There are 28 billion stars similar to the Sun in the Milky Way, but ours is truly special (at least to us). Its gravity holds the Solar System together , and its energy makes life on Earth possible . It also influences us in many other ways, as it is responsible for the weather and the changing seasons, ocean currents, and the beautiful phenomenon of the auroras.
But, like everything in the universe, sooner or later the Sun will cease to exist. What would happen if the Sun went out? What if the Sun disappeared right now?
How and when will the sun rise?
Stars like the Sun are made up of very large amounts of gas, especially hydrogen and helium. The immense pressure resulting from these vast masses produces such high temperatures that electrons are stripped from the atoms, and the ionized gases are called plasma.
Under these conditions, nuclear fusion occurs : as its name indicates, in this reaction the nuclei of atoms fuse. Thus, from two hydrogen cations (each consisting of one proton), a helium nucleus (containing two protons) is formed. This process releases a great deal of energy in the form of light and heat (our Sun has a temperature of 15 million °C, about 6 times that of a light bulb!). By releasing this energy in a perpetual explosion, a star manages to avoid being crushed by the weight of the gas that composes it. It is said to not collapse.
Inside stars like the Sun, hydrogen will run out after a few billion years. Once this "fuel" is depleted, nuclear fusion is no longer able to prevent the star's collapse, which releases even more heat and light due to the compression of the helium inside (because increased pressure corresponds to increased temperature).
However, due to this increase in energy, the star expands into a red giant , a star up to hundreds of times larger than our Sun and considerably cooler on the surface.
Cold stars?
At this point, the hydrogen in the star's outer layers is also depleted. This triggers another collapse of the celestial body, initiating another nuclear fusion cycle that, however, involves helium, forming nuclei of heavier elements such as carbon (with six protons) and oxygen (with eight protons). The resulting energy is insufficient to counteract the high pressure due to the weight of these elements, so the core shrinks. We are therefore talking about a white dwarf , a "cool" star with dimensions similar to those of Earth. Around it, it leaves behind its outer layers, which create a nebula : a very bright cloud of gas and dust that lasts for about 10 years.
It is estimated that the Sun will burn out in about 8.000 billion years —as many as the number of humans who inhabit Earth today. Our planet, along with Mercury and Venus, will likely vaporize much sooner, when the Sun becomes a red giant in 5.000 billion years. But we won't have to worry about that. Since the Sun expands by 10% every billion years, by then Earth will be so hot that its oceans will have evaporated, along with all life on Earth.
But imagine that the Sun simply goes out suddenly, like a light bulb. What would happen?
How long would life on Earth last if the sun went out?
If the Sun were to suddenly stop emitting light and heat, life on Earth would last much longer than we might expect. But let's take a closer look at what would happen.
The first weeks without sunlight
If the sun went out, how long would it take us to notice? At first, nothing would happen. For about eight minutes and twenty seconds, everything would be normal: in that time, the last rays of sunlight would reach the Earth's surface. Then, suddenly, darkness would fall everywhere; our satellite, the moon, would also go dark.
Since our galaxy, the Milky Way, is about 1/300th the brightness of the Moon, we would essentially only see by artificial light. However, due to the delay of sunlight at great distances, looking at the sky we would observe other planets in the Solar System as if they were unaffected by the problem: for example, Jupiter would remain illuminated for about half an hour after the Sun had set.
And what would happen to the temperature?
Within minutes of sunrise, temperatures would drop by 4 to 5 degrees Celsius in the sunlit areas. Meanwhile, strange behaviors would begin to be observed in animals whose life cycles depend on the passage of days: for example, some birds would stop singing. The daytime and nighttime activities of the fauna would continue to be fairly synchronized with the 24-hour cycle, thanks to internal clocks that regulate the daily functions of organisms even independently of the presence of stars. But many animals need the sun to know where to go, so they would begin to move erratically.
As soon as the light disappeared, photosynthesis would stop . Almost all the oxygen in the atmosphere comes from this process, but it would take a very long time to run out: thousands of years for it to be consumed by the living things that depend on it. But most plants, unable to photosynthesize, would die within a few weeks, and the animals that eat them would begin to starve. This would cause many physiological changes in the animals: increased energy storage, at the expense of growth and other functions not strictly related to survival. But that's not all.
The feeding frenzy
Hungry animals behave differently than usual : their priority becomes finding food, so they neglect activities like socialization, reproduction, and anti-predator behaviors (actions used to avoid being preyed upon) in order to explore wider territories. Therefore, if the sun were to go out, the increasing scarcity of plants and the species that feed on them would lead the surviving animals to behave more recklessly, and the dynamics between individuals and between different species would also be altered.
For example, the hermit crab Dardanus pedunculatus typically hosts symbiotic anemones that allow it to blend in, protecting it from predation. However, when the crustacean is hungry, it may feed on the anemones. All's fair in love and war.
The first year without sunlight
If the sun were to go out, the average temperature of our planet in Kelvin would drop by half every month: in just four weeks, we would go from about 300 K (27 °C) to 150 K (-123 °C). While larger plants have enough sugar reserves to survive in darkness for years, the temperatures would cause them to freeze in a matter of weeks . Likewise, within the first few months, almost all living things would become extinct due to the freezing temperatures.
But not all of our planet's heat comes from the Sun. Radioactive decay causes the Earth's core to reach temperatures of 5°C (approximately twice that of a light bulb); in addition, 20% of the Earth's subsurface heat comes from the high pressure exerted on rocks. Some of this heat also reaches the Earth's crust, particularly in geothermal zones , areas where geothermal energy finds a way to escape. These are volcanoes, fumaroles, hot springs, hydrothermal vents, and geysers: near these areas, life might last a little longer.
And we?
To survive, humans would have to establish settlements near these areas and, as temperatures drop even further, construct underground or thermally insulated buildings powered by nuclear power plants. Anyone not in these areas would certainly be dead within a year. Of course, a major problem would remain: how to eat? We could salvage some vegetables with artificial lighting, but it would be difficult to find solutions to feed many people well and for an extended period. Barring exceptional inventions, our species would likely only survive a few years.
If the sun went out: the first twenty years
Three years after the Sun's disappearance, the oceans would appear completely frozen: the water would have solidified kilometers below the surface. But ice is less dense than water and is a very good thermal insulator, so these ice sheets would float on a layer of water, protected from freezing. This would allow water to remain liquid on our planet for billions of years.
Some underwater organisms, including some deep-sea animals, do not depend on the sun for survival. Furthermore, the seabed would be warmed by heat from the Earth's interior: at depth, the ocean has an average temperature of 0-3°C. However, most of these organisms depend on decaying matter from the surface to survive: with the extinction of other life forms that rely on the sun and photosynthetic organisms, these creatures would also disappear.
However, underwater ecosystems such as hydrothermal vents—underwater "chimneys" from which geothermally heated water emerges—would be preserved. So too would the chemolithotrophic microbes that inhabit them: capable of converting heat, methane, and sulfur into energy, these organisms would survive as if nothing had changed.
Many other microorganisms could continue to live for millions of years even if the sun sets. There are hundreds of species of extremophiles (organisms that live only in extreme conditions) and extremely tolerant organisms (that survive even in such cases). Some psychrophilic microorganisms , that is, those that love the cold, live in the ice: they could represent the only form of life on the surface for the rest of Earth's existence. These include, for example, some Antarctic fungi . And then there are some organisms capable of remaining cryopreserved , suspended, waiting to return to life in warmth.
Because on the surface, meanwhile, the air would also cool more and more. To the point that, ten years after the Sun rose, the gases that make up the atmosphere would condense: there would be liquid air and then snow . After another ten years, the elements that make up the atmosphere would be completely frozen.
What if the sun disappeared?
In addition to the effects observed from the absence of light, if the Sun were to disappear completely, there would be effects dependent on the absence of gravity . Since gravitational waves travel at the speed of light, again no effects would be observed during the first eight minutes and twenty seconds. After that, our planet would begin to move almost in a straight line, tangent to its orbit, due to inertia.
At that point, it would be a lottery. Like Earth, all objects in the Solar System would begin traveling in a straight line . On its journey, at a speed of 30 km per second, our planet would encounter thousands of celestial bodies. It could collide with one, bringing about the end of everything, or be drawn into a new gravitational center, like another star acting as a Sun. In this case, the small glimmers of life that our planet still harbors could give rise to new species.