EXPLORE: Life Cycle of the Sun

1. Life as a Protostar

A protostar forms when gravity and pressure condense matter in a nebula's center. No nuclear fusion occurs yet. When nuclear fusion begins, a star is "born." Our Sun began burning hydrogen 4.5 billion years ago. Initially, it was smaller, cooler, and less bright, with enough hydrogen to burn for 11 billion years.

Key terms:

• GigaYear (Gyr): 1 billion years
• MegaYear (Myr): 1 million years
• Astronomical Unit (AU): The distance from Earth to the Sun (~150 million kilometers)

2. Main Sequence

The Sun is in its main sequence phase, fusing hydrogen into helium. This stage accounts for most of a star's life.

• Age: 4.55 billion years
• Mass: 1.99 × 10³³ grams
• Radius: 700,000 km
• Luminosity: 3.83 × 10²⁶ Watts
• Temperature: 5779 K
• Fuel Supply: ~50% of core hydrogen consumed

3. Sun in the Next Few Billion Years

As the Sun ages, it grows slightly larger, brighter, and hotter.

• 1.1 Billion Years: 10% brighter; Earth's atmosphere dries up.
• 3.5 Billion Years: 40% brighter; Earth becomes uninhabitable.

Next stage: the Red Giant phase.

4. Red Giant

At 12.2 billion years old, the Sun will expand into a Red Giant, engulfing Mercury. The outer layers expand, while the core contracts, initiating helium fusion.

Once helium is depleted, the Sun will shrink and cool, marking the end of this phase.

5. Planetary Nebula

During this brief phase, the Sun ejects its outer layers, forming a planetary nebula. Unlike a supernova, this process is less dramatic. Eventually, the remaining core becomes a dense white dwarf.

6. White/Black Dwarf

The Sun's core becomes a white dwarf, a dense remnant of its former self. Over trillions of years, it will cool and darken, transitioning into a black dwarf.

Scientists have not observed black dwarfs, as the universe is not old enough for them to form.