The Sun

Is the Sun going to die?

Is the Sun going to die?

Yes, it is. The Sun is only one of about 200 billion stars in our galaxy, Milky Way. Apart from holding the planet Earth, Sun is not exceptional and, like other celestial bodies, had a beginning and will have an end. But it will not take place anytime – the Sun still has about 5 billion years of life left.

The Sun
The Sun (Source: NASA). Credit: NASA/SDO

All the stars are sentenced to death – sooner or later . Some of them will live long and relatively peacefully. Some of them will have a short and turbulent life, culminating in a cosmic cataclysm. Generally, the smaller the mass of a star, the longer it will exist burning hydrogen and then possibly helium and heavier elements. But if the star is very massive, it is condemned to a fast, spectacular death in supernova explosion or contraction into a black hole.

Our Sun is just an average star. It has a rather low mass and total luminosity. It is a single and almost perfectly spherical body. Even its spectral type is just average – it is equal to G2. That is just somewhere in between the highest possible temperatures of very hot and young, blue or white stars (about 10000K) and very low temperatures of relatively cold, old stars with much more red spectra (~ 1000K). Sun is so-called yellow dwarf with a mean surface temperature of 5778K (5505°C). Finally Sun, like most stars in our galaxy, is a main-sequence star generating energy by nuclear fusion of hydrogen nuclei into helium in its extremely hot and dense core. Astronomers calculated that the rate of this fusion is equal to about 600 million metric tons of hydrogen each second. So the use of solar mass transformed into radiative energy is huge. How is it possible that Sun shines for so long? Its age is estimated at 4.6 billion years. The answer is simple: its total mass is just enormous.

The Sun in different wavelenghts
The Sun in different wavelenghts (Source: NASA). Credit: NASA/SDO/Goddard Space Flight Center

To understand the fate of the Sun one has to comprehend the overall processes governing the evolution of stars. They are quite complex. However, every star forms from the gravitational collapse of a large molecular cloud. Most of this matter soon accumulates at the center, while the rest creates so-called protoplanetary disk which is the base of a future planetary system (some stars do not have planets orbiting them). Then, the central and still gravitationally collapsing mass becomes very hot and dense, eventually initiating first possible thermonuclear reaction in its core – conversion of hydrogen into helium that provides a certain amount of radiated energy. This is the same energy we observe indirectly as solar light and heat.

According to the researchers Sun is currently at about half of its life and evolves at a stage called main-sequence. This means a phase of dominant reaction of nuclear fusion of hydrogen into helium. This phase will last for about 5 billion more years. During this period humanity and all other life forms on Earth will be relatively safe, if only we do not destroy the planet ourselves.

What is next? We can say that nothing lasts forever. In about 5 billion years the hydrogen contained in the center of our star will begin to run out. Instead there will be quite a lot of helium, which will begin its gravitational contraction due to the actual lack of radiative energy which is needed to balance the forces and stop the further collapse. It will start the next process of hydrogen burning in a shell surrounding the solar interior. At this interesting phase the Sun will be growing up slowly while its surface temperature will decrease and the observed spectrum will turn to red. Sun will expand more or less rapidly for few billion years until it is over two hundred times larger than now. Astronomers say that such a star enters the red giant branch phase. Finally, the helium core will start its own contraction due to the lack of fuel required to maintain the fusion. It will eventually reach the temperatures and densities needed to ignite new fusion of three helium nuclei into a carbon nucleus. Almost simultaneously carbon fuses with another helium nucleus, giving oxygen.

Star soon becomes larger and more luminous again. This phase if often called asymptotic giant branch and lasts relatively short. For our Sun it may take about 20 million years and after that Sun will become unstable. Its mass loss will be very fast and the star may begin to experience the huge thermal pulses increasing its size and luminosity. Some numerical models predict that after few such pulses lasting together for “only” about 500 000 years, a typical sun-like star loses a half of its initial mass. The most outer star layers lose strong connection with the star and move away, forming so-called planetary nebula composed of ionized gas ejected from star envelope due to the pulsations and/or stellar wind. The nebulae gradually melt in space and finally merge with the interstellar medium, enriching it with a large number of heavy elements produced in the core of the former star: carbon, nitrogen and oxygen.

Layers of the Sun
Layers of the Sun (Source: NASA). Credit: NASA

And what happens to the nucleus of the sun-like star after formation of the nebula? Because its mass is now too low to ignite the fusion of previously created carbon, further nuclear reactions cease and it becomes a white dwarf star composed of carbon and oxygen. Such star evolves without any source of incoming energy and it is only stable because of the internal pressure resulting from the complex process of so-called electron degeneracy. The exposed core of the former star initially has the temperature of over 100000 K but soon it starts to cool down through radiation due to the lack of fusion or other internal heating. White dwarf becomes more and more red and finally it should turn into black dwarf emitting no heat and no light at all. So this is the fate of our Sun and this is what its death will eventually look like. However, as scientists calculated, the duration of this phase of stellar evolution takes a long, long time. Longer than the age of the Universe (approximately 13.8 billion years). Therefore we do not yet observe any black dwarfs. Anyway, the death of the Sun is likely not to be spectacular but rather a calm and a very long process.

Author: Elzbieta Kuligowska

References & further reading:

The death of the Sun (EnchantedLearning)
Stellar Evolution – The Birth, Life, and Death of a Star (NASA)
Foreseeing the Sun’s Fate (NOAO)
White dwarf (Wikipedia)
White Dwarfs (Goddard Space Flight Center)