The life cycle of a star: how will our solar system end?


The Sun formed about 4.6 billion years ago and is expected to survive in its present state, roughly, for another 4.5 to 5.5 billion years. And while we can’t predict what will happen billions of years from now, knowledge of how stars evolve has allowed astronomers to largely infer how the Sun’s life is likely to unfold. More massive stars could end their lives in an explosion known as a supernova, but that’s not the likely scenario awaiting ours.

1. Hydrogen combustion phase

Every second, the Sun converts 600 million tons of hydrogen into four million tons of energy: the rest is converted into helium “ash”. Throughout its lifetime, the Sun’s energy output has steadily increased, and it is believed to have become 30% brighter in the 4.6 billion years since its formation. Over the next billion years, as more hydrogen is converted into helium, the Sun is expected to become about 10% brighter, leading to an increase in thermal energy. If we consider the effect that human-caused climate change is already having on our planet’s weather patterns, imagine the effect of such an increase.

The rising heat will cause the polar ice caps to melt and the oceans to warm, sending water vapor into our atmosphere. This water vapor will trap more heat, creating a “wet greenhouse” effect that will further increase global temperatures. In about 3.5 billion years, the Sun will be 40% brighter than it is today, causing our oceans to boil, the ice caps to melt completely and our atmosphere to crumble. The Earth will become like Venus: scorched, arid and lifeless.

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2. Subgiant phase

As horrifying as this scenario is, it is only the beginning of the Sun’s disappearance. In about five billion years, the Sun will have reached the end of the main sequence of its lifespan and will have consumed all the hydrogen in its core.

Without a fusion process to counter the force of gravity, the core will begin to contract and densify over time. As it does, its temperature will increase and eventually ignite the remaining hydrogen outside the core.

This new fuel source will generate huge amounts of energy that will push the outer layers outward, causing the Sun to expand two to three times its current diameter, turning it into a subgiant star.

Illustration of a view of Earth as the Sun dies in about 5 billion years © Detlev Van Ravenswaay/Science Photo Library

3. Red Giant Phase

As the surface layers of the Sun are pushed further, they will continue to trap heat from the dense core buried deep within this ever-expanding shell, and the star will grow into a huge luminous object called a red giant.

These aging stars can grow to sizes between 100 and 1,000 times that of the Sun, and surface expansion will cause the temperature of the outer layers to cool to about 3,000°C (the Sun’s surface is around 5,500°C today). The cooler temperature means these stars glow in the redder part of the color spectrum; hence the name “red giant”.

During this process, the Sun will expand beyond the orbits of the inner planets Mercury and Venus, engulfing them completely, and may even reach Earth’s orbital path. However, our home planet may not be completely destroyed, as during this expansion the Sun will continue to lose mass: some estimates suggest that at its largest size, only 65 to 70 might remain. %.

The gravitational pull will consequently be weakened and the orbits of the remaining planets in the solar system will begin to drift outwards. Maybe Earth will make a lucky escape. All the while, the Sun’s core will get smaller and hotter, until 12 billion years after its formation, a new nuclear reaction takes place.

4. A New Red Giant

The core will continue to contract until temperatures reach around 100 million°C – hot enough to ignite the helium produced when hydrogen is consumed and convert it to carbon and oxygen. As the dense core will not be able to expand to accommodate this increased energy output, the helium will burn with intense ferocity, producing a brief explosion known as a “helium flash”. This will reduce core density and provide temporary stability, as helium can now burn at a more controlled rate.

However, it won’t take long for the new fuel source to run out; only about 100 million years old. As the helium continues to burn, it will generate fierce energy and, just like with the burning of hydrogen, this will once again cause the Sun to expand into a second red giant phase.

5. Planetary Nebula

Despite all the expansion and contraction, mass loss and fuel consumption, the Sun’s life cycle is not yet complete. The red giant will continue to convert helium into carbon and oxygen, but the core will never reach the 600 million °C needed to ignite that carbon, so it will begin to contract again.

As the helium is depleted, the outer layers will be pushed further out and lost to space so that, about 12.5 billion years after its formation, half the mass of the Sun will remain. . The expanding outer layers will be illuminated by the hot core within, creating a glowing cosmic cloud known as a “planetary nebula.”

These phenomena are well known to astronomers and are typical of an aging star of the mass of our Sun, but have nothing to do with planets. Their name comes simply from their round, puffy shape.

6. White Dwarf

As the outer layers of the Sun have finally dissipated, all that remains is a hot, dense core known as a white dwarf. These objects are among the densest in the Universe, but are generally only slightly larger than our own planet. Nevertheless, they can reach temperatures above 100,000°C.

At the center of the South Ring Nebula is a hot, dense white dwarf. As it transformed into a white dwarf, the star periodically ejected mass – the shells of matter you see here. © NASA/ESA/CSA/STScI

Much of the heat that was generated in the core throughout the Sun’s aging process will be trapped in this stellar remnant, and it will take tens or even hundreds of billions of years to cool.

7. Black Dwarf

The rest of the white dwarf will eventually expend all of its remaining heat and light energy and (perhaps in hundreds of billions of years) fade into its final stage: that of a lifeless black dwarf. Currently, black dwarfs are only hypotheses because the 13.8 billion year old Universe is not yet old enough to have created one, but it is believed that this will be the final fate of our Sun.

As if to make the story even more tragic, the low mass of our once mighty star will have lost much of its gravitational pull, causing the planets to drift away, nothing more than frozen, charred rock.

But, as the remnants of our solar system are lost in space, particles from our own dead Sun could merge and start the process of star formation all over again. This can result in the formation of planets with rocky bodies, atmospheres, and liquid water primed for new life.

Illustration of the different stages that our Sun will go through

An illustration of the Sun’s life cycle © Mark Garlick/ Science Photo Library

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Requested by: Simon Gruffudd, Flintshire

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