We know that the Sun is large: it is a sphere with a radius of about 700,000 kilometers, that is, a hundred times larger than the Earth, which means that it would take a 747 plane 200 days to make a complete revolution around our beloved luminary. It also has a lot of mass, on the order of 2,000 trillion trillion tons, 300,000 times more than Earth, and it radiates light at a rate of 400 trillion trillion watts, which means that it emits 10 trillion times more energy than Earth. the one that consumes the whole humanity.

The Sun comes out of all conceivable human scales. To make things more accessible, it may be good to see our immense Sun from another point of view. If we divide its mass by its volume, or what is the same, we calculate its density, we will discover that it is only one and a half times more dense than liquid water . And if we divide its luminosity by its mass we find that each ton of solar material produces 0.2 watts. That is, the amount of solar stuff that can fit into a trailer container would produce less than a watt of power. Compared to the seven watts of Christmas tree bulbs, the truth is that the Sun is a poor producer of light . In fact, the human being emits thousands of times more energy in the form of body heat than the same amount of solar matter.

It was born, along with the entire Solar System, inside a cloud of gas and dust 5 billion years ago. But it was not born alone but within what is called an open cluster, a group of stars born from that same gas cloud. The cluster ended up disintegrating and each star went to live its life. Since then, the Sun and its entire cohort of planets revolve around the center of the Milky Way, making a complete revolution every 230 million years; Earth is about to be 22 galactic years old , and our recorded history, spanning 10,000 years, is only one ten-thousandth of the way. On our journey around the galactic center we pass through empty and densely populated regions, such as the spiral arms, which we encounter every 60 million years. During all that time our star has shone more or less constantly. That yes, every 11 years it suffers something like a spring rash , filling with spots, and each year it loses a hundredth of a billionth of its mass in the form of solar wind. It doesn’t sound like much, but that mass corresponds to more than 300 planets like Earth.

the life of the sun

At present, the life of the Sun runs peacefully and calmly, and will continue to do so for at least another 5 billion years. If we reduce its lifespan to a more human scale, say a century, we find that the Sun spent only two days in its initial protostar phase. Its youth and maturity will pass quietly burning hydrogen for about 80 years. Then, in a couple of weeks, it will become a red giant, and then it will have eight years of retirement left living off its energy savings, until death overtakes it like a cold black star. Today the Sun is at the height of its maturity: this huge ball of hydrogen and helium gas inside which nuclear fusion reactions take place is currently 40 years old.

Its life, like that of any star, is a continuous struggle against gravity , which tends to concentrate all the mass in the center. What prevents the collapse is the energy released in the fusion. However, just as cars run out of fuel, the Sun will run out of hydrogen. What will happen then? The core will contract while the rest, the envelope, will slowly expand. And slowly it will engulf and vanish Mercury, and it will engulf and vanish Venus. In this expansion, the solar surface will become increasingly cold and will acquire a reddish hue.

Until one day, in 6 billion years, on Earth there will be a last perfect day. After that, the temperature will rise, the oceans will boil, and all traces of life and civilization will disappear. The Sun will engulf the Earth and perhaps overtake our neighbor Mars. Then its expansion will stop.

For any extraterrestrial astronomer the Sun will not be a small yellow star but a red giant . Our Sun will have entered the last million years of its life.


R. Cohen (2010) Chasing the Sun, Random House

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