The Future of the Sun

Updated 26 December 2025

The Sun is the star at the centre of our Solar System and its sunlight provides energy which is essential for life on Earth. The two key natural units of measuring periods of time, the year and the day, are both based upon the Earth’s motion with respect to the Sun and in many ancient cultures the Sun was regarded as a god.

Helios, the ancient Greek Sun god, who drove the chariot of the sun from the East to West across the sky each day.  

Our Solar System formed about 4.6 billion years ago, when a large cloud of dust and gas collapsed. More than 99% of the matter from the original cloud formed an object which became so massive and dense that nuclear reactions could start its core -the Sun. The remaining material which was left over became the planets, asteroids, comets and other bodies within the Solar system.

This post talks about what will happen to the Sun for the remainder of its lifetime, how it will evolve over time and the implications for life on Earth.

What kind of star is the Sun?

The Sun is one of 400 billion stars in the Milky Way galaxy [1].  If we plot how the brightness of stars varies with surface temperature then we get a diagram like the one below, known as a Hertzsprung Russell diagram.

In the diagram above the axes are as follows

• The x-axis gives the surface temperature of the star.
• The y-axis gives the brightness of the star, on a scale where the brightness of the Sun = 1. So a value of 100 means that the star is 100 times brighter than the Sun, 10,000 means that the star is 10,000 times brighter than the Sun etc.

Most of the stars in the galaxy lie in the region of the diagram which is labelled “Main Sequence”. In the top left of the main sequence are very hot bright blue stars, known as blue giants. In the bottom right of the diagram are relatively cool (even though they still have a surface temperature of around 3,000 degrees) dim stars known as red dwarfs. The Sun, marked with an “X” in the diagram, has a surface temperature of around 5,500 degrees Celsius and is an average yellow star on the main sequence. The stars above and below the main sequence are stars at the end of their lives and are called white dwarfs, red giants and super-giants and I’ll talk about them later in this post.

How does the Sun generate its energy?

Before I talk about how the Sun generates its energy I’ll give a brief overview of atoms. For more details see my post: A Brief History of the Universe.

All ordinary matter is made up of atoms which consist of a nucleus, which has a positive electric charge, surrounded by negatively charged electrons.

The nucleus, which contains nearly all the mass of an atom, consists of a number of positively charged particles called protons and neutrons which have no charge. Because the electrons have a negative charge, and the number of protons and electrons in an atom is always the same, the atom has a net charge of zero.

• The number of protons is called the atomic number and determines which element it is. You may remember from high school chemistry that this is its position in the periodic table.
• The number of neutrons in the nucleus does not affect the chemical properties of the atoms. In fact, all elements have isotopes, which have a different numbers of neutrons but the same number of protons.

The simplest possible atomic nucleus is that of hydrogen, which consists of a single proton.  Atoms which have 2 protons (regardless of the number of neutrons) are helium atoms, 3 protons are lithium atoms and so on.  The naturally-occurring element with the highest atomic number is uranium, which has 92 protons.

An atom of the most common isotope of carbon.

At very high temperatures, such as those found in the Sun, the electrons are not bound to the nucleus but can move around freely. This is known as a plasma.

The Sun generates energy by combining the nuclei of four hydrogen atoms to produce a nucleus of a single helium-4 atom, which has two protons and two neutrons (See Note 1). These nuclear fusion reactions can only  take place at the dense core, where the temperature is high enough for them to occur, about 15 million degrees Celsius.

The diagram above shows the internal structure of  the Sun. The core where all the Sun’s energy is generated is coloured white and  is quite small, only about 20% of its diameter. This works out as less than 1% of is volume. As you get further from the core the temperature drops and the surface it is a mere 5,500 degrees Celsius!

Future evolution of the Sun

In about 5 billion years time the Sun will have exhausted all the hydrogen at its core. No more nuclear reactions will take place. The core which by then will consist almost entirely of helium-4 nuclei, will shrink. There will still be plenty of hydrogen outside the Sun’s core and nuclear reactions will take place in a large shell surrounding it.The heat generated will cause outer regions of the Sun to greatly expand and it will become a red giant.

The diagram above shows the Sun in the red giant phase of its lifetime.

It is unclear exactly how large the Sun will get when it becomes a red giant. Current estimates are that it will expand to 100-250 times its current diameter [2]. If we take the lower value, the innermost planet Mercury (but not Venus and the Earth) will be swallowed up by the Sun.  At the higher value, the Earth would also be inside the Sun.

With the lower value, where the Sun expands to 100 times its current radius  value, on Earth the Sun would appear 10,000 times larger than it is today. The surface temperature of the Earth would be around 1500 degrees Celsius, hot enough for it glow a dull red colour. The Earth would have lost its atmosphere long before this and will be a bone dry scorched airless desert on which it will be impossible for life to exist.

What sunrise might look like in 5 billion years time. The sky would be black in daytime because the Earth would have no atmosphere

After the Sun becomes a red giant

The Sun will remain as a red giant, converting hydrogen to helium in an expanding shell around its core for about 1 billion years. During this time more and more helium will accumulate around the core, making the mass of the core grow. The core will gradually get hotter and more dense due to the weight of helium around it.

Producing Carbon in the Triple Alpha Process

When the temperature of the core reaches 100 million degrees, the intense temperatures and pressures will start another type of nuclear reaction in the core as shown in the diagram below. In this reaction, which occurs in stages, the overall effect is to combine three helium-4 nuclei to make a single carbon-12 nucleus (which has six protons and six neutrons)

This reaction is called the triple alpha process because another name of a helium-4 nucleus is an alpha partice. The name is a little misleading because it implies that three alpha particles all come together at the same time to make a carbon-12 nucleus. This is not the the case – the reaction proceeds as follows. (For simplicity in the remainder of this post when I refer to helium I will mean helium-4. Although, helium has another stable isotope helium-3 which has two protons and one neutrons, it is extremely rare)

• Two helium nuclei fuse together to form an isotope of beryllium, beryllium–8 (which has four protons and four neutrons)
• Beryllium-8 is extremely unstable.It has a half-life of about 8 x 10^-17 seconds i.e, it decays back into two helium nuclei almost the instant it is formed
• The very high temperatures and density in the red giant core means that ccasionally a third helium nucleus smashes into the Beryllium-8 nucleus before it decays. When this happens a carbon-12 nucleus is formed.
• This carbon-12 nucleus is formed in a excited state known as the Hoyle state after the British Astronomer Fred Hoyle (1915 -2001) who proposed it
• Carbon-12 in the Hoyle State  is very unstable. It has a half life of only 2.4×10⁻¹⁶ seconds. 99.96 % of the time it decays back into three helium nuclei. However 0.04% of carbon-12 in the Hoyle State  decay into carbon-12 in the ground state which is stable.

The Planetary Nebula Stage

When all of the helium in the core has been converted into carbon, nuclear reactions in the core will once again stop. The Sun will start to convert helium into carbon in a shell outside its core but will become unstable. It will vary widely in brightness as it flares up, ejects some of its outer layers into space and then contracts again. Eventually the whole of the outer regions of the Sun will be blown away forming a glowing shell of plasma called a planetary nebula.

 A planetary nebula

The planetary nebula will continue to cool and expand, getting fainter and fainter. After about 10,000 years it will have faded away altogether and spread out into the space between the stars – the interstellar medium. The planetary nebula will contain carbon, small amounts of nitrogen and oxygen plus other heavier elements  which were manufactured in the Sun. See note 2. Some of this material may in billions of years time find it way into gas clouds which condense and form new stars and planets. A form of stellar recycling!

 The White Dwarf Stage of the Sun

At the centre of the planetary nebula the remnant of the Sun’s core will collapse into an extremely dense hot star called a white dwarf.  The mass of the white dwarf will only be 30% of the original mass of the Sun. The remaining 70% of the Sun’s mass will have been lost into space as stellar wind or blown away as the planetary nebula.

White dwarfs are very compact. The white dwarf formed from the collapse of the Sun’s core will only be roughly the same size as the Earth, but will have about 100,000 times more mass – 1 litre of white dwarf material would have a mass of around 550 tons. At such densities ordinary atomic matter cannot exist, so white dwarfs are thought to consist of a special form of matter called degenerate matter which does not consist of atoms but rather a sea of atomic nuclei packed together with the electrons floating freely between them.

A white dwarf is essentially a dead star and does not undergo any nuclear reactions. It is very hot when it is first formed from the core of a star and the light we see from a white dwarf is this stored heat being radiated away. Over billions of years a white dwarf gradually cools down and becomes fainter and fainter and eventually will become undetectable. For a non technical overview of white dwarfs see the NASA website [4]

Implications for life on Earth

It is clear that any form of life which is found on Earth could not persist while the Sun is in its red giant phase. In fact the Earth is likely to become uninhabitable well before then because the Sun’s brightness is gradually increasing at a rate of 1% every 100 million years (ref 3). At some point within the next billion years a tipping point will be reached where a runaway greenhouse effect will occur to make the Earth so hot as to be uninhabitable.

However, these timescales are so huge that I believe that long before this occurs life on Earth is likely to have spread to other stars and planetary systems within our galaxy. This is such a large topic that I will discuss this further in my post on The Future of Humanity

Notes

1  Although the net result is that four hydrogen nuclei or protons combine to form a helium nucleus. This does not occur in a single step. In this reaction which is called a proton-proton chain reaction takes place in a number of stages. See http://hyperphysics.phy-astr.gsu.edu/hbase/astro/procyc.html for more details.

2 Although the main nuclear reactions taking place in the Sun produce helium and, in is later red giant stage, carbon, other less common nuclear reactions take place which produce small amounts of other elements such as oxygen, neon, nitrogen and magnesium.

Related Posts

I hope you have enjoyed this post. Other related posts include…

• The future of humanity – what will happen to life on Earth over the next few billion years.
• History of the Universe in the  standard big bang theory
• The End of the Universe. This discusses the ultimate fate of the Universe
• Fred Hoyle mentioned in this post, was one of architects of the Steady State theory in which the Universe does not change over time.

References

[1] Cain, F (2013) How Many Stars are There in the Universe?, Available at:http://www.universetoday.com/102630/how-many-stars-are-there-in-the-universe/ (Accessed 28 December 2025)

[2] http://www.scientificamerican.com/article/the-sun-will-eventually-engulf-earth-maybe/ (Accessed 28 December 2025)

[3] http://www.redorbit.com/news/science/1113029594/earth-oceans-will-dry-up-in-a-billion-years-121713/ (Accessed 28 December 2025)

[4] NASA (2010). White Dwarfs. [online] Nasa.gov. Available at: https://imagine.gsfc.nasa.gov/science/objects/dwarfs2.html. (Accessed 28 December 2025)

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