The question of when our Sun will “blow up” touches on the eventual end of our star. The Sun is a G-type main-sequence star, a massive sphere of hot plasma that has been the steady source of energy for our solar system for billions of years. It is roughly 4.6 billion years old, having spent nearly half its expected lifetime in its current stable state. The dramatic end of its life involves a massive physical expansion rather than a sudden explosion, and current models predict this transformation will begin in approximately 5 billion years.
The Sun’s Present Life Cycle
The Sun exists in a long-lived, stable phase of stellar evolution known as the main sequence. This stability is maintained by a balance between the inward pull of gravity and the outward pressure generated by nuclear fusion in the core. In the core, heat and pressure cause hydrogen atoms to fuse together, creating helium and releasing tremendous amounts of energy.
This energy, which radiates outward as light and heat, provides the necessary pressure to resist the star’s own gravity. The Sun’s core currently converts about 600 billion kilograms of hydrogen into helium every second. As the Sun continues to burn its fuel, the core slowly accumulates helium, which does not participate in the fusion reaction at this stage.
Throughout its time on the main sequence, the Sun gradually increases its luminosity as the helium buildup modifies the core conditions. Stars like the Sun spend about 90% of their total lives in this hydrogen-burning phase. The Sun has about 5.4 billion years remaining before it exhausts the primary fuel in its central region and transitions to its next evolutionary stage.
The Great Expansion: Becoming a Red Giant
The Sun’s stable existence will end when the hydrogen fuel in its core is depleted. Without the outward pressure from hydrogen fusion, gravity will overcome the remaining forces, causing the inert helium core to contract dramatically. This contraction releases gravitational energy, which heats the layers of hydrogen surrounding the dense helium core.
The temperature in this shell surrounding the core will rise high enough to ignite a new round of fusion, initiating shell hydrogen burning. This shell burning will be far more energetic than the previous core fusion, causing the Sun’s outer layers to expand massively. This expansion transforms the Sun into a Red Giant star.
The Sun’s radius will swell to over 100 times its current size, its surface will cool, and its light will shift toward the red end of the spectrum, giving the star its new red giant appearance. The transition to the fully expanded Red Giant phase is a gradual process spanning approximately one billion years. At its maximum extent, the Sun’s outer atmosphere will reach well past the orbit of Venus and likely encompass the orbit of Earth.
The Sun’s Final Form
The Red Giant phase is an extended but temporary stage in the Sun’s evolution. As the core continues to contract and heat up, it will eventually reach about 100 million Kelvin. This immense pressure and heat will trigger the fusion of helium atoms into carbon and oxygen, an event known as the Helium Flash.
The onset of helium fusion will momentarily stabilize the Sun, causing it to shrink slightly and become less luminous as it settles into a new, short-lived phase of core helium burning. Once the helium fuel in the core is exhausted, the Sun will undergo a second, even larger expansion, fueled by helium and hydrogen burning in shells around an inert carbon-oxygen core. During this final instability, the star will expel its outer layers into space through powerful stellar winds.
This ejected material will form an expanding shell of gas known as a planetary nebula. The remaining core, composed mostly of carbon and oxygen, will be left behind as a dense, hot remnant called a white dwarf. Since the Sun is not massive enough to fuse carbon, it will not end its life in a supernova explosion. The white dwarf will slowly cool and fade over trillions of years, eventually becoming a cold, dark object.
What Happens to Earth?
The fate of Earth is determined long before the Sun reaches its maximum Red Giant size. As the Sun ages, its luminosity slowly increases, and this gradual warming will have dramatic effects within the next one to two billion years. The increased heat will trigger a runaway greenhouse effect, causing Earth’s surface temperatures to soar and its oceans to boil away.
By the time the Sun fully transforms into a Red Giant, Earth will already be a dry, uninhabitable world. During the Red Giant expansion, the Sun’s outer layers will extend past Earth’s orbit. The drag created by this stellar material will cause Earth’s orbit to decay, resulting in the planet spiraling inward and being vaporized.
The Sun’s expansion will fundamentally reshape the entire solar system. Mercury and Venus are certain to be engulfed and destroyed. Conversely, the outer planets and their moons will receive significantly more heat than they do today, potentially creating new, temporary zones where liquid water could exist on their surfaces.