Astronomers speculate on the future of the solar system

The BBC's Science Focus Magazine website published an Aug. 21 article titled "The Life Cycle of Stars: How Will Our Solar System End? by Ian Todd. The full article is excerpted below.

The Sun formed about 4.6 billion years ago and will continue to exist for about 4.5 to 5.5 billion years. Although we cannot predict what will happen in the next billions of years, knowledge of how stars evolve has allowed astronomers to make broad inferences about how life on the Sun will probably develop. More massive stars could end their lives in an explosion known as a supernova, but this is unlikely to be the scenario that awaits our sun.

1. Hydrogen burning phase

The Sun converts 600 million tons of hydrogen into 4 million tons of energy every second: the rest is converted into helium "ash". The Sun's energy output has been increasing throughout its life cycle. It is believed that in the 4.6 billion years since its formation, its brightness has increased by 30%. Over the next billion years, the sun's brightness will increase by about 10% or so as more hydrogen is converted to helium, leading to an increase in heat energy. If we consider the effect that anthropological climate change is already having on our planet's weather patterns, you can imagine how strong this effect from the sun itself will be.

The increasing heat will cause the polar ice caps to start melting and the oceans to start warming, which will send water vapor into our atmosphere. The water vapor will absorb more heat, creating a "moist 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 be stripped away. The Earth will become like Venus: scorched, arid, and lifeless.

2. Sub giant phase

As frightening as this scenario is, it is only the beginning of the Sun's demise. In about 5 billion years, the Sun will reach the end of the main sequence phase of its life cycle and will have depleted all the hydrogen in its core.

With no fusion process to counteract the effects of gravity, the core will begin to shrink and become denser over time. During this process, the sun's temperature will rise and eventually ignite the remaining hydrogen outside the core.

This new fuel source will generate enormous amounts of energy, pushing the outer layers outward and expanding the Sun's diameter to two to three times its current size, turning it into a sub-giant star.

3. Red giant stage

As the Sun's surface layer pushes outward further, the Sun will develop into a giant luminous body called a red giant.

These aging stars can reach 100 to 1,000 times the size of the Sun, and the expanding surface area will cause the temperature of the outer layers to drop to about 3,000 degrees Celsius (the Sun's surface is currently 5,500 degrees Celsius). The lower temperatures mean that these stars glow in the redder parts of the color spectrum, hence the name "red giant".

As the Sun goes through this process, it will extend beyond the orbits of the inner planets Mercury and Venus, engulfing them completely and possibly even reaching the orbit of Earth. Our planet may not be destroyed, however, because the Sun will continue to lose mass during this expansion: some estimates suggest that at its maximum, only 65 to 70 percent may remain.

Gravity will thus weaken, and the orbits of the remaining planets in the solar system will begin to drift outward. Maybe the Earth will get away with it. Meanwhile, the Sun's core will get smaller and hotter, and a new nuclear reaction will occur 12 billion years after it forms.

4. A new red giant

The sun's core will continue to shrink until it reaches a temperature of about 100 million degrees Celsius - hot enough to ignite the helium produced during hydrogen depletion and convert it to carbon and oxygen. Since the dense core cannot expand to increase its energy output, the helium will burn violently, producing a brief explosion called a "helium flash. This will reduce the density of the core and bring temporary stability because the helium will now be able to burn at a more controlled rate.

However, it won't be long before the new fuel source is exhausted; it will only take about 100 million years. As helium continues to burn, it will generate enormous amounts of energy, just like the burning of hydrogen, which will cause the Sun to expand again and enter a second red giant phase.

5. Planetary nebulae

Despite the expansion and contraction, mass loss, and fuel depletion, the Sun's life cycle is not yet over. The red giant will continue to convert helium into carbon and oxygen, but the core will never reach the 600 million degrees Celsius needed to ignite the carbon, so it will shrink again.

As the helium runs out, the sun's outer layers will push further outward and disappear into space, so that about 12.5 billion years after the sun formed, it has half its mass left. The expanding outer layers will be illuminated by the hot core, creating a luminous cosmic cloud called a "planetary nebula".

These phenomena are well known to astronomers and are typical of aging stars with masses comparable to that of the Sun, but not related to planets. They are called "planetary nebulae" only because of their round, expanding shape.

6. White dwarf phase

As the Sun's outer layers eventually dissipate, only a hot, dense core known as a white dwarf remains. White dwarfs are the densest objects in the universe but are usually only slightly larger than the Earth. However, they can reach temperatures of more than 100,000 degrees Celsius.

During the aging process of the Sun, most of the heat generated in the core will be trapped in this stellar remnant, and it will take tens or even hundreds of billions of years for it to cool down.

7. Black dwarf phase

The remnant of the white dwarf will eventually use up all the remaining heat and light energy and (perhaps after hundreds of billions of years) gradually enter its final stage: a lifeless black dwarf. For now, black dwarfs are only hypothetical, since the universe is 13.8 billion years old and not yet old enough to produce any, but it is thought 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 cause it to lose most of its gravitational pull, causing the planet to drift farther away, leaving only frozen, burnt rock.

But as the remnants of our solar system disappear into space, particles from our dead sun could merge and restart the star-formation process. This could lead to the formation of planets with rocky bodies, atmospheres, and liquid water, ready for new life.