The Universe Revealed

How does the Sun burn so long?

How does the Sun burn so long?

This is a question that early man asked because they thought that the Sun was actually burning like a fire. The fact is that if that were the case, the Sun would have burned out a long time ago.

It's hard to imagine that the Sun, that bright orb in the sky that provides both light and heat to the Earth, has been doing its thing for 4 billion years with very little change. How is that even possible?

The Sun has an equatorial radius of 686,342 kilometers. It's roughly a million miles in diameter. The Sun's volume is 1.41 times 10 to the 18 power kilometers cubed. The mass contained in this volume is 1.966 times 10 to the 30 power kilograms, which calculates to an average density of 1.046 grams per cubic centimeter. The density increases as we go down to the core, which has a density of 162.2 grams per centimeter cubed.

That bright object in the sky that you have trouble looking at is 93 million miles away. Could you imagine how bright it would be up close?

The sun converts 2.26 million metric tons of mass to energy every second. A metric ton is 1000 kilograms. So that's 10^3 (10 to the 3 power) times 2.26 times 10^6 kilograms per second or 2.26 times 10^9 kilograms per second. And, 60 seconds times 60 minutes times 24 times 365 is the amount of seconds in a year or 3.1536 times 10^7 seconds. Thus 7.127 times 10^19 kilograms are burned every year, which equals 1.966 times 10^30 kilograms. Dividing this into the mass we find that the Sun could theoretically last 2.768 times 10^13 years. That's a long time. I suspect that not all of the Sun's mass can be fused, so this is much longer than it will actually last. Astrophysicists believe that there is enough stuff in the Sun's core to continue shining in our sky for a very long (4 billions years) time.

The really strange thing about light that comes from the Sun is that the photons take a long time to get to the surface but only a short time to get here to the Earth. The reason for this is complicated, but the main reason is the fact that the core of the Sun is very compacted. Its pressure is millions of times that on the surface of the Earth, and the core's temperature is 15 million Kelvin degrees. The fusion of two hydrogen nuclei into a helium nuclei creates a very powerful gamma ray photon, which has a heck of a time getting out of the core. It is absorbed by nuclei and then readmitted at slightly lower energy. It takes anywhere from 10,000 to 170,000 years for this photon to get to the photosphere of the sun, and then only 8 minutes to bet the Earth. That means that the Sun isn't in a hurry.

The surface of the Sun is almost 6000 degree Kelvin and it's very bright. The Sun's luminosity is 3.848 times 10^28 lumens. A 100-watt incandescent light bulb puts out 1,600 lumens. Believe me, ten to 28 lumens is blinding even at 93 million miles away, and it has been burning this bright for over 4 billion years.

The Sun constantly blows particles and radiation into space. This is the solar wind and it is both a blessing and a curse. The solar wind blows out for hundreds of billions of miles until it runs into the cosmic radiation from interstellar space. This boundary is called the heliosphere, which is what Voyager 1 just passed through. This solar wind creates a magnetic field that gets twisted and curved so that it deflects a lot of cosmic radiation that would otherwise get here to Earth and bombard us. Some scientists believe that this is why life was able to develop and thrive on Earth. On the other hand, this solar wind could ablate our atmosphere away if the magnetic field that surrounds our planet were to fail. That's what happened to Mars.

Hopefully, our Sun will keep shining for a long time. We need it.

Thanks for reading.