all stars in the Milky Way. The core transitions back to a normal gas, causing a powerful expansion that releases enormous amounts of energy. After this collapse, the Sun cools exponentially, transforming into a subgiant with a double energy source: a core burning carbon and oxygen and an outer shell burning helium.

The subgiant stage is short-lived, as the Sun soon swells into a red giant. This expansion continues until the outer layers expand beyond the orbit of Jupiter. The core becomes even more electron degenerate, smaller, and denser. The layers surrounding the core begin to separate, gradually blowing away as a planetary nebula. This process occurs over a period of about 100,000 years, with each shell burning fuel at different rates.

In the end, after all the layers are blown away, only a hot stellar corpse remains: a white dwarf. This white dwarf will gradually cool until it no longer emits any light or heat, becoming a black dwarf. This fate will be common for all stars like the Sun.

Implications for Life on Earth

The survival of life on Earth will depend heavily on how far the Sun expands and whether it engulfs our planet. If the Sun expands to the orbit of Earth, it could cause significant changes, leading to the end of life as we know it. However, current scientific estimates suggest that this scenario may not occur, as the Sun's expansion is constrained by its mass.

Conclusion

The Sun's transition into a red giant is a dramatic and fascinating event that will unfold over billions of years. Understanding this evolutionary process helps us appreciate the finite nature of stellar lifespans and the vast timescales involved in astronomical phenomena.